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Krelina EPJ Quantum Technology (2021) 8:24
https://doi.org/10.1140/epjqt/s40507-021-00113-y
R E V I E W Open Access
Quantum technology for military
applications
Michal Krelina1,2*
*Correspondence:
michal.krelina@cvut.cz
1Faculty of Nuclear Sciences and
Physical Engineering, Czech
Technical University in Prague,
Brehova 7, Prague, Czech Republic
2Quantum Phi s.r.o., Bryksova 944,
Prague, Czech Republic
Abstract
Quantum technology is an emergent and potentially disruptive discipline, with the
ability to affect many human activities. Quantum technologies are dual-use
technologies, and as such are of interest to the defence and security industry and
military and governmental actors. This report reviews and maps the possible
quantum technology military applications, serving as an entry point for international
peace and security assessment, ethics research, military and governmental policy,
strategy and decision making. Quantum technologies for military applications
introduce new capabilities, improving effectiveness and increasing precision, thus
leading to ‘quantum warfare’, wherein new military strategies, doctrines, policies and
ethics should be established. This report provides a basic overview of quantum
technologies under development, also estimating the expected time scale of delivery
or the utilisation impact. Particular military applications of quantum technology are
described for various warfare domains (e.g. land, air, space, electronic, cyber and
underwater warfare and ISTAR—intelligence, surveillance, target acquisition and
reconnaissance), and related issues and challenges are articulated.
Keywords: Quantum warfare; Quantum technology; Quantum computing;
Quantum sensing; Quantum network; Quantum radar; Quantum imaging; Military
applications; Quantum security; Dual-use technology
1 Introduction
Although fourth generation modern warfare is characterised by decentralisation and the
loss of states’ monopoly on war [1,2], armies of advanced countries characteristically have
access to state-of-the-art military technologies. This includes the appearance of quantum
technologies on the horizon.
The term quantum technology (QT) means the technology mostly arising out of the so-
called secondquantumrevolution. Earlier,the first quantumrevolutionbroughttechnolo-
gies that are familiar to us today, such as nuclear power, semiconductors, lasers, magnetic
resonance imaging, modern communication technologies or digital cameras and other
imaging devices. The first quantum technology resulted in nuclear weapons and energy;
then, the classical computer gained a significant role. Presently, laser weapons are being
implemented and tested [3].
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Krelina EPJ Quantum Technology (2021) 8:24 Page 2 of 53
The second quantum revolution [4] is characterised by manipulating and controlling
individual quantum systems (such as atoms, ions, electrons, photons, molecules or var-
ious quasiparticles), allowing to reach the standard quantum limit; that is, the limit to
measurement accuracy at quantum scales. In this report, the term quantum technology
refers to the technology from the second quantum revolution. Quantum technology does
not bring fundamentally new weapons or standalone military systems, but rather signifi-
cantly enhances measurement capability, sensing, precision and computation power and
efficiency of the current and future military technology. Most of the quantum technolo-
gies typically are technologies of dual use. Consequently,there is tremendous potential for
military applications of quantum technology. Various studies and recommendations are
emerging, signalling the increasing likelihood of such technology being realised; see, for
example, [5–8].
Thisreport provides a more in-depth contextin which to understand the term ‘quantum
warfare’,discussing the possibility of its affecting theintelligence,securityanddefence sec-
tors, and describing new possible capabilities or improvements. The goal is not to provide
a precise forecast of quantum-based technologies, but rather to show possible directions
andtrendsinimplementation and applications. Quantum technologies in general arecon-
sidered emerging technologies, with the potential to change the conduct of warfare and
the outcomes of battles [8]. Although the current quantum technologies mostly have low
Technology Readiness Levels (TRL), they are believed to have disruptive potential [9].The
mapping of quantum technologies’ conceivable military applications is also important for
the further assessment of threats to global peace and in the discussion of ethics policies
or quantum-based preventive arms control.
This report comprises eight sections. In Sect. 2, the terms ‘quantum technology’ and
‘quantum warfare’ are defined, with the quantum technology taxonomy and quantum
technologies being introduced.Section 3providesthe basic quantumtechnologyoverview
that is the foundation for a particular application, including the expected time of deploy-
ment and utilisation impact. Section 4presents the general considerations and expecta-
tions regarding quantum technology development and deployment in the military do-
main. In Sect. 5, the applications of individual quantum technologies in the military are
presented for various domains (e.g. cyber, underwater, space and electronic warfare). Sec-
tion 6identifies and discusses the quantum hype as well as the realistic possibilities. Sec-
tion 7contains the initial discussion on related military, peace and ethical aspects as well
as the technical consequences and challenges. Section 8concludes the paper.
Sections 5and 4concern national security and defence issues. Although Sect. 3is based
on state-of-the-art research and provides related references, Sect. 5is based more on
various military or government reports, policy briefs and international security analyses
such as [5–8,10–13]. Here, the reader should be wary of the hype surrounding quantum
technology and avoid exaggerated expectations; this aspect is addressed in Sect. 6and by
[14]. For many of the presented quantum technology military applications, it is uncertain
whether all challenges connected with the demands of high-end military technologies will
be resolved, or even that the technology will actually be deployed.
2 Definitions
The term quantum technology is defined as follows:
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Krelina EPJ Quantum Technology (2021) 8:24 Page 3 of 53
Quantum technology (QT) is an emerging field of physics and engineering based on
quantum-mechanical properties—especially quantum entanglement, quantum su-
perposition and quantum tunnelling—applied to individual quantum systems, and
their utilisation for practical applications.
As follows from the definition, quantum technology describes the various physical princi-
ples of quantum-mechanical systems, with numerous applications; for instance, the tech-
niqueoftrapped ions can serve as aquantumbit for quantumcomputersand asaquantum
sensor for magnetic fields or quantum clocks.
Dual-use technology refers to fields of research and development with potential ap-
plication in both defence and commercial production [15].
Quantum technology is a typical dual-use technology which has been of considerable in-
terest not only for military but also for governmental actors [16] and peacekeeping organ-
isations.
Quantum warfare (QW) is warfare that uses quantum technologies for military ap-
plications that affect intelligence, security and defence capabilities of all warfare do-
mains, and it ushers in new military strategies, doctrines, scenarios and peace as well
as ethics issues.
There have been attempts also to define the quantum domain [17] as a new domain for
warfare. However, in this text, we will consider quantum technology as a factor that im-
proves all currently defined domains, rather than as a standalone warfare domain.
Subsequently,itishelpful to definethe term quantumattack,which refers tousingquan-
tum technologies to break, disrupt or eavesdrop on either classical or quantum security
systems. Typical examples are eavesdropping using quantum key distribution or quantum
computers breaking the Rivest–Shamir–Adleman (RSA) encryption scheme.
Although there is plenty of QT literature, there is no explicit agreement on quantum
technology taxonomy. We will use the following taxonomy:
•Quantum Computing and Simulations
– Quantum Computers (digital and analogue quantum computers and their
applications,suchasquantumsystemsimulation,quantum optimisation,...)
– Quantum Simulators (non-programmable quantum circuits)
•Quantum Communication and Cryptography
– Quantum Network and Communication (quantum network elements, quantum
key distribution, quantum communication)
– Post-Quantum Cryptography (quantum-resilient algorithms, quantum random
number generator)
•Quantum Sensing and Metrology
– QuantumSensing(quantummagnetometers,gravimeters,...)
– Quantum Timing (precise time measurement and distribution)
– Quantum Imaging (quantum radar, low-SNR imaging, ...)
Aside from the general quantum technology taxonomy presented above, we introduce
a new division of quantum technologies according to their benefits and utilisation. The
following classification can be generalised; however, we place more emphasis on military
applications. The quantum technology utilisation impact classification is as follows:
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Krelina EPJ Quantum Technology (2021) 8:24 Page 4 of 53
•Must have: quantum technology that has to be implemented to protect against future
quantum attacks (e.g. post-quantum cryptography);
•Effectiveness: quantum technologies that increase the effectiveness of the current
technology and methods (e.g. quantum optimisations, quantum machine learning or
artificial intelligence);
•Precision: quantum technologies that increase the precision of the current
measurement technology (e.g. quantum magnetometry, quantum gravimetry,
quantum inertial navigation, timing);
•New capabilities: quantum technologies offering new capabilities that were beyond
the scope of the present technology (e.g. quantum radar, quantum simulation for
chemistry, quantum cryptoanalysis, quantum key distribution).
Note that this classification is not mutually exclusive.
3 Quantum technology overview
This section provides a basic description of quantum technologies, with related refer-
ences. For each quantum technology, the current development status is presented, the
utilisation impact determined, expected time to deployment estimated,1and the main
challenges are sketched. For quantum computing application, the approximate number
of required logical qubits is provided.
Differentquantum technologies and theirapplications are atdifferent TRLs2fromTRL1
(e.g., some types of qubits) to TRL 8 (e.g., quantum key distribution).
Wearenotaiming for completenesshere, nor do wepresentany theoretical background,
but just introduce the basics, the effects and the current state of development, as needed
to follow the discussed military applications.
3.1 Quantum information science
Quantuminformationscience (QIS) is an informationsciencerelated to quantumphysics,
and deals with quantum information. In classical information science, the elementary car-
rier of information is a bit that can be only 0 or 1. The quantum information elementary
carrier of information is the quantum bit, qubit in short. A qubit can be |0or |1,oran
arbitrary complex linear combination of states |0and |1called the quantum superposi-
tion.
Theother crucialpropertyisthequantum entanglement. Quantum entanglementrefers
to a strong correlation between two or more qubits (or two or more quantum systems in
general)withnoclassical analogue. Quantum entanglementisresponsibleformanyquan-
tum surprises. Another feature is the no-cloning theorem [18], which says that quantum
information (qubit) cannot be copied. This theorem has profound consequences for qubit
error correction as well as for quantum communication security.
Quantum information science describes the quantum information flow in quantum
computing and quantum communications, although in a broader sense it can be applied
in quantum sensing and metrology, see [19,20].
1Short-term: 0–5 years, Mid-term: 6–10 years, Long term: 10–20 years.
2See Technology Readiness Levels according to EU,https://ec.europa.eu/research/participants/data/ref/h2020/wp/
2014_2015/annexes/h2020-wp1415-annex-g-trl_en.pdf
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Thereisconsiderableacademic interest, and several quantum algorithms have been cre-
ated [21]. However, only a few are expected to be valuable for defence and security appli-
cations.
3.2 Quantum computing
•Status: commercially available with very limited number of physical qubits
•Utilisation impact: new capability, effectiveness, precision
•Timeline expectation: one million physical qubits in ten years
•Main challenges: improving the quality of qubits (coherence, error resistance, gate
fidelity), upscaling the number of qubits, logical qubits
Quantum computing refers to the utilisation of quantum information science to perform
computations. Such a machine can be called a quantum computer. The classification of
quantum computers can be very complex. For the purposes of this report, we simplify the
classification as follows:
•Digital quantum computer (also called a gate-level quantum computer) is universal,
programmable and should perform all possible quantum algorithms and have
numerous applications described below. Classical computers can fully simulate the
gate-level based quantum computer. The difference is in resources and speed. For
instance, the simulation of fully entangled qubits increases the requirement of
classical resources exponentially. This means that the simulation of 45 qubits is
practically impossible on the classical (super)computers.
•Analogue quantum computer (also called Hamiltonian computation) is usually
realised using quantum annealing (as the noise version of the adiabatic quantum
computing). Quantum annealer differs from the digital quantum computer by the
limited connectivity of qubits and different principles. Therefore, the utilisation of
analogue quantum computers is more constrained but is still suitable for tasks such as
quantum optimisations or Hamiltonian-based simulations.
•Quantum simulator is used for the study and simulation of other quantum systems
that generally are less accessible and is usually built as a single-purpose machine. In
comparison with a quantum computer, the quantum simulator can be imagined as a
non-programmable quantum circuit.
In general, quantum computing will not replace classical computation. Quantum comput-
ers will be practical and useful for a limited type of problems only, typically problems with
high complexity. The actual deployment of quantum computing applications depends on
the quality (coherence, error resistance, gate fidelity) and the number of qubits. Some
of the essential parameters to follow are: the number of qubits, qubit coherence time,
quantum-gate fidelity and qubit inter-connectivity. The set of quantum instructions ap-
plying quantum gates on individual qubits is called a quantum circuit. A quantum circuit
is a practical realisation of the quantum algorithm.
Following[7], quantumcomputerscan beclassifiedintothree evolutionary stages: Com-
ponent quantum computation (CQC), Noisy intermediate-scale quantum (NISQ) com-
puting and Fault-tolerant quantum computing (FTQC). The CQC stage covers quantum
computing demonstrators and maturing the basic elements. CQC has a very limited com-
putational capability that is sufficient for demonstration of some proofs of principle. The
NISQ stage quantum computer should have a sufficient number of qubits to demonstrate
the advantages of quantum computing. Continuous research should lead to increasing
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Krelina EPJ Quantum Technology (2021) 8:24 Page 6 of 53
the number and quality of qubits. The FTQC stage starts when a perfect logical qubit is
reached (for an explanation, see below).
Physical qubits can be realised by numerous quantum systems. The most recent ad-
vanced are quantum computer based on superconducting qubits and the trapped-ion
qubits that are in or close to the NISQ stage. All other technologies, such as cold atoms,
topological, electron spin, photonic or NV centre-based qubits, are still in CQC stage or
theory only. The individual quantum computers and their performance differ significantly
(in, e.g. speed, coherence time, the possibility to entangle all qubits, gate fidelity). Various
metrics and benchmarks, such as Quantum Volume metric [22], have been developed for
their comparison.
The problem, common to all types of qubits, is their quality. A qubit is very fragile and
has a limited coherence time (a time scale during which it will not lose the quantum infor-
mation). Every operation performed on a qubit has limited fidelity. Researchers accord-
ingly need to employ the error correction codes. The error correction for qubits is much
more complicated than error correction of classical bits, because qubits cannot be copied,
as the no-cloning theorem explains. Two types of qubits are distinguished: the physical
qubit realised by a physical quantum system and the logical qubit consisting of several
physical qubits and error correction codes. A logical qubit is a perfect or near-perfect
qubit with very long-to-infinity coherence time, very high fidelity and higher environment
resistivity. For example, based on surface error correction protocol, for one logical qubit,
depending on the algorithm, up to 10,000 physical qubits [23] will be needed. For a recent
overview of quantum computing, see, for example, [24].
Examplesofleading-edge quantum computersare thequantumcomputerwith53 physi-
calsuperconductingqubits,manufacturedby Google (whichclaimedquantumsupremacy
in 2019 [25]), and the one by IBM. The best trapped-ion quantum computers are of 32
qubits by IonQ or six qubits by Honeywell. In the case of photonic qubits, there is a 24-
qubit quantum computer by Xanadu. The anticipated timelines as imagined by the quan-
tum computing roadmap by IBM and Google are the following: IBM plans a 433-qubit
quantum processor in 2022 and 1121 qubits by 2023 [26]. Google has announced a plan
to reach a quantum module of 10,000 qubits. All other quantum processors would consist
of such modules up to 1 million qubits in 2029 [27]. Based on a survey among leaders in
key relevant areas of quantum science and technology, it is likely that quantum computers
will start to become powerful enough to pose a threat to most of the public key encryption
schemes(formore details, see Sect. 3.2.2)inabouttwodecades[28]. Examples of analogue
quantum computers are the quantum annealer by D-Wave Systems with over 5000 qubits
and the coherent Ising machine by Toshiba.
The difference between analogue and digital quantum computers lies in their different
physical principles and their limitations. The digital quantum computer is limited by re-
sources and not by noise (noise can be corrected using more resources). In contrast, the
analogue quantum computer is limited by noise which is difficult to understand, control
and characterise (especially for a quantum annealer). Therefore, analogue quantum com-
puters’ applicability is limited [24].
In reality, the tasks accomplished by quantum computers will be mostly only subpro-
grams or subroutines of the classical computer programs. The classical program will not
only control quantum computers but will also provide a lot of computation that it would
be impractical to carry out on a quantum computer. This includes the recent applications
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Krelina EPJ Quantum Technology (2021) 8:24 Page 7 of 53
of quantum simulation in chemistry using, for example, the Variational quantum eigen-
solver (VQE) [29], which is a hybrid combination of classical and quantum computing.
Also, quantum computers are large machines, many of which require cryogenics. It is un-
likely therefore that in the decades to come most customers will acquire a personal quan-
tum computer, but rather they will access these as a service in the cloud. The cloud-based
models of quantumcomputing (often called Quantum Computing-as-a-Service – QCaaS)
are commercially available nowadays, even for free, and they allow access to anyone in-
terested in quantum computing. The cloud access to quantum computers is offered by
individual quantum hardware manufacturers. Some platforms, such as Microsoft Azure
QuantumorAmazon Braket, offeraccessto quantumcomputersof various manufacturers
within one ecosystem.
It is also helpful to clarify the terms of quantum supremacy, advantage and practicality.
Quantum supremacy isacasewhereaparticularproblemissolvedbyaquantumcom-
puter significantly faster than by a classical computer. However, the problem is likely to
be theoretical rather than practical. Quantum advantage refers to a case when a quantum
computer is able to solve real-world problems that classical computers cannot. Quantum
practicality is similar to quantum advantage, with the only difference being that the quan-
tum computer solves real-world problems faster than the classical computer.
We provide below a basic overview of possible quantum computer applications. The
reader should keep in mind that quantum computing is a fast developing sector, and new
revolutionary quantum algorithms are still waiting to be discovered. Note that, in the con-
text of quantum computing applications, the term ‘qubit’ implies a logical qubit. However,
small quantum circuits can be run with only physical qubits, with reasonable precision.
3.2.1 Quantum simulations
•Status: algorithms in development, small-scale applications
•Utilisation impact: new capability (e.g. quantum chemistry computation)
•Timeline expectation: short-term, usability scales up with the number of qubits
•Qubits requirement:∼200 (e.g. for nitrogen fixation problem)
•Main challenges:numberoflogicalqubits
Long before the first quantum computer was created, the main task for the quantum com-
puter was considered the simulation of other quantum systems [30]. Molecules are such
a quantum system. Despite advancement of extant computing power, the full simulation
of only simpler molecules can be performed using present computational chemistry, or of
larger molecules for the price of many approximations and simplifications. For example,
for a system with nelectrons, the classical computer would need 2nbits to describe the
state of electrons, whereas the quantum computer needs only nqubits. Therefore, quan-
tumsimulationsare the first and still perhaps the most promisingapplicationforquantum
computers.
The most dominant approaches are two: quantum-phase estimation [31]andquantum
variational techniques (VQE) [32,33]. The latter approach in particular has the highest
likelihood of success on NISQ computers; for example, in 2020, Google performed the
biggest quantum chemical simulation up to date (of H12 molecule using the VQE) [34].
Algorithms for quantum chemistry simulations are being developed. They can be ap-
plied to more complex simulations, hand in hand with the number of qubits. Therefore,
even at this early stage of quantum computing, there is significant interest from the chem-
ical and pharmaceutical industries. In general, such simulations allow the discovery and
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Krelina EPJ Quantum Technology (2021) 8:24 Page 8 of 53
design of new drugs, chemicals and materials. Recently considered topics, for instance,
are high-temperature superconducting, better batteries, protein folding, nitrogen fixation
and peptides research.
3.2.2 Quantum cryptoanalysis
•Status: algorithms ready
•Utilisation impact: new capability (e.g. public-key cryptography schemes breaking)
•Timeline expectation: mid- to long-term
•Qubits requirement:∼6200 for 2048 bit RSA factorisation [35], ∼2900 for 256 bit
ECDLP-based3encryption [36]
•Main challenges:numberoflogicalqubits
One of the most well-known quantum computer applications is the factorisation of large
primenumbersbyexponential speedupdescribed by Shor’salgorithm [37]. Thisisa threat
for public-key cryptography schemes, such as RSA, DH and ECC,4based on the large
prime number multiplications, the discrete logarithm problem or the elliptic-curve dis-
crete logarithm problem-based schema that are considered computationally intractable
or very hard for classical computers.
Although the resources of existing NISQ quantum computers are far from what is
needed forRSAbreaking,thethreat isquite real. Anadversaryorforeign intelligencecould
intercept and store encrypted traffic until the quantum cryptoanalysis becomes available.
Because the time of declassification of many secrets is far beyond the expected timelines
for powerful quantum computer delivery, such a threat can be considered genuine, nowa-
days.
Quantum cryptoanalysis also offers improved tools for a brute-force attack on the sym-
metric encryption schemes. For example, the well-known Grover’s searching algorithm
[38] reduces the key security by half against a brute-force attack; a 256-bit AES5key could
be resolved by brute force in roughly 2128 quantum operations. Despite the huge resource
requirements of quantum computers, doubling the symmetric key length [39]is recom-
mended, nevertheless. Moreover, Simon’s algorithm and superposition queries [40]can
completely break most message authentication code (MAC), and authenticated encryp-
tion with associated data (AEAD), such as HMAC-CBC and AES-GCM6[41,42].
Further, there is active research on cryptoanalytic attacks upon symmetric key systems
based on the structure present in symmetric cryptosystems, which can offer up to super-
polynomial speedup [43]. However, these algorithms suffer from excessive resource re-
quirements on the quantum computer.
3.2.3 Quantum searching and quantum walks
•Status: algorithms under development
•Utilisation impact: effectiveness (e.g. faster searching)
•Timeline expectation:short-tomid-term
•Qubits requirement:∼100, depends on the searched system size
3Cryptography based on Elliptic Curve Discrete Logarithm Problem.
4cryptography schemes named after Rivest–Shamir–Adleman, Diffie–Hellman, Elliptic Curve Cryptography
5Advanced Encryption Standard
6Hash-based Message Authentication Code-Cipher Block Bhaining, AES with Galois/Counter Mode
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Krelina EPJ Quantum Technology (2021) 8:24 Page 9 of 53
•Main challenges:numberoflogicalqubits
One of the most famous searching quantum algorithms is the Grover’s algorithm [38],
which offers quadratic speedup in database searching, or generally in inverting a function.
For an unsorted list or database, the classical searching algorithms are about complexity
O(N) (meaning proportional to the number of Nentities), although Grover’s algorithm is
about O(√N).
Quantum searching algorithms are an important topic for the so-called Big Data (un-
structured data) analysis. Working on a large amount of data requires a large quantum
memory. However, there is no reliable quantum memory that would keep the quantum
information for an arbitrarily long time and in large amounts. Second, the transformation
of classical data to the quantum form is time-consuming and ineffective. Therefore, only
the searching on data generated algorithmically is considered practical at the moment.
Theother approach tosearchingcan be based on the quantumrandom walk mechanism
[44], which offers similar speedup as Grover’s algorithm.
3.2.4 Quantum optimisations
•Status: algorithms in development
•Utilisation impact: effectiveness (e.g. faster solution of NP problems)
•Timeline expectation:short-tomid-term
•Qubits requirement:∼100, depends on the problem complexity
•Main challenges:numberoflogicalqubits
Quantumoptimisationis a very activelyexploredtopic,giventhe possibility of solving NP-
level7complex problems. An example of such an NP problem is the travelling salesman
problem. Here, given a list of places and the distances between them, the goal is to find the
shortest (and optimal) route. Naively, one can try all possibilities, but such an approach
has severe disadvantages, and may even become impossible, with increasing complexity.
Therefore, the most common solutions are based on heuristic algorithms that are not nec-
essarily guaranteed to find the best solution but at least one close to it.
Quantum computing introduces a new perspective on the issue and offers different ap-
proaches and techniques. The most dominant methods are currently based on a varia-
tional approach, such as the Quantum approximate optimisation algorithm (QAOA) [45].
Part of QAOA is the sub-technique called Quadratic unconstrained binary optimisation
(QUBO) [46], which is also suitableforanaloguequantumcomputers. Other methods are,
for example, the quantum analogy of the least-squares fit [47] or semidefinite program-
ming [48].
So far, it is not clear whether the quantum optimisation will offer some speedup against
the classical heuristic methods. However, there is consensus that if at all some speedup
is achievable, it will not be more than polynomial [48]. A new paradigm introduced by
quantum computing leads to new quantum-inspired classical algorithms, such as in the
case of QAOA [49] that delete the quantum speedup. On the other hand, we can speak
about quantum-inspired algorithms as of the first quantum computing practical result.
There have been many demonstrations, use cases and proofs of concept for quantum
optimisations, especially in connection with analogue quantum computing that currently
7NP is a complexity class characterised by the fact that it cannot be solved in polynomial time but can be verified in
polynomial time. Specifically, the NP-hard problems are not only hard to solve but are difficult to verify as well. Examples
of NP-hard problems are the Travelling Salesman Problem and Graph Colouring problems.
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Krelina EPJ Quantum Technology (2021) 8:24 Page 10 of 53
offers the most quantum computing resources for such applications. The typical demon-
strations were optimisations for traffic, logistics or the financial sector.8
3.2.5 Quantum linear algebra
•Status: algorithms in development
•Utilisation: effectiveness (e.g. faster linear equation solving)
•Timeline expectation:short-tomid-term
•Qubits requirement: depends on the solved system size
•Main challenges:numberoflogicalqubits
It has been shown that quantum computers can reach super-polynomial speedup for solv-
ing linear equations also. Such a speedup was estimated especially for the HHL (Harrow-
Hassidim-Lloyd) [50] algorithm for sparse matrices. However, the estimated speedup de-
pends on the size of the problem (matrix). There are also large resource requirements,
which for some problems can be considered too impractical [51]. On the other hand, for
the system of linear equations of 10,000 parameters, for instance, 10,000 steps are needed
to solve it, whereas the HHL can provide an approximate solution after 13 steps.
At present, many numerical simulations in planning, engineering, construction and
weatherforecastingsimplify complexproblems as a large set of linear equations. Formany
of them, being statistical in nature, the approximated solution could be sufficient.
Note that the HHL algorithm was shown as universal for quantum computing and was
demonstrated for various applications such as k-mean clustering, support vector ma-
chines, data fitting, etc. For more details, see [52].
One of the major caveats of quantum algorithms working with a large amount of input
data is data loading. Classical data, especially binary data or bits, need to be transferred
into quantum states for follow-on processing by efficient quantum algorithms. This pro-
cess is slow, and the classical data loading itself can take longer than the coherence time.
The solution is a quantum memory or quantum RAM [52,53].
3.2.6 Quantum machine learning and AI
•Status: algorithms in development
•Utilisation impact: effectiveness (e.g. better machine learning optimisations)
•Timeline expectation:short-tomid-term
•Qubits requirement:∼100, depends on the problem complexity
•Main challenges:numberoflogicalqubits
Due to the hype around classical machine learning and artificial intelligence (ML/AI), it
can be expected that there will be quantum research on this topic also. First, note that one
cannot expect full quantum ML/AI, considering the very low efficiency of working with
classical data [54], all the more so if the missing quantum memory and very slow loading
and coding of classical data (e.g. data of picture) into quantum information format are
considered.Itis simplynotpractical. Another situationwillemerge when ML/AIisapplied
to quantum data; for instance, from quantum sensors or imaging [55].
Nevertheless, quantum-enhanced ML/AI [56,57] can be introduced, where quantum
computing can improve some machine learning tasks such as quantum sampling, linear
8For examples of developed quantum optimisation applications at D-Wave’s quantum annealer, see
https://www.dwavesys.com/applications.
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Krelina EPJ Quantum Technology (2021) 8:24 Page 11 of 53
algebra (where machine learning is about the processing of complex vectors in a high-
dimensional linear space) or quantum neural networks [54]. One example is the quantum
support vector machine [58].
In fact, the ML/AI topic covers various techniques and approaches, and it is no different
in connection with quantum computing. Quantum ML/AI or quantum-enhanced ML/AI
is the subject of many research works nowadays. For a survey of quantum ML/AI algo-
rithms and their possible speedup, see [59].
3.3 Quantum communication and cryptography
Quantum communication refers to a quantum information exchange via a quantum net-
work that uses optical fibre or free-space channels. In most cases, quantum communica-
tion is realised using a photon as the quantum information carrier. However, due to the
limitations of photons, such as losses at large distances, the quantum network contains
other elements such as a quantum repeater or quantum switch.
The goal of quantum cryptography is to replace conventional (mainly asymmetric) en-
cryption schemes with quantum-resistant algorithms with the quantum key distribution.
The typical quantum features used for quantum communication are the following: quan-
tum entanglement, quantum uncertainty, and the no-cloning theory which states that
quantum information cannot be copied [18,60].
3.3.1 Quantum network
•Status: in research (commercially available for QKD with trusted nodes only)
•Utilisation impact: new capability, effectiveness (e.g. ultra secure communication,
quantum-resilient cryptography)
•Timeline expectation:mid-term
•Main challenges: quantum repeater and switch (quantum memory)
Theaim of the quantumnetwork(sometimescalledquantuminternet [61]orquantumin-
formationnetwork (QIN)) is to transmitquantuminformation via numerous technologies
across various channels. The quantum information (qubit) is usually carried by individual
photons, and as such the quantum information transmission is fragile. Moreover, many
quantum network applications rely on quantum entanglement.
Theusualchannelsfor quantum information transmission are specialised low-loss optic
fibres or the current telecommunication optic fibre infrastructures with higher loss. The
case of two communicating endpoints close to each other is as simple as using one optical
fibre. The complexity of the network increases with more end nodes or large distances,
where components such as a quantum repeater or quantum switch are required. Note
thatvery modest (onequbit) quantum processors aresufficientfor most quantum network
applications.
The free-space quantum channel is more challenging. Optical or near-optical photons
are of limited utility in the atmosphere due to the strong atmospheric attenuation. There-
fore, the most commonly considered and realised quantum network scenario is using
quantum satellites [62,63]. The advantage of satellites is the possibility of utilising optical-
photon communication for transmission of the quantum information, where the losses in
the satellite–ground link are lower than the loss between two ground nodes far apart.
Nevertheless, the optical photons’ communication in the free-space channel for short dis-
tances can be realised using, for instance, drones [64].Thebestwaywouldbetousethe
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Krelina EPJ Quantum Technology (2021) 8:24 Page 12 of 53
microwave spectrum as employed by classical wireless communication. However, com-
munication that uses the microwave spectrum at the level of individual photons is even
more challenging [65]. Microwave single-photon technology involves greater difficulty in
generating and detecting individual photons. Another problem is a noisy environment in
microwave bands.
Quantum communication at long distances requires quantum repeaters due to photon
loss and decoherence. A quantum repeater is an intermediate node that works similarly to
the amplifier in classical optical networks but needs to obey the no-cloning theorem. In
fact, thequantum repeaterallowsentangling qubits ofend nodes.When two endnodesare
entangled, the effect of quantum teleportation [66] can be exploited. This means that the
quantum information can be teleported without a physically sent photon; just a classical
communication is required. Utilising quantum entanglement, the quantum information
can then flow through a quantum network or part of it, which can even be under eaves-
dropper control without any chance of revealing the transmitted quantum information.
For correct functioning of the quantum repeater, quantum memory is required. However,
no reliable and practical quantum memory is available yet.
As an intermediate step, a trusted repeater can be used. The trusted repeater will not
entangle end nodes and is used for the quantum key distribution (QKD, see the next sec-
tion,3.3.2) only.To imagine how it works, let us consider two parties Aand Bandatrusted
repeater R. Then the key kAB is encrypted with key kAR. The trusted repeater Rdecrypts
kAR to get kAB. At this point, the trusted repeater Rknows the key kAB,andAand Bhave
to trust that the key is safe and not under the control of the eavesdropper. Finally, Rre-
encrypts kAB using the kRB key and sends it to B.ThisisatechniqueusedinpresentQKD
networks.
The next step, currently tested in experiments, is the measurement device-independent
QKD (MDI-QKD) [67,68]. It is a quantum protocol that not only replaces trusted re-
peaters (still not quantum, no support of entanglement) with secure repeaters, but also
serves as a switch. That means the usual star network topology and infrastructure can
start to be built. Note that in the MDI-QKD network, attacks on the central node physi-
cally cannot reveal the key nor reveal sensitive information. Later, the central nodes will
be replaced by the quantum switch and repeater, and the fully functional quantum infor-
mation network will be achieved.
Quantum networks will work in parallel with the classical networks, since not all trans-
mitted information needs to be encoded in quantum information. In fact, parallel classical
network is required, for instance, for quantum teleportation. Quantum networks can be
used for the following applications:
-Quantum key distribution (QKD), a secure transmission of cryptographic key (see
Sect. 3.3.2);
-Quantum information transmission between quantum computers or quantum
computing clusters at large distances or for sharing of remote quantum capabilities;
-Blind quantum computing [69,70] allowing to transmit a quantum algorithm to
quantum computer, perform computations and retrieve results without the owner or
eavesdropper knowing what the algorithm or result was;
-Network clock synchronisation [71], see Sect. 3.4.2;
-Secure identification [72] allowing identification without revealing authentication
credentials;
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Krelina EPJ Quantum Technology (2021) 8:24 Page 13 of 53
-Quantum position verification [73] allowing to verify the position of the other party;
-Distributed quantum computing [74,75] for several quantum computers, allowing to
compute tasks as one quantum computer;
-Consensus and agreement tasks refering to the so-called Byzantine Agreement
(problem of decision of group on one output despite the intervention of an adversary).
The quantum version [76] can reach agreement in O(1) complexity in comparison
with classical complexity O√n/log n.
-Entangled sensor network [77,78] allowing improvement in the sensitivity of the
sensors and reduction of errors, and evaluating global properties rather than
gathering data about specific parts of a system.
A quantum network allows direct secure quantum communication between quantum
computers, where quantum data can be directly exchanged. This can be useful for effec-
tive redistribution of computing tasks according to individual quantum computer perfor-
mance, mainly when an enormous task can be divided into smaller tasks. Another case is
thequantumcloud,wherequantumdatacanbesharedbetweenseveralquantumcom-
puters. Moreover, it is questionable whether it will be possible to build one standalone
high-performance quantum computer. The realisation will be more likely via distributed
quantum computing [74,75], where many quantum computers will be connected via the
quantum network.
3.3.2 Quantum key distribution
•Status: commercially available (with trusted repeaters)
•Utilisation:newcapability
•Timeline expectation:short-term
•Main challenges: secure quantum repeater (quantum memory), security certification
of the physical hardware
Quantum key distribution (QKD) is the most mature application of quantum communi-
cation. The goal is to distribute a secret key between two or more parties for encrypted
data distributed via classical channels. Due to the no-cloning theorem, any eavesdropper
has to perform a measurement that is detectable by communicating parties.
The dominant classes of protocols are two: one based on BB84 (Bennett-Brassard 1984)
protocol [79] and the other the E91 (Ekert 1991) protocol [80]. The dominant BB84
protocol is technically simpler but requires a quantum random number generation (see
Sect. 3.3.4), and the providing party has to prepare a key before the distribution. Protocol
E91 utilises quantum entanglement that generates the key during the process of distribu-
tion, and all parties know the key simultaneously. In this protocol, the quantum random
number generator is not required. However, the technical solution with quantum entan-
glement is more challenging. Both classes of protocols are information-theoretically se-
cure.
Theoretically, the QKD is impenetrable during the transmission. However, the typical
vector of attack can focus on the final (receiver/transmitter) or intermediate nodes where
the hardware of the software layer can contain vulnerabilities such as bugs in control soft-
ware, an imperfect single-photon source, parties verification problem, etc. This is an area
of very active research. For example, the imperfect physical hardware can be abused by
the so-called photon-number-splitting [81], or Trojan-horse [82] attacks. Here, security
certification of the hardware and software is necessary and will take time.
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Krelina EPJ Quantum Technology (2021) 8:24 Page 14 of 53
Apartfromtrustedrepeaters,theotherweakpointisthequbittransferrate,whichis
too slow to distribute long keys. A new high transfer rate of single-photon sources can
resolve the issue.
At present, QKD technology is commercially available as a point-point connection at
short distances or by using trusted repeaters at large distances. The trusted repeater can
be a space satellite, as was demonstrated by China [62,63].
3.3.3 Post-quantum cryptography
•Status: algorithms ready
•Utilisation impact:musthave
•Timeline expectation:short-term
•Main challenges: standardisation, implementation
Post-quantum cryptography (sometimes referred to as quantum-proof, quantum-safe
or quantum-resistant cryptography) represents an area of encryption techniques that
should resist future quantum computer attacks. Presently, this is not true for most of the
asymmetric encryption that uses public-key technology. On the other hand, most of the
symmetric cryptographic algorithms and hash functions are considered relatively secure
against attacks by quantum computers [83]. Nevertheless, doubling the symmetric key
length is recommended [39].
Now, several approaches are considered as quantum-resistant. For example, lattice-
based cryptography [84], supersingular elliptic curve isogeny cryptography [85], hash-
based [86] cryptography, multivariate-based [87] cryptography, code-based cryptography
[88] and symmetric key quantum resistance.
Unlike QKD, all these algorithms are not provably secure from a mathematical per-
spective. Therefore, within the process of standardisation, all these algorithms are rigor-
ously tested and analysed, including the implementation. There is no worst case where a
quantum-resistant algorithm with bugs in implementation could be cracked by a classical
computer [89]. The most followed standardisation process is the one by the U.S. National
Institute of Standards and Technology (NIST). The standardisation process is in the third
round [90], with three finalists (algorithms based on the lattice, code-based and multi-
variate) and several alternate candidates. The NIST standardisation process is expected
to conclude in 2023-24. Regardless, more and more commercial vendors are offering new
quantum-resistant encryption solutions now.
3.3.4 Quantum random number generator
•Status: commercially available
•Utilisation impact: new capability (truly random number generation)
•Timeline expectation:short-term
•Main challenges: increasing bit rate
Random number generators (RNG) are essential for many applications such as Monte
Carlosimulationsand integration, crypto operations, statistics and computergames.Nev-
ertheless, the RNG in a classical computer, since it acts deterministically, is not truly ran-
dom, and is called pseudo-random number generation. However, for many applications,
the pseudo-RNG is sufficient.
On the other hand, generating strong keys is the cornerstone of security, which can be
achieved only by truly random RNG. One solution is a quantum random number gener-
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Krelina EPJ Quantum Technology (2021) 8:24 Page 15 of 53
ator (QRNG) that is hardware-based. Moreover, QRNG is a crucial part of BB84-based
QKD protocols, to be provably secure.
QRNG can be used for any cryptography and makes all cryptography better. One of the
advantages of QRNG is that it can be verified and certificated [91], unlike any other RNG.
3.4 Quantum sensing and metrology
Quantum sensing and metrology is the most mature quantum technology area, which im-
proves timing, sensing or imaging. For example, atomic clocks from the first quantum
revolution have been part of the Global Positioning System (GPS) for almost half a cen-
tury. The current quantum clocks are coming up with much higher time measurement
precision.
Quantum sensing stands for all quantum technologies that measure various physical
variables such as external magnetic or electric fields, gravity gradient, acceleration and
rotation. Quantum sensors can produce very precise information about an electric signal,
magnetic anomalies and for inertial navigation.
Quantum imaging is a subfield of quantum optics exploiting photon correlations, allow-
ing suppression of noise and increasing the resolution of the imagined object. Quantum
imaging protocols are considered for quantum radar, detecting objects in the optically
impermeable environment, and in medical imaging.
Quantum sensing and metrology technology relies on one or more of the following fea-
tures: quantum energy levels, quantum coherence and quantum entanglement [92]. Indi-
vidual quantum sensors have various metrics that vary with the application. The common
metrics are: sensitivity (a signal that gives unity signal-to-noise ratio after 1 second of inte-
grationtime), dynamic range(minimaland maximal detectablesignal),sampling rate(how
often the signal is sampled), operating temperature, etc. Derived key metrics include, for
example, spatial resolution at a certain distance and the time required to achieve a spec-
ified sensitivity. The typical measure quantities are magnetic and electric fields, rotation,
times, force, temperature and photon counting.
3.4.1 Quantum electric,magnetic and inertial forces sensing
•Status: laboratory prototypes
•Utilisation impact: precision, new capability
•Timeline expectation:short-tolong-term
•Main challenges: miniaturisation, cooling
Manysensing quantumtechnologiesareuniversal andcanmeasurevariousphysicalquan-
tities. A detailed description of each technology is outside the scope of this report; how-
ever, a basic overview is provided. Many applications include various quantum technolo-
gies. For example, quantum inertial navigation consists of three types of sensing: accelera-
tion, rotation and time. In general, precise quantum-based timing is required for many
applications, not only for quantum technologies. For quantum timing, see Sect. 3.4.2.
Themostpromisingtechnologiesare:atomic vapour,cold-atom interferometry,nitrogen-
vacancy centres, superconducting circuits and trapped ions.
Cold-atom interferometry (measured quantities: magnetic field, inertial forces, time).
Atoms cooled at very low temperatures exhibit wave-like behaviour and are sensitive to
all forces that interact with their mass. The changes are observed in the interference pat-
tern [5,92,93]. The particular realisation can be in the form of Raman atom interferom-
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Krelina EPJ Quantum Technology (2021) 8:24 Page 16 of 53
etry, atom Bloch oscillation or others [94–96]. For example, in gravimetry, the quantum-
based gravimeter has the potential to reach about several orders of magnitude higher pre-
cision [5] than the best classical counterparts. Such a precise gravimeter allows very de-
tailed mapping of the Earth’s surface and underground with a resolution at the centimetre
level. Regarding inertial navigation, the shaken lattice interferometry has the potential to
overcome shortcomings of the state-of-the-art atom interferometry techniques and can
work as accelerometer and gyroscope at once [97]. Several challenges remain. Some of
the biggest challenges are the integration of the quantum sensor into one quantum iner-
tial measurement unit, miniaturisation of laser cooling apparatus used for cooling down
atoms and simultaneously maintaining the coherency (suppression of the interaction with
the noisy environment), or the dynamic range of the cold atom sensor outside the labora-
tory. However, significant advances also can be found in this area, e.g. [98]. For a review
see [99].
Trapped ions (measured quantities: electric and magnetic field, inertial forces, time).
Trapped ions are one of the most universal sensing platforms [100–102]. Well-controlled
trapped ionsformacrystal with quantised modes ofmotion.Anydisturbance can be mea-
sured through the transition between these modes. A single trapped ion can serve as a
precise measurement of time or as a qubit in a quantum computer. For inertial navigation,
the optical lattice technology of trapped cold atoms in 1, 2 and 3-dimensional arrays po-
tentially offers a sub-cm level in size. Besides allowing measurement of gravitational and
inertial parameters, it can measure Casimir or van der Waals forces. More recently, using
quantum-entangled trapped ions, measurement of electric fields has reached a sensitiv-
ity of ∼240 nV/ms–1 [103], which is several orders of magnitude better than the classical
counterpart.
Nitrogen-vacancy (NV) centres (measured quantities: electric and magnetic field, ro-
tation, temperature, pressure). Nitrogen-vacancy centre in a diamond crystal works as
an electron spin qubit that couples with external magnetic fields. In addition, negatively
charged NV centres using Berry’s phase can measure rotation. In general, NV centre-
based sensors offer high sensitivity, cheap production and operation in a wide range of
conditions[92,104,105].In particular,NVcentre-basedtechnologycan also work atroom
temperature and higher. A novel proposed 3D design allows to sense all three compo-
nents of magnetism, acceleration, velocity, rotation or gravitation simultaneously [106].
The strengths of NV centres in diamond-based sensing are spatial resolution and sen-
sitivity. On the other hand, the challenge is choosing, implementing and manufacturing
individual NV centres or their ensembles. In the case of electric field sensing, it is chal-
lenging to define a sensitivity [107].
Superconducting circuits (measured quantities: electric and magnetic field). The tech-
nology of superconducting circuits based on the Josephson effect describes the quantum
tunnelling effect between two superconductors [92]. This technology allows manufactur-
ing a quantum system at the macroscopic scale and can be controlled effectively with mi-
crowave signals. The superconducting quantum interference device (SQUID) is one of the
best magnetometric sensors. However, the disadvantage is the requirement of cryogenics.
Note that for the measurement of magnetic-field variations smaller than the geomagnetic
noise, the preferred design is based on an array of sensors to cancel the spatial-correlation
with applications, such as in medical and biomedical applications (e.g. MRI or molecule
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Krelina EPJ Quantum Technology (2021) 8:24 Page 17 of 53
tagging). The recent development shows that the superconducting qubits used in quan-
tum computers can be used to measure electric and magnetic fields [92] as well.
Atomic vapour (measured quantities: magnetic field, rotation, time). Spin-polarised
high-density atomic vapour undergoes state transition under external magnetic field
which can be measured optically [92,108,109]. An advantage is a deployment at room
temperature. The atomic vapour is suitable for rotation sensing, known as the Atomic
Spin Gyroscopes (AGS). AGS can be chip-scale [5]. For comparison, the best classical
rotation sensors are very precise (e.g. ring laser gyroscope). The expected quantum sen-
sor will be about twice as precise. However, the mentioned best classical gyroscope has a
size of 4 ×4m,whichisimpractical[110]. Atomic vapour cell magnetometers based on
atomic ensembles have the potential to outperform SQUID magnetometers and work at
room temperature [92].
3.4.2 Quantum clocks
•Status: laboratory prototypes
•Utilisation impact:precision
•Timeline expectation:short-tomid-term
•Main challenges: miniaturisation
Atomic clocks have been with us for several decades; for example, as part of GPS satel-
lites. The current atomic clocks are based on atomic physics, where the electromagnetic
emissions from electrons when changing energy level utilise a ‘tick’. As such, the atomic
clock is a very mature technology. Today, the atomic clocks based on atomic fountain or
thermal atomic beam and magnetic state selection principles can reach a relative uncer-
tainty ∼10–15 –10
–16 [111], or state-of-the-art chip-size atomic clocks have uncertainty
2×10–12 [5].
The second quantum revolution comes with new principles for atomic or quantum
clocks. Quantum logic clock is based on single-ion, a technology related to trapped-ion
qubit for quantum computing [101]. Quantum logic clock was the first with clock uncer-
tainty below 10–18 [112]. Quantum clocks can also benefit from quantum entanglement
[113].
Later, the quantum logic clock was superseded by experimental optical lattice clocks.
Note that the current atomic clocks work with microwave frequencies, i.e. the transition
between energy levels emits a microwave photon. The measurement of level transition
withthe emittedphotonin opticalfrequenciesisharder toachieve, althoughitoffers better
performance. Optical clocks are still in development, with systems being based on: single
ionsisolated inaniontrap, neutral atoms trapped in an optical latticeandatomspacked in
a 3D quantum gas optical lattice. The 3D quantum gas optical lattice clocks in particular
have demonstrated frequency precision 2.5 ×10–19 [114]. Recently, it was demonstrated
that quantum entanglement could enhance the clock stability [115].
Another research focuses on vapour-cell (or gas-cell) atomic clock that provides chip-
size realisation [116]; solid-state (for instance, the NV centre in diamond) clock [117]; or
nuclear clock with a similar principle as microwave or optical atomic clock, except that
it uses nuclear transition instead of electron transition in an atom’s shell [118], with the
potential for unprecedented performance, outstripping atomic optical clocks [119].
Various clock technologies have their own challenges, such as precise frequency comb,
laser systemforcontrol and cooling down and blackbody radiationshift(inthe caseofop-
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Krelina EPJ Quantum Technology (2021) 8:24 Page 18 of 53
tical clocks). Also, miniaturisation usually comes at the cost of lower frequency precision.
Another common type of challenge is the synchronisation of those clocks.
Precise timing is essential for many technologies, such as satellite navigation, space
systems, precise measurement, telecommunication, defence, network synchronisation, fi-
nance industry, energy grid control, and in almost all industrial control systems. However,
very precise timing is crucial for quantum technologies, especially for quantum sensing
and imaging. For instance, a very high precision clock allows new measurements, such
as gravity potential measurement down to the centimetre level at the Earth’s surface or
searching for new physics.
3.4.3 Quantum RF antenna
•Status: laboratory prototypes
•Utilisation:effectiveness
•Timeline expectation:short-tomid-term
•Main challenges: miniaturisation, cooling
Radio frequency (RF) antennas serve as receivers or transmitters of various signals.
They can be simple dipole antennas to complex AESA9modules. Their size limitation
is bounded by the wavelength of the produced or received signal. For example, a 3 GHz
signal has a wavelength of ∼10 cm and the size of the antenna should be no less than
approximately 1/3 of this wavelength. This is called the Chu–Harrington limit [120,121].
Rydberg atoms’ technology allows breaking this limit and having an antenna of the size
of a few micrometres independently on the receiving signal wavelength. Rydberg atoms
are highly excited atoms with a correspondingly large electric dipole moment, and there-
fore high sensitivity to external electric field [122,123]. Note that Rydberg atoms-based
antenna can only receive a signal.
Therecent prototypes of Rydbergatoms-basedanalyserwere demonstrated forfrequen-
cies 0 to 20 GHz for AM or FM radio, WiFi and Bluetooth signals [124]. The combination
ofmoreantennas can detect theangle-of-arrival ofthesignal [125]. Atthelaboratorylevel,
Rydberg atoms technology is available commercially.
Quantum RF receiver as a single cell (for targeted frequency, narrow bandwidth) or ar-
rayed sensor (broad frequency span) can find its applications in navigation, active imaging
(radar), telecommunications, media receiver or passive THz imaging.
3.4.4 Quantum imaging systems
•Status: laboratory prototypes and proof-of-concept verifications
•Utilisation:newcapability
•Timeline expectation:short-tolongterm
•Main challenges: improving resolution, high rate single-photon sources
Quantum imaging systems are a wide area covering 3D quantum cameras, behind-the-
corner cameras, low brightness imaging and quantum radar or lidar (for quantum radar,
see Sect. 3.4.5).
SPAD (Single Photon Avalanche Detectors) Array is a very sensitive single-photon de-
tector connected with a pulsed illumination source that can measure the time-of-flight
9AESA (active electronically scanned array) module consists of an array of small transmitters and receivers that are con-
trolled by a computer. Simply put, each cell of AESA can behave as an independent radar module.
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Krelina EPJ Quantum Technology (2021) 8:24 Page 19 of 53
from source to an object and hence the range of the object. Then, putting SPAD into an
array can work as a 3D camera. SPAD works with the optical spectrum with extension
developed to the near-infrared spectrum.
SPAD array can be used to detect objects out of the line of sight, too (e.g. hidden behind
the corner of a wall). The idea is based on laser and camera cooperation, where the laser
sends a pulse in front (e.g. a spot on the floor) of the SPAD camera. From the spot, the
laser pulse will scatter in all directions, including behind the corner, where the photons
can be reflected to the spot in front of the SPAD camera and then to the camera. SPAD is
sensitive enough to detect such a three-scattered signal [126].
Quantum ghost imaging [127–129], also known as coincidence imaging or two-photon
imaging, is a technique that allows imaging an object that is out of the line of sight of the
camera. In the source, two entangled photons are created, each of a different frequency.
Theone in theopticalfrequency isrecordeddirectlybya high-resolution photon-counting
camera. The second photon having a different frequency (e.g. the infrared) is sent toward
the object. The reflected photon is detected by a single-photon detector (the so-called
‘bucket’ detector). The image is then created from the correlations between both photons.
Theghost imaging protocol was demonstratedwithoutquantum entanglement, too (using
classical correlation), although with worse resolution.
Such a schema allows imaging an object at extremely low light levels. Also, infrared light
can better penetrate some environments with a better signal-to-noise ratio (SNR) [130].
Ghost imaging experiments that use x-ray or ultra-relativistic electrons were demon-
strated recently [131,132].
Sub-shot-noise imaging [133] is another quantum optics schema allowing detection of
a weak absorption object with a signal below the shot noise. Shot noise is the result of
fluctuations in the detected number of photons. For example, the shot noise is the limit
for lasers. This limit can be overcome using correlated photons. The detection of one
‘herald’ or ‘ancilla’ photon signifies the presence of the correlated photon that probes the
object or environment.
Quantum Illumination (QI) [134] is a quantum protocol to detect a target using two
correlated (entangled) photons. One photon, called the ‘idler’, is kept. The other, called the
‘signal’ photon, is sent toward the target and reflected, and both photons are measured.
The advantage of this protocol remains even when the entanglement is destroyed by a
lossy and noisy environment. QI protocol is one of those mainly adapted for the quantum
radar, but it can also be applied to medical imaging or quantum communication.
3.4.5 Quantum radar technology
•Status: laboratory prototypes and proof-of-concept verification
•Utilisation:newcapability
•Timeline expectation: long-term and more
•Main challenges: high rate single-photon source, quantum microwave technology
Quantumradar,inprinciple,workssimilarlyto classical radar,in thesensethat a signal has
to be sent toward the target, and the radar system needs to wait for the reflected signal.
Nevertheless, theoretically improved precision and new capabilities can be achieved by
quantum mechanical approaching.
Thereare several protocols consideredforquantum radar,such asinterferometricquan-
tum radar [135], quantum illumination (QI) [134], hybrid quantum radar [136,137]or
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Krelina EPJ Quantum Technology (2021) 8:24 Page 20 of 53
Maccone-Ren quantum radar [138]. None of the mentioned protocols is perfect. Inter-
ferometric quantum radar, for example, is too sensitive to noise and requires quantum
entanglement preservation. QI is an ideal protocol for a noisy environment and is even
laboratory-verified for microwave spectrum [139],butitrequiresknowledgeofthedis-
tance to the target, and such as it has no ranging function. Nevertheless, the QI-based
approach to quantum target ranging is under development [140]. This ranging problem is
also solved by the hybrid quantum radar, but at the expense of sensitivity. The Maccone-
Ren protocol has QI properties and ranging function, but it is only a theoretical concept
so far.
The biggest challenge common to all protocols is the high rate of generation of entan-
gled photons in (not only) a microwave regime. The quantum version of the radar equa-
tion [141] still holds the dominant term 1/R4,whereRis the radar–target distance. As a
result, the number of demanded entangled photons (modes) is several orders of magni-
tude higher than is available currently [142]. In a sense, quantum radar is similar to noise
radars and shares many properties such as the probability of interception, low probability
of detection, efficient spectrum sharing, etc., see [137] and references therein.
Another related challenge is target finding. Theoretical work [143]showsthatquantum
entanglement can outperform any classical strategy in finding the unknown position of
the target. Moreover, the presented method can work as a quantum-enhanced frequency
scanner for the fixed target range.
3.4.6 Other sensors and technology
•Status: laboratory prototypes
•Utilisation: new capability (e.g. chemical and precise acoustic detection)
•Timeline expectation:short-tomediumterm
•Main challenges: improving resolution
Quantum technologies can be used for ultra-precise sound sensing up to the level of a
phonon, a quasiparticle quantising sound waves in solid matter [144,145], using photoa-
coustic detection. Precise detection of acoustic waves is essential for many applications,
includingmedicaldiagnostics, sonar,navigation,trace gas sensingandindustrial processes
[146,147].
Photoacoustic detection can be combined with quantum cascade laser and used for gas
or general chemical detection. Quantum cascade laser (QCL) is yet a mature technology
[148]. QCL is a semiconductor laser that emits in the mid- and long-wave IR bands and,
as with many other quantum technologies, requires cooling far below –70◦C. However,
recent development allows chip-level implementation working at around –23◦C, which
can be achieved by a portable cooling system [149].
4 Quantum technology in defence
Military technologies have more demanding requirements than industrial or public appli-
cations. This requires greater caution, considering possible deployment on the battlefield.
Section 5presents various possible military applications with different TRLs, time expec-
tations and with multiple risks of realisation.
It will be simpler and less risky for technologies that are easily implemented and fit into
current technologies, such as quantum sensors where, simply put, we can replace a clas-
sical sensor with a quantum sensor.
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Krelina EPJ Quantum Technology (2021) 8:24 Page 21 of 53
On the contrary, QKD is an example of a technology that is already commercially avail-
able but is challenging to deploy. A lot of new hardware, systems and interoperability with
current communication systems are needed. Thus, this technology carries more signifi-
cant risks in terms of military deployment.
We can expect an advantage in lowering SWaP and scaling up quantum computers and
quantum networks in the long term. That will make the deployment easier and proba-
bly necessary if the nation/army wants to compete with other nations/armies with edge
(quantum) technologies.
4.1 Quantum strategy
The future users of military quantum technologies will have to think carefully about
whether, where and when to invest time and resources. The goal of the defence forces
is not to develop military technology but usually only to specify requirements and their
acquisition. However, they can participate significantly in development, especially if they
are the end user.
As a foundation, it is ideal to have a national quantum ecosystem in place composed of
industry and academic institutions. Such an ecosystem should be supported generally at
the government level, i.e. having a national quantum plan, but should also be motivated
to develop technologies for the defence sector. This can be achieved through appropriate
grant funding and even various thematic challenges, in which individuals and startups
can participate and perhaps bring new disruptive ideas and solutions. This will naturally
lead to closer cooperation with industry and academia. The quantum industry is quite
interesting, where there is a great deal of cooperation between academia and industry.
The first step is to establish a quantum technology roadmap or quantum strategy. The
roadmap/strategy should specify all the next steps, from identifying disruptive quantum
solutions, market survey, technology and risk assessment and development itself to pro-
totype testing and eventually solution deployment. The roadmap or quantum strategy can
consist of three parts:
1. Identification,
2. Development,
3. Implementation and deployment.
The most critical part is the identification of the most advantageous and disruptive
quantum technologies for the considered warfare domains. This step also includes the
technological and scientific assessment to balance technological risk (limited deployabil-
ity, performance below expectations, or impossibility of transfer from the laboratory to
the battlefield) versus the potential advantage of individual quantum technologies. This
process of identification should be repeated in cycles in order to react relatively quickly to
new discoveries and disruptive solutions. It is important to remember that many applica-
tions are yet to be identified or discovered.
The next step is the usual process of research and development (R&D). The R&D
should be sufficiently supported financially, but also with minimal bureaucratic obsta-
cles. It should involve fast development cycles with close interaction with the end user
of the military technology (specifications and performance consultations, prototype test-
ing,preparingforcertifications,...).Attheendofthisphase,thenewsystemattheinitial
operating capability should be ready.
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Krelina EPJ Quantum Technology (2021) 8:24 Page 22 of 53
The last step is to reach full operational capability, including modification or creation
of new military doctrines, preparing new military scenarios, strategies and tactics fully
exploiting the quantum advantage.
The final note pertains to the Identification phase. Here, the decision maker needs to
also assume the long-term perspective. So far, many quantum technologies have been
considered individually: sensors, QKD, quantum computing, etc. However, the long-term
vision considers the interconnection of quantum sensors and quantum computing via the
quantum network. Here, the theoretical and experimental works demonstrate additional
quantum advantage exploiting quantum entangled sensors and computers [77,78]. More
similarapplicationsmay yet be discovered or invented. Thisisimportanttoconsider when
the optical-fibre/quantum networks are being built. Later, the current elements such as
trusted repeater can be replaced by fully quantum repeaters and switches, allowing to
reach the full potential of the quantum network.
4.2 TRL and time horizon
As has been mentioned several times, various quantum technologies are at different TRL,
varying from 1 to 8. The TRL variation and time horizon expectations are even more com-
plex when considering various applications and deployment platforms, especially for mil-
itary purposes. Some TRL and time horizon estimates were provided in [150]. However,
some estimations, such as quantum precision navigation at TRL 6, seem too optimistic
based on what is described in this report.
Here, we provide our own TRL and expected time horizon in Table 1,whichcorrespond
to the findings of this work.
The reader can compare these with other timelines in [11,150].
The actual military deployment can take some time to overcome all technological ob-
stacles and meet military requirements. Take, for example, the quantum gravimeter for
undeground scanning. The first generation will likely be deployed as a static sensor placed
on a truck, and the range/spatial resolution will be rather low. In time, the next generation
will improve sensitivity and spatial resolution. Along with reduction of SWaP, the sensor
will be capable of being placed aboard an aircraft, and later on a drone and maybe on an
LEO satellite. However, it is also possible that the sensor’s limits will be reached earlier,
resulting in deployment becoming impossible, e.g. on a drone or LEO satellite.
Table 1 TRL and time horizon expectations. These expectation reflect general TRL rather than just
military TRL. Note that various quantum technologies are at different TRL within the same application
Technology TRL Horizon
Quantum computer (annealer) 4-5 (5-6) 2030
QKD (satellite) 7-8 (6-7) 2025 (2030)
Post-quantum cryptography 7-8 2025
Quantum communication network 1-3 2030-2035
Quantum inertial navigation 4-5 2025-2030
Quantum clocks 4-6 2030
Quantum radar 1-2 none
Quantum RF antenna 4 2025-2030
Quantum magnetic and gravity sensing 5-6 2025
Quantum imaging 5 2025-2030
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Krelina EPJ Quantum Technology (2021) 8:24 Page 23 of 53
4.3 Quantum technology countermeasures
A standalone section on quantum technology countermeasures is warranted, although
this topic will be touched upon, e.g. in Sect. 5.6 about the quantum analogy of the classical
electronic warfare. This topic is less studied, and few texts deal with this subject; besides,
a detailed description is beyond the scope of this report.
Briefly, this topic refers to the methods and techniques of spoofing, disabling or destroy-
ingquantumtechnologies, whether itisquantumcomputers, quantum networks or quan-
tum sensors and imaging systems. Quantum technologies exploit the quantum-physical
properties of individual quanta. As such, they are very susceptible to interference and
noise from the environment, and so can potentially be spoofed or paralysed. Especially in
relation to quantum networks and in particular to QKD, we speak about quantum hacking
[151–155], which has developed hand in hand with QKD itself.
Authorsand decision makers onquantumstrategy should keep inmindthat when quan-
tum technologies are deployed in the military field, various countermeasures will very
likely emerge sooner or later. What is currently unknown is the possible effectiveness of
quantum technology countermeasures and their impact.
5 Quantum technology military applications
Quantum technologies have the potential to significantly affect many areas of human ac-
tivity. This is especially true for the defence sector. Quantum technologies can impact all
the domains of modern warfare. The second quantum revolution will improve sensitivity
and efficiency, and introduce new capabilities and sharpen modern warfare techniques
rather than lead to new types of weapons.
The following text maps the conceivable quantum technology applications for military,
security, space and intelligence in different aspects of modern warfare, as sketched in
Fig. 1. It also mentions the industrial applications which may suggest quantum technolo-
Figure 1 Sketch of quantum warfare utilising various quantum technology systems
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Krelina EPJ Quantum Technology (2021) 8:24 Page 24 of 53
gies’ capabilities and performances, especially when no public information on military
applications is available.
It is important to notice that many applications are still more theoretical than realistic.
The significant quantum advancement achieved in the laboratory does not always result
in similar progress outside the laboratory. The transfer from laboratory to practical de-
ployment involves other aspects too, such as portability, sensitivity, resolution, speed, ro-
bustness, low SWaP (size, weight and power) and cost, apart from a working laboratory
prototype. The practicality and cost-effectivenessofquantumtechnologies will determine
whether particular quantum technologies are manufactured and deployed.
The integration of quantum technology into a military platform is even more challeng-
ing. Apart from quantum computers that will mostly be located at data centres similarly as
for civil use, the integration and deployment of quantum sensing, imaging and networks
faces several challenges posed by the increased demands of military use (in comparison
with civil/industry or scientific requirements). Forexample, the military level requirement
of precise navigation necessitates fast measurement rates that can be quite limiting for the
current quantum inertial sensors. There are more examples, and probably more are yet to
come.
Moreover, this area is still very young, and new technological surprises, both in a bad
and a good sense, could impose other quantum advantages or disadvantages.
5.1 Quantum cybersecurity
Key points:
• Necessity of quantum crypto-agility implementation.
• Operations that want to take advantage of Shor’s algorithm should start to collect the
data of interest before the quantum-safe encryption is deployed.
• The implementation of QKD needs to be carefully considered.
• In QKD, the endpoints will be the weakest part of the system.
Quantum advantage in cyber warfare can provide new, but on the one hand very effec-
tive (with exponential speedup), vectors of attack on the current asymmetric encryptions
(based on integer factorisation, the discrete logarithm or the elliptic-curve discrete loga-
rithm problem) and, theoretically, on symmetric encryption [90,156]. On the other side
are new quantum-resilient encryption algorithms and approaches, as well as quantum key
distribution. For an overview, see, for example, [157–160].
The current trend also is the development and employment of machine learning or arti-
ficial intelligence for cyber warfare [161]. For more details on the quantum opportunities,
see Sect. 5.2.
5.1.1 Quantum defence capabilities
The post-quantum cryptography implementation is the ‘must-have’ technology that
should be carried out as soon as possible. The risk that hostile intelligence is gathering
encrypted data with the expectation of future decryption using the power of quantum
computers is real, high and present [162]. This applies to military, intelligence and gov-
ernment sectors as well as to industry or academia where secrets and confidential data
are exchanged or stored. The current trend is to start preparing the infrastructure for
implementing quantum crypto-agility when the certified (standardised) post-quantum
cryptography becomes ready to deploy [90,156].
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New quantum-resilient algorithms can offer not only a new mathematical approach dif-
ficult enough even for quantum computers, but also a new paradigm of working with en-
crypted data. For instance, fully homomorphic encryption (FHE) allows the data to never
getdecrypted—evenif they arebeing processed [163]. Although the security applications,
such as for genomic data, medical records or financial information, are the most men-
tioned, applicationsfor intelligence, military or governmentareevident, too.Assuch,FHE
is a good candidate for cloud-based quantum computing to ensure secure cloud quantum
computation [164].
Note that post-quantum cryptography should be implemented in the Internet of Things
(IoT), or the Internet of Military Things (IoMT) [165], as a rapidly growing sector with
many potential security breaches. For an overview of post-quantum cryptography for IoT,
see [166].
Quantum key distribution (QKD) [160,167,168] is another new capability that allows
safe encryption key exchange where the security is mathematically proven. Although it
is impossible to eavesdrop on the quantum carrier of the quantum data (key), the weak-
nesses can be found at the end nodes and trusted repeaters, due to imperfect hardware
or software implementation. Another question is the cost, considering the quantum data
throughput, security and non-quantum alternatives independently if the solution is opti-
cal fibre-based or utilising quantum satellites. The QKD solution seems to be preferred in
EU [169], while the post-quantum encryption solution finds favour in US [170].
The last note refers to quantum random number generators. QRNG increases security
[171] and denies attacks on pseudorandom number generators [172].
5.1.2 Quantum attack capabilities
WithShor’salgorithm-basedquantum cryptoanalysisof Public key encryption (PKE)—for
instance, RSA, DH, ECC—the attacker can decrypt the encrypted data collected earlier.
There is no precise forecast when the so-called ‘Q-Day’, the day when a quantum com-
puter breaks the 2048-bit RSA encryption, will happen. However, the general opinion is
it will take about 10–15 years (based on a survey in 2017) [173]. A similar threat applies
to most message authentication codes (MAC) and authenticated encryption with asso-
ciated data (AEAD), such as HMAC-CBC and AES-GCM, because of Simon’s algorithm
and superposition queries.
One has to assume that such offensive operations already exist or that intense research
is being done. In 10 years, most sensitive communication or subjects of interest will be
using the post-quantum cryptography or QKD implemented in the next six years. That
means by the time a quantum computer able to crack PKE becomes available, most of the
security-sensitive data will be using a quantum-safe solution.
In theory, Grover’s algorithm weakens the symmetric key encryption algorithms; for
example, DES and AES. However, the quantum computing, and in particular quantum
memory, requirements are so huge that it seems to be unfeasible in the next few decades
[174].
Another vector of attack uses the classical hacking methods of classical computers that
will remain behind quantum technologies. In general, quantum technology is a techno-
logically young sector where plenty of new quantum system control software is being
developed. The new software and the hardware tend to have more bugs and security
breaches. For example, the current QKD quantum satellites working as trusted repeaters
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Krelina EPJ Quantum Technology (2021) 8:24 Page 26 of 53
controlled by a classical computer can be an ideal target for a cyber attack. Moreover, spe-
cificphysical-based vectors of attackagainst quantum networks (e.g. QKD) are thesubject
of active research [175], such as photon-number-splitting [81] or the Trojan-horse attack
[82], and future surprises cannot be excluded. For an overview of quantum hacking, see,
e.g. [157].
5.2 Quantum computing capabilities
Key points:
• Quantum computing capabilities will increase with the number of logical qubits.
• Most likely, quantum computing will be used as part of a hybrid cloud.
• Small, embedded quantum computing systems are desirable for direct quantum data
processing.
• General use for quantum optimisations, ML/AI enhancements and faster numerical
simulations.
Quantum computing will introduce new capabilities to the current classical computing
services, helping with computational problems of high complexity. Further, besides the
quantum simulations described above, quantum computing covers quantum optimisa-
tions, machine learning and artificial intelligence (ML/AI) improvement, quantum data
analysis, and faster numerical modelling [11,24]. The military problems that could be
solved with near-term quantum computers were presented in [10]. They are: Battlefield
or war simulations; Analysis of radio frequency spectrum; Logistics management; Supply
chain optimisation; Energy management; and Predictive maintenance.
To get the most effective results, future quantum computing implementation will be
in computing farms along with classical computers, which will create a hybrid system.
A hybrid quantum-classical operating system will analyse the tasks to be computed using
ML/AI, and split individual computations into resources such as CPU, GPU, FPGA,10 or
quantum processor (QPU), where the best and fastest result can be obtained.
A small, embedded quantum computer that could be placed, for example, in an au-
tonomous vehicle or mobile command centre is questionable. The current most advanced
qubit designs need cryogenic cooling. Therefore, more efforts should be focused on the
other qubit designs as photonic, spin or NV centres that can work at room temperature.
The embedded quantum chip could perform simple analytical tasks or serve for simple
operations related to quantum network applications where a straightforward quantum
data process is desired. Nevertheless, the machine learning and model optimisation of
autonomous systems and robotics can also benefit from ‘large’ quantum computers.
Quantum computing is likely to be efficient in optimisation problems [10,176,177].
In the military sector, examples of quantum optimisations could be logistics for overseas
operations and deployment, mission planning, war games, systems validation and verifi-
cation, new vehicles’ design and their attributes such as stealth or agility. At the top will be
an application for enhanced decision making, supporting military operations and func-
tions through quantum information science, including predictive analytics and ML/AI
[178]. Specifically, quantum annealers have proven themselves in verifying and validating
complex systems’ software code [179,180].
10Central Processing Unit; Graphics Processing Unit; Field-Programmable Gate Array
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Krelina EPJ Quantum Technology (2021) 8:24 Page 27 of 53
Quantum computers are expected to play a significant role in Command and Control
(C2) systems. The role of C2 systems is to analyse and present situational awareness or
assist with planning and monitoring, including simulation of various possible scenarios to
provide the best conditions for the best decision. Quantum computers can improve and
speed up the scenario simulations or process and analyse the Big Data from ISR (Intel-
ligence, Surveillance and Reconnaissance) for enhanced situational awareness. This also
includes the involvement of quantum-enhanced machine learning and quantum sensors
and imaging.
Quantum information processing will probably be essential for Intelligence, Surveil-
lance, and Reconnaissance (ISR) or situational awareness. ISR will benefit from quantum
computing, which offers a considerable boost to the ability to filter, decode, correlate and
identify features in signals and images captured by ISR. Quantum image processing in
particular is an area of extensive interest and development. It is expected that in the near
term situational awareness and understanding can benefit from quantum image analysis
and pattern detection utilising neural networks [13].
Quantum computing will enhance classical machine learning and artificial intelligence
[54], including for defence applications [178]. Here, quantum computing will surely not
be practical to carry out the complete machine learning process. Nevertheless, quantum
computing can improve ML/AI machinery (e.g. quantum sampling, linear algebra, quan-
tum neural networks). A recent study [181] shows that quantum ML provides an advan-
tage just for some kernels fitting particular problems. Quantum computing can possi-
bly enhance, in principle, most classical ML/AI applications in defence; for example, au-
tomating cyber operations, algorithmic targeting, situation awareness and understanding
and automated mission planning [182,183]. The most immediate application of quantum
ML/AIisprobablyquantum data; for instance,dataproducedbyquantumsensing or mea-
suring apparatus [55]. Actual applicability will grow with quantum computer resources,
and in eight years, quantum ML/AI can be one of the important quantum computing
applications [184]. Such applicability can be accelerated by hybrid classical-quantum ma-
chine learning where tensor network models could be implemented on small near-term
quantum devices [185].
Quantum computers, through quantum neural networks, can be expected to provide
superior pattern recognition and higher speed. This may be essential, for instance, in bio-
mimeticcyber defencesystemsthat protect networks, analogously to the immunesystems
of biological organisms [13].
Besides, through faster linear algebra (see 3.2.5), quantum computing has the poten-
tial to improve the current numerical linear equation-based numerical modelling in the
defence sector, such as war games simulations, radar cross section calculations, stealth
design modelling, etc.
In the long term, the quantum systems can enable Network Quantum Enabled Capa-
bility (NQEC) [13]. NQEC is a futuristic system that allows communication and shar-
ing information across the network between individual units and the commander to re-
spond quickly to battlefield developments and for coordination. Quantum enhancement
can bring secured communication, enhanced situational awareness and understanding,
remote quantum sensor output fusing and processing, and improved C2.
5.3 Quantum communication network
Key points:
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Krelina EPJ Quantum Technology (2021) 8:24 Page 28 of 53
• Various security applications (e.g. QKD, identification and authentication, digital
signatures).
• The adoption of security applications will happen as quickly as all new technology
security aspects are explored, carefully.
• Quantum clock synchronisation allows utilising higher precision quantum clocks.
• Quantum internet is the most effective way of communication between quantum
computers and/or quantum clouds.
Quantum internet stands for a quantum network with various services [186] which have
significant, andnotonly security,implications. However,many progressive quantumcom-
munication network applications require quantum entanglement; that is, they require
quantum repeater and quantum switch. Recall that the trusted repeaters can be used for
QKD only (see Sect. 3.3.1). Future combinations of optical fibre and free-space channels
will interconnect various end nodes such as drones, planes, ships, vehicles, soldiers, com-
mand centres, etc.
5.3.1 Security applications
Quantumkeydistribution is one ofthemost matured quantumnetwork applications. This
technology is going to be interesting for the defence sector later, when long-distance com-
munication using MDI-QKD or quantum repeaters becomes possible. Currently, basic
commercial technology that uses trusted repeaters is available. These pioneers can serve
as a model of how quantum technologies can be employed. Here, QKD companies pro-
motethetechnology as the most secure, andmoreand more use cases appear,especially in
the financial and healthcare sectors. On the other hand, the numerous recommendation
reports and authorities are more circumspect; for example, the UK National Cyber Secu-
rity Centre [187] that does not endorse QKD for any government or military applications
in its current state.
ApartfromQKD,which distributes the key only,the quantum networkcouldbe usedfor
quantum-secure direct communication (QSDC) [188–191] between space, special forces,
air, navy and land assets. Here, the direct messages encrypted in quantum data take ad-
vantage of security similar to QKD. One obstacle could be a low qubit rate, which will only
allow sending simple messages and not audiovisual and complex telemetry data. In that
case, the network switch to the QKD protocol for distributing the key and the encrypted
data will be distributed over classical channels. Other protocols such as quantum dialogue
[192] and quantum direct secret sharing [193] aim to use the quantum network for prov-
able secure communications as QSDC. Note that QKD and QSDC are considered to be a
native part of 6G wireless communication networks and discussed accordingly in [194].
Another significant contribution of the quantum approach to security is the quantum
digital signature (QDS) [195]. It is the quantum mechanical equivalent of a classical digital
signature. QDS provides security against tampering of a message after a sender has signed
the message.
Next, quantum secure identification exploits quantum features allowing identification
withoutrevealingauthenticationcredentials [72]. Non-quantum identificationisbased on
the exchange of login and password or cryptographic keys, which allows intruders to at
least guess who has tried to authenticate.
The other application is position-based quantum cryptography [196,197]. Position-
based quantum cryptography can offer more secure communication, where the accessed
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Krelina EPJ Quantum Technology (2021) 8:24 Page 29 of 53
informationwillbeavailable only from aparticulargeographicalposition,suchascommu-
nication with military satellites only from particular military bases. Position-based quan-
tumcryptography canalsoprovide secure communicationwhenthe geographical position
of a party is its only credential.
5.3.2 Technical applications
Quantum network will perform network clock synchronisation [71,198]thatisalreadya
major topic in classical digital networks. Clock synchronisation aims to coordinate other-
wise independent clocks, especially atomic clocks (e.g. in GPS) and local digital clocks
(e.g. in digital computers). A quantum network that uses quantum entanglement will
reach even more accurate synchronisation, especially when quantum clocks come to be
deployed (for Time standards and frequency transfer see Sect. 5.4). Otherwise, the high
precision of quantum clocks would be utilised locally only. Precise clock synchronisation
isessentialforthe cooperation of C4ISR (Command, Control,Communications,Comput-
ers, Intelligence, Surveillance and Reconnaissance) systems for accurate synchronisation
of various data and actions across radar, electronic warfare, command centres, weapon
systems, etc.
A short note is dedicated to blind quantum computing [69,70]. This class of quantum
protocols allows for a quantum program to run on a remote quantum computer or quan-
tum computing cloud and retrieve results without the owner knowing what the algorithm
or result was. This is valuable when secret computation is needed (e.g. military operation
planning or new weapon technology design) and no own quantum computer capability is
available.
Distributed quantum computing via the quantum network—see Sect. 3.3.1—will be im-
portant for the military and governmental actors owning quantum computers, to build
high-performance quantum computing services or quantum cloud.
A quantum network capable of distributing entanglement can integrate and entangle
quantum sensors [77] for the purpose of improving the sensitivity of the sensors, reducing
errors, and most importantly to perform a global measurement. That provides an advan-
tage in cases where the parameters of interest are global properties of the entire network;
forexample,when a signal’sangle of arrival needs measurement from threesensors,where
each measures a signal with a certain amplitude and phase. Afterwards, each sensor’s out-
put can be used to estimate the angle of arrival of the signal. Quantum entangle sensors
can evaluate this globally. This process can then be improved by machine learning [78].
Quantum protocols for distributed computing agreement [76] can have advantageous
military application for a swarm of drones, or in general for a herd of autonomous vehi-
cles (AVs). Here, quantum protocols can help achieve agreement between all AVs at the
same time scale, independent of their quantity. Nevertheless, open space quantum com-
munication between all rapidly moving AVs will be a challenge that has to be solved first.
Note that the first experiment of quantum entanglement distribution from a drone was
successfully carried out, recently [64].
5.4 Quantum PNT
Key points:
• All quantum PNT technologies have in common the demand for a highly accurate
quantum clock.
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• Quantum inertial navigation could bring few orders of magnitudes higher precision
than its classical counterpart.
• Quantum inertial navigation can be extended by the quantum augmented navigation
using quantum magnetic or gravity mapping.
• Promising quantum navigation based on Earth’s magnetic anomalies.
Quantum technologies are expected to significantly improve positioning, navigation and
timing (PNT) systems, especially inertial navigation. Time standards and frequency trans-
fer (TFT) is a fundamental service that provides precise timing for communication,
metrology, but also global navigation satellite system (GNSS). Although present TFT sys-
tems are well established, the performance of optical atomic or quantum clocks in combi-
nation with TFT utilizing quantum networks [199,200] will keep pace with the increasing
demandsofthe present applications(communication,GNSS, financial sector,radars, elec-
tronic warfare systems) and enables new applications (quantum sensing and imaging).
New quantum-based technologies andapproachessupport the development ofsensitive
precision instrumentsforPNT. Thequantum advantage willbemanifestedforGPS denied
or challenging operational environments, enabling precise operations. Examples of such
environments are underwater and underground, or environments under GPS jamming.
CurrentGNSS (GPS,GLONASS,Galileo,BeiDou, ...)rely onprecisetimingprovided
through multiple atomic clocks in individual satellites that are corrected by the more sta-
ble atomic clocks on the ground. The higher precision of the quantum clock will increase
the accuracy of positioning and navigation as well. Over the long term, the GNSS satellites
should be connected to the quantum internet for timing distribution and clock synchroni-
sation. Chip-size precise mobile clocks could help discover GNSS deception and spoofing
[201].
Some quantum GNSS (not only quantum clock) have been considered and investigated;
for instance, interferometric quantum positioned system (QPS) [199,202,203]. One of
the schemes of QPS [202,203] has a structure similar to the traditional GNSS where there
are three baselines, each consisting of two low-orbiting satellites, with the baselines are
perpendicular to each other. However, although theoretically the accuracy of positioning
is astonishing, significant engineering must be done to design a realistic QPS.
Most of the current navigation relies on GPS, or in general GNSS, which is the most
precise available technology for navigation. GNSS technology is prone to jamming, de-
ception, spoofing or GPS-deprived environments such as densely populated areas with
high electromagnetic spectrum use. Moreover, for underground or underwater environ-
ments, GNSS technology is not available at all. The solution is inertial navigation. The
problem with classical inertial navigation is its drifting, a loss of precision over time. For
example, the marine-grade inertial navigation (for ships, submarines and spacecraft) has a
drift 1.8 km/day and navigation grade (for military aircraft) has a drift 1.5 km/hour [204].
In2014,DARPAstartedaMTO-PTN project with agoalto reach drift 20 mand1 ms/hour
[205]. Even so, some expectations are very high, that quantum inertial navigation will offer
errorofonlyapproximatelyhundredsofmeterspermonth[5,206].
The full quantum inertial navigation system consists of a quantum gyroscope, ac-
celerometer and atomic/quantum clocks. Although the individual sensors required for
quantum inertial navigation are tested out of laboratories, it is still challenging to create a
complete quantum inertial measurement unit. For navigation for highly mobile platforms,
sensors need fast measurement rates of several 100 Hz, or to improve the measurement
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Krelina EPJ Quantum Technology (2021) 8:24 Page 31 of 53
bandwidth of quantum sensors [204,207]. The key component that needs the most im-
provement is the low-drift rotation sensor. The classical inertial sensors are based on var-
ious principles [208]. One common chip-size technology is the MEMS (Micro Electro
Mechanical Systems) technology, where MEMS gyroscopes have demonstrated instabili-
ties at level ∼10–7 rad ·s–1 that is suitable for military applications [99]. The instability
limit for the best current cold-atom gyroscopes is about ∼10–9 –10
–10 rad ·s–1 (at inte-
gration time 1000 s) [209]. The uncertainty is in the precision of the in field-deployable
quantum sensors in comparison to the presented laboratory experiments’ precision. The
intermediate step between classical and quantum inertial navigation can be a hybrid sys-
tem fusing the outputs of classical and quantum accelerometers [210]. With the size of the
quantuminertialnavigationdevicedecreasingtochip size, itsdeploymentcanbeexpected
on smaller vehicles, especially unmanned autonomous vehicles or missiles. However, the
miniaturisation we can reach is unknown. There are many doubts about chip-sized quan-
tum inertial navigation. It is certainly a next-generation technology, although a very big
challenge.
Currently, the individual elements, such as gyroscope or accelerometer, are also tested
on various platforms; for instance, on board an aircraft [211], or more recently a [212].
For many years, the US National Oceanic and Atmospheric Administration (NOAA)
were mapping the Earth’s magnetic anomaly and creating a magnetic anomalies map. Us-
ingsensitivequantum magnetometers in combinationwith Earth’smagnetic anomaly map
is another way to realise quantum non-GNSS navigation [213,214].
Gravitational map matching [215] works on a similar principle, and one can expect im-
proved performance using the quantum gravimeter. Together, quantum gravimeter and
magnetometer could be a basis for a submarine quantum augmented navigation, espe-
cially in undersea canyons, wrinkled seabeds, or littoral environments.
In general, quantum inertial navigation or augmented navigation has vast potential,
since there is no need for GPS, infra or radar navigation and it is not susceptible to jam-
ming, or in general to electronic warfare attacks. However, the claim of ‘no need for GPS’
is not quite accurate. These systems will always need some external input on their initial
position, most probably from GNSS.
5.5 Quantum ISTAR
Key points:
• Intense involvement of quantum computing to gather and process information.
• Desired deployment on low-orbit satellites, but the resolution is questionable.
• Vast applications for undersea operations.
• Expected advanced underground surveillance with uncertain resolution.
• New type of 3D, low-light or low-SNR quantum vision devices.
ISTAR (intelligence, surveillance, target acquisition and reconnaissance) is a crucial capa-
bility of a modern army for precise operations. Quantum technologies have the potential
to dramatically improve situational awareness of multi-domain battlefields.
In general, a large impact can be expected from quantum computing that will help with
acquiringnewintelligence data, processing BigDatafromsurveillanceandreconnaissance
and identifying targets using quantum ML/AI [178,183].
Apart from the processing part of ISTAR, dramatic advancement can be expected from
quantum sensing placed on individual land/sea/aerial vehicles and low-orbit satellites.
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Krelina EPJ Quantum Technology (2021) 8:24 Page 32 of 53
Quantum gravimeters and gravitational gradiometers promise high accuracy that can
improve or introduce new applications: geophysics study, seismology, archaeology, min-
erals (fissile material or precious metals) and oil detection, underground scanning and
precise georeferencing and topographical mappings (e.g. of the seabed for underwater
navigation) [7].
Another significant type of sensing is quantum magnetometry. The applications of
quantum magnetometry are partially overlapped by applications for quantum gravimetry,
thus introducing new applications: Earth’s magnetic field including magnetic anomalies,
localmagneticanomaliesdue tothepresence, such asmetallicobjects(submarines,mines,
etc.), or weak biological magnetic signals (applications mainly for medical purposes) [7].
The third field interesting for ISTAR is quantum imaging. Quantum imaging offers
plenty of diverse applications; for example, quantum radar (see Sect. 5.7), imaging devices
for medicine, 3D camera, stealth rangefinder, etc.
The potential quantum computing applications in ISR and situational awareness are de-
scribed in Sect. 5.2.
5.5.1 Quantum Earth’s surface and underground surveillance
Quantumsensingbased onmagnetometry,gravimetry and gravity gradiometry at the first
level helps with the study of continents and sea surface, including underground changes of
natural origin. Both magnetic anomaly and gravity-based sensing provide a different pic-
ture of the Earth’s surface. The Earth is very inhomogeneous (ocean, rocks, caves, metallic
minerals,...),includingthemassiveconstructionsorvehiclesmadebypeoplewhichgen-
eratea unique gravitational (depending on themass)andmagnetic(dependingon metallic
composition) footprint.
The discussed quantum sensing technologies—magnetometry, gravimetry and gravity
gradiometry—can reach very high precision, at least in the laboratory. For example, the
precision of absolute gravimetry out of the laboratory is about 1 μGal (10 nm·s–2)[216].
Note that the sensitivity of 3.1 μGal corresponds to a sensitivity per centimetre of height
above the Earth’s surface. However, the problem is the spatial resolution that usually is
anti-correlated with the sensitivity (higher sensitivity is at the cost of lower spatial resolu-
tion and vice versa). Spatial resolution and sensitivity are the critical attributes that define
whatyouwillrecognise (large-scale naturalchangesorsmall underground structures) and
from what distance (from the ground, drone or satellite-based measurement). Examples
of the current spatial resolution are about 100 km [217] for satellite-borne gravity gra-
diometer or 16 km [218] additional width using radar satellite altimetry (for sea areas), or
5km[219] for airborne gravimetry. For more information, see e.g. [5].
For many quantum sensing applications, it would be essential to place sensors on low
Earth orbit (LEO) satellites [220]. However, the current sensitivity and spatial resolution
allow only the applications for Earth monitoring (mapping resources such as water or oil,
earthquake or tsunami detection).
Apart from low-orbit satellites, the mentioned quantum sensors are considered for de-
ployment on airborne, sea or ground vehicle platforms. Nowadays, quantum sensing ex-
periments are performed outside the laboratory environment, such as in a truck [221],
on drones and aeroplanes [222,223]oraboardships[217]. For example, the quantum
gravimeter could be mounted on drones to search for human-made structures such as
tunnels used to smuggle drugs [223]. Placing quantum sensing devices on a drone (this
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Krelina EPJ Quantum Technology (2021) 8:24 Page 33 of 53
may be an unmanned aerial vehicle (UAV), Unmanned Surface Vessel (USV), Remotely
Operated Vehicle (ROV) or unmanned underwater vessel (UUV)) needs more engineer-
ing to reach the best sensitivity, resolution and operability simultaneously.
Low-resolution quantum sensing could be used for precise georeferencing and topo-
graphical mappings to help with underwater navigation or mission planning in rugged
terrain. Also, the detection of new minerals and oil fields can become a new centre of in-
terest, especially under the seabed [224]. This can be a source of international friction,
despite the fact that borders are clear in most cases.
High-resolution quantum magnetic and gravity sensing [217,225–227] is considered in
numerous reports and articles [7,225,228–231]tobeableto:detectcamouflagedvehi-
cles or aircraft; effectively search for a fleet of ships or individual ships from LEO; detect
underground structures such as caves, tunnels, underground bunkers, research facilities
and missile silos; localise buried unexploded objects (landmines, underwater mines and
improvised explosive devices); achieve through-wall detection of rotating machinery.
However,noteagainthat itishighly uncertain wherethe technical limits areandwhether
the mentioned quantum gravimetry and magnetometry applications will reach such sen-
sitivity and resolution (especially for using from LEO) as to realise all the aforementioned
ideas. Quantum sensors will be delivered to the market in many generations, each with
better sensitivity and resolution and lower SWaP, allowing more extensive deployment
and application.
5.5.2 Quantum imaging systems
Besides quantum radar and lidar (see Sect. 5.7), there are other military-related appli-
cations of quantum imaging. In general, all-weather, day-night tactical sensing for IS-
TAR for long/short-range, active/passive regime, invisible/stealth using EO/IR/THz/RF
frequencies features and advantages are considered. Quantum imaging systems can use
various techniques and quantum protocols; for example, SPAD, quantum ghost imaging,
sub-shot-noise imaging, or quantum illumination as was described in Sect. 3.4.4.Ingen-
eral, it is not a problem to construct quantum imaging systems of small sizes. The criti-
cal parameters are the flux of the single-photon/entangled photon emitter or the single-
photondetectionresolutionand sensitivity.Moreover,alarge-scaledeploymentof a quan-
tum imaging system with high photon flux will require powerful processing that can limit
the system deployability and performance.
Quantum 3D cameras exploiting quantum entanglement and photon-number correla-
tions will introduce fast 3D imaging with unprecedented depth of focus with low noise
aiming at sub-shot noise or long-range performance. This capability can be used to in-
spect and detect deviation or structural cracks on jets, satellites and other sensitive mil-
itary technology. Long-range 3D imaging from UAV can be used for reconnaissance and
to explore mission destination or hostile facilities and equipment.
Another commercially available technology is quantum gas sensors [232]. Technically it
is a single-photon quantum lidar calibrated to detect methane leakage. The next prepared
product is a multiple gas detector able to also detect carbon dioxide (CO2). With proper
improvement and calibration, it could serve for human presence detection, too.
A specific feature at short range is the possibility of behind-the-corner or out of the
line-of-sight visibility, [126]. These methods can help to locate and recover trapped peo-
ple, people in hostage situations or to improve automated driving by detecting incoming
vehicles from around a corner.
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Quantum imaging can serve as a low-light or low-SNR vision device; for example, in
an environment such as cloudy water, fog, dust, smoke, jungle foliage or in the night-
time, leading to an advantage. Low-SNR quantum imaging could help in target detection,
classification and identification with low signal-to-noise ratios or concealed visible sig-
natures and potentially counter adversaries’ camouflage or other target-deception tech-
niques. Quantum imaging will be very useful for helicopter pilots when landing in dusty,
foggy or smoky environments [9].
One significant product will be a quantum rangefinder [233,234]. Conventional
rangefinders use a bright laser and can be easily detected by the target. A quantum
rangefinder will be indistinguishable from the background both temporally and spectrally
when viewed from the target. In other words, the quantum rangefinder will be invisible
and stealthy, including at night time, whereas the classical rangefinder can be visible to
the target or others.
Under some circumstances, quantum ghost imaging can play the role of quantum lidar
[235], especially when the target does not move or moves very slowly and infinite depth
of focus is required for 3D imaging.
5.6 Quantum electronic warfare
Key points:
• Enhancement of current EW by smaller universal quantum antennas, precise timing
and advanced RF spectrum analysers.
• The problem with detection of quantum channels.
• When the quantum channel is localised, several types of attacks are considered and
developed.
Quantum electronic warfare (EW) can be divided into quantum-enhanced classical EW
and quantum EW focusing on countermeasures, counter-countermeasure and support
against quantum channels. By a quantum channel is meant any transfer of photons car-
rying quantum information for quantum internet, quantum radar or another quantum
system that uses the free-space or optical fibres channel.
Classical EW systems for electronic support measures can benefit from the quantum
antenna. Quantum antenna based on Rydberg atoms can offer a small size independent
of the measured signal wavelength (frequency) [122,123]. This means that even for low-
frequency (MHz to kHz [124,236]) signal interception a few-micrometres of quantum
antennais sufficient. Therecanbean array of quantumantennas for multi-frequency mea-
surement for different bandwidths or one antenna dynamically changing bandwidth ac-
cording to the interest. Moreover, Rydberg atoms-based antennas can measure both AM
and FM signals, offer self-calibration, and measure both weak and very strong fields and
detect the angle-of-arrival [125]. In the future, quantum antennas could look like an array
(matrix) of Rydberg atom cells. Different cells can measure different signals, and in the
joint measurement of two or more cells, the angle-of-arrival of the signal could be deter-
mined.Theweakest aspect of suchantennas is thecryogenics required forcoolingRydberg
atoms that need to be scaled down to an acceptable size. In general, quantum RF sensors
are a key enabler for advanced (LPD/LPI11) communications, over-horizon directional RF,
resistance to RF interference and jamming, RF direction finding, or RF-THz imaging. As
11Low Probability of Intercept/Low Probability of Detection (LPI/LPD)
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Krelina EPJ Quantum Technology (2021) 8:24 Page 35 of 53
an example, an arrayed quantum RF sensor is developed as a potential upgrade for fighter
F-35 [237].
Classical EW can also benefit from quantum computing, offering improved RF spec-
trum analysers for electronic warfare where quantum optimisations and quantum ML/AI
techniques can be applied. Higher effectiveness can be reached by the processing and
analysing directly of quantum data [55]fromRFquantumsensors(Rydbergatoms,NV
centres), where the impact of a quantum computer can be more significant. Moreover,
other quantum-based solutions and approaches are under development, such as NV cen-
tre based RF spectrum analysis or SHB based rainbow analyser [238].
The current EW systems will also benefit from quantum timing. Quantum timing can
enhance capabilities such as signals intelligence, counter-DRFM (digital radio frequency
memory) and other EW systems that require precise timing; for instance, counter-radar
jamming capabilities.
The other area of quantum EW will be signals intelligence (SIGINT) and communica-
tions intelligence (COMINT) (detecting, intercepting, identifying, locating) and quantum
electronic attack (jamming, deception, use of direct energy weapons). Quantum channels
(forquantumcommunication or quantumimaging)have specific characteristics.First,the
simplesignalinterceptionis problematic becausethequantumdata are carriedbyindivid-
ualquanta,andtheir interception can beeasilydetected. Second, typical quantumimaging
technologies use a low signal-to-noise ratio, which means that it is challenging to recog-
nise signal and noise without extra knowledge. Third, coherent photons, usually used as
a signal, behave like a laser that is very focused. Finding such a quantum signal without
knowing the position of at least one party is very challenging. These characteristics make
the classical EW obsolete and blind against quantum channels.
The situation is difficult even for potential quantum electronic warfare systems, since it
is open to question whether it will be possible to detect the presence of a quantum (free-
space) channel. This will require the development of quantum analogy of laser warning
receivers [239]. For quantum EW, it will be critical to get intel on the position of one or
both parties using the quantum channel.
Classical EW would intercept and eavesdrop on the free-space classical channel. How-
ever, this is not possible for the quantum channel where it would be detected promptly.
One possible attack is a man-in-the-middle type attack [240,241], since the early quan-
tum network parties can have a problem with authentication or trusted repeaters. Other
types of attacks are considered at the quantum physics level; for example, a photon num-
ber splitting attack relies on utilising coherent laser pulses for the quantum channel [81]
ortheTrojan-horseattacks[82], or the collecting of scattered light and its detection [242].
However,these types of attacks arevery sophisticated,andtheir practicability,forexample
in space, is uncertain.
It is more probable that the quantum EW attack will be just a type of denial of ser-
vice, where the quantum channel is intercepted, leading to stoppage of use of the channel.
Another possibility is the sophisticated jamming of the receivers on one or both sides,
leading to enormous noise. When the position of the receiver or transmitter is known,
another countermeasure of the classical EW is to make use of directed energy weapons
such as laser, leading to damage or destruction of sensors. Such an attack also could help
eavesdroppers [155].
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In general, new approaches and methods will need to be developed to realise the capa-
bilities of quantum electronic warfare and address the corresponding requirements.
5.7 Quantum radar and lidar
Key points:
• Long-range surveillance quantum radar is unlikely with existing quantum microwave
technology.
• Possible applications in the optical regime - quantum lidar.
• Quantum radar could be used for space warfare.
The perception of the quantum radar topic [141,243,244]isaffectedbythehypeinthe
media claiming quantum radar development in China [245,246] or by optimistic labora-
tory experiments. Indeed, the theoretical advantages and features of quantum radar are
significant (some of them depend on individual quantum protocols):
- Higher resistance to noise—that is, better SNR (signal-to-noise ratio)—higher
resistance to jamming and other electronic warfare countermeasures;
- Based on individual photons; that is, the output signal power is so low that it will be
invisible to electronic warfare measures;
- Target illumination; thatis, a radar allowing identification of the target.
Based on the list of unique quantum radar features, it could be a powerfully disruptive
technology that could change the rules of modern warfare. Therefore, attention is being
paid to this topic internationally, despite the immaturity of the technology, and the many
doubts about whetherthequantum radar could work as thestandardprimary surveillance
radar.
Moreover, many people immediately imagine quantum radar as a long-range surveil-
lance radar with a range of hundreds of kilometres, whereas such an application of quan-
tum radar seems unlikely [247,248]. Such an optimal, long-term surveillance quantum
radar would be extremely expensive (many orders of magnitude higher than the classical
radar cost for any range) [247], and it would still not fulfil all the advantages and features
listed above.
Briefly, the practical problems are the following [247]. Quantum radar too is subject to
the radar equation, where the received power is lost with the distance’s fourth power. In
parallel, to keep the quantum advantage, it is desirable to have one or fewer photons per
mode.Insummary,the relativelyhigh power madeoflow-photonmodesinthe microwave
regime is needed to be generated. This requires a lot of quantum signal generators, cryo-
genics, largeantennasizes,etc.All this leads toextremelyhigh cost, andimpracticaldesign
[137,247]. Scientists need to come up with more practical quantum microwave technol-
ogy to overcome these difficulties.
Apartfrom the highprice,scepticism also remainsaboutthe detection ofstealthytargets
or jamming resistance. Quantum radar can be advantageous against a barrage jammer,
but not necessarily against a DRFM or other smart jammer [247]. In summary, the long-
range surveillance quantum radar is unlikely to be achieved even as a long-term prospect.
For its realisation, one would need to evolve new technology allowing smaller cryogenics,
RF quantum emitter working at a higher temperature or more efficient cryogenics cool-
ing, and a more powerful emitter (high rate of low photon pulses). Note that even if the
room-temperature superconducting materials were developed, it would not help in the
Josephson parametric amplifier (JPA) method of entangled microwave photon generation
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Krelina EPJ Quantum Technology (2021) 8:24 Page 37 of 53
[249]. Nevertheless, JPA is not the only method to obtain entangled microwave photons
[137]. It is not entirely impossible that a new theory and designs of quantum radar will
be discovered in the future. The long-range surveillance quantum radar described above
would suffer from large size, weight and power consumption, and it is questionable if such
a radar would be stealthy [247].
Another problem is the ranging in the case of quantum illumination (QI) protocol. QI
protocol requires knowledge of the target in advance, and therefore it requires some ex-
tension for ranging, whether classical or quantum [6].
For several years, it was believed that the quantum radar cross section (RCS) is higher
than the RCS of classical radars [250,251]. A new precise study of quantum RCS [252]
shows that the previously claimed advantage of quantum RCS over the classical RCS re-
sults from erroneous approximation. Quantum and classical RCS seem to be equal, at the
moment.
Another approach can be the quantum-enhanced noise radar [137,253,254]. Noise
radar uses noise waveform as a transmission signal, and detection is based on the cor-
relation between the transmitted signal and the received noise waveform radar returns.
The advantage is the low probability of interception (LPI), being nearly undetectable by
today’s intercept receivers. The quantum noise radar design needs more study to see prac-
tical applicability. However, a potential use here is especially for the microwave regime.
Still,the current theory and researchhaveapplications intheradar sector,especiallythat
which uses the optical or near-optical photons; that is, quantum lidar. Here, a short-range
quantum lidar could be used for target illumination at short distances. Experiments with
single-photon imaging were demonstrated from 10 [255]to45km[256]. In this range,
quantum lidar could operate as an anti-drone surveillance radar or as part of a SHORAD
(Short Range Air Defense) complex.
Space can be another example of an advantageous environment for quantum radar/lidar
[257] which is low noise for the optical regime, and it even almost eliminates the decoher-
ence problem in the case of entangled photons. For example, Raytheon performs simula-
tions of the quantum radar in the optical regime for space domain [258,259]. The idea is
to place a quantum radar on a satellite and detect small satellites that are difficult to detect
because of their small cross-sectional area, reflectivity, and environmental lighting con-
ditions. The deployment of quantum radar/lidar for the space environment can provide
almost all the advantages listed above.
A small note is dedicated here to quantum-enhanced radar. Classical radar can be
equipped with an atomic or quantum clock. Such quantum-enhanced radars show high
precision and reduced noise, and thus demonstrate an advantage in detecting small, slow-
moving objects such as drones [260].
5.8 Quantum underwater warfare
Key points:
• Submarines can be one of the first adopters of quantum inertial navigation.
• Quantum magnetometers as the main tool for detection of submarines or underwater
mines.
Quantum technologies can significantly interfere in underwater warfare, with enhanced
magnetic detection of a submarine or underwater mines, novel inertial submarine navi-
gation and quantum-enhanced precise sonars. In general, in the maritime environment,
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Krelina EPJ Quantum Technology (2021) 8:24 Page 38 of 53
sensing based on quantum photo-detectors, radar, lidar, magnetometers, or gravimeters
can be applied [257]. For a general overview of the implications of quantum technology
for nuclear weapon submarines’ near invulnerability, see [261].
Submarines and other underwater vehicles will benefit from quantum inertial naviga-
tion described in Sect. 5.4 about PNT. Large submarines can probably be one of the first
adopters of quantum inertial navigation because they can afford to install larger quantum
devices, including cryogenics cooling. Moreover, sensitive quantum magnetometers and
gravimeters can help map surroundings such as an undersea canyon, icebergs and a wrin-
kled sea bottom without using sonar that can be easily detected. An example of another
type of inertial navigation especially suitable for underwater arctic navigation is based on
quantum imaging [262].
The basic tool for anti-submarine warfare could be the quantum magnetometer. Re-
searchers anticipate that the SQUID magnetometers in particular could detect a sub-
marine from 6 kilometres away, with still improving noise suppression [263,264]. Note
that the current classical magnetic anomaly detectors, usually mounted on a helicopter
or a plane, have a range of only hundreds of meters. An array of quantum magnetome-
ters, such as along the coast, could cover significant areas, leading to denial area for sub-
marines. Moreover, an array of quantum magnetometers seems to work better with more
suppressed noise.
Quantum magnetometers can also be used to detect underwater mines using, for in-
stance, an unmanned underwater vessel [230].
However, the main discussion is about the detection range, sensitivity, etc., as in
Sect. 5.5.1. Even other underwater domain technology such as sonar offers longer detec-
tion range [229]. It was also pointed out in [261] that quantum technologies will have little
impact on SSBN (ballistic missile submarines). It is possible that quantum magnetome-
ters could work with other sensors to aid in detection, identification and classification of
targets [229].
5.9 Quantum space warfare
Key points:
• Important for long-distance quantum communication.
• Low Earth orbit will be important for the future deployment of quantum sensing and
imaging technologies.
• Space warfare will lead to new quantum radar/lidar and quantum electronic warfare
technologies for deployment in space.
The space domain is gaining in importance and will be an important battlefield used by
advanced countries. Space used to be a place mainly for satellites for navigation, mapping,
communication and surveillance, often for military purposes. Nowadays, space is becom-
ingmoreweaponised[265]; for example, satellites with laser weapons or ‘kamikaze’ satel-
lites are placed in Earth orbit, and anti-satellite warfare is growing in parallel. Another
surging problem is the amount of space garbage, with the number of satellites estimated
at 2,200 and several more planned to be released [266].
Space also will be key for placing quantum sensing and communication technology in
satellites [267–271], as well as for space countermeasures.
For many quantum technology applications described in previous sections, it would be
desirable to place quantum sensing technology such as quantum gravimeter, gravity gra-
diometer or magnetometer on satellites in Earth orbit, especially the low one (LEO). Such
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Krelina EPJ Quantum Technology (2021) 8:24 Page 39 of 53
applicationsare indevelopment;forexample, alow-powerquantumgravity sensingdevice
thatcanbedeployed in space on board a small satellite for accurately mappingresources or
to aid in assessing the impact of natural disasters [272]. However, such an application does
not require too high spatial resolution. See Sect. 5.5.1 for a detailed discussion. The same
applies to satellite-based quantum imaging. For example, China claimed the development
of a spy satellite that uses ghost imaging technology [273]. However, what spatial resolu-
tion it has is uncertain. Nevertheless, quantum ghost imaging would have the advantage
of being usable in cloudy, foggy weather or at night as well.
Ontheother hand, utilisation of satellitesforquantum communication has already been
demonstrated [62,274]. Satellite-based quantum communication will be essential for the
near-term integrated quantum network at long distances [275]. The present quantum
communicationsatellitessufferfromthesameproblemsastrustedrepeatersforopticfibre
channel. In fact, present quantum satellites are trusted repeaters. The issue with trusted
repeaters is that they keep the doors open to possible cyber attacks on the satellite control
system. Abettersecurity situationiswiththepresentlydemonstrated MDI-QKDprotocol
[276], where the central point works as a repeater or switch, but in a safe regime, and later
with quantum repeaters. For a space quantum communication overview, see [270,271].
A new required military capability will be technology to detect other satellites, space-
borne objects, space garbage and track them. classical radars are used for this purpose;
for instance, the Space Fence project as part of the US Space Surveillance Network [277].
However, most of these space surveillance radars have problems with objects with a size of
about 10 cm and smaller [266](inthecaseofSpaceFence,theminimalsizeisabout5cm),
and another problem is the capacity, as to how many objects they can track. This is the
case with most of the space garbage that is only a few centimetres in size. Instead of classi-
cal radar, quantum radar or lidar is considered [6,257,259] as an alternative. Specifically
for the space environment, the quantum radar in optical regime is considered [259], since
the optical photons do not suffer from losses such as in the atmosphere. Space quantum
radar can offer most of the advantages of quantum radar as described in Sect. 5.7,includ-
ing stealth. According to simulations [259], quantum radar in space can offer at least one
order of magnitude higher detection sensitivity and object tracking sensitivity in space
in comparison with GEODSS (Ground-based Electro-Optical Deep Space Surveillance).
Space quantum radar would be very useful for tracking small, dark and fast objects, such
as satellites, space garbage or meteoroids.
The increasing presence of quantum sensing and communication devices in space will
lead to increased interest in quantum electronic warfare as described in Sect. 5.6.
5.10 Chemical and biological simulations and detection
Key points:
•∼200 qubits are sufficient to carry out chemical quantum simulation research.
• The capability of achieving more complex simulations increases with the number of
logical qubits.
• Chemical detection in the air or in samples.
• Suitable for detecting explosives and chemical warfare agents.
The defence-related chemical and biological simulations are primarily interesting for the
military and national laboratories, the chemical defence industry or CBRN (Chemical, Bi-
ological, Radiological and Nuclear) defence forces. Research on new drugs and chemical
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Krelina EPJ Quantum Technology (2021) 8:24 Page 40 of 53
substances based on quantum simulations will require an advanced quantum computer,
classical computing facility and quantum-chemical experts. The quantum simulations for
chemicalandbiologicalchemicalwarfareagents,inprinciple,have the same requirements
as civil research, such as the already ongoing protein folding, nitrogen fixation and pep-
tides research.
The number of required qubits depends on the number of spatial basis functions (vari-
ous basis sets exist, e.g., STO-3G, 6-31G or cc-pVTZ); for example, using the 6-31G basis,
the Benzene and Caffeine molecules can be simulated by approx. 140 and 340 qubits, re-
spectively [278]. Then, the Sarin molecule simulation, for instance, requires about 250
qubits. Based on quantum computer roadmaps [27,279] and logical qubit requirements,
one can come to 100 logical qubits in 10 years, but probably earlier with more effective
error corrections and error-resisting qubits. This is sufficient for medium-sized molecule
simulations.
The threat could be the design and precise simulation of structures and the chemical
properties of new small- to medium-sized molecules that could play the role of chemical
warfare agents similar to, for example, Cyanogen, Phosgene, Cyanogen chloride, Sarin or
Yperit. On the other hand, in general the same knowledge can also be used for CBRN
countermeasures and new detection technique development.
The research on protein folding, DNA and RNA exploration, such as motifs identifi-
cation, Genome-wide association studies and De novo structure prediction [280]could
impact the research on biological agents as well [281]. However, more detailed studies are
needed to assess the real threat from quantum simulations.
Photoacoustic detection with quantum cascade laser will be effective as a chemical de-
tector.Forexample, quantum chemical detectorscandetectTNT andtriacetonetriperox-
ide elements used in improvised explosive devices (IED) that are a common weapon used
in asymmetric conflicts. The same system for detecting Acetone can be used to discover
baggage and passengers with explosives boarding aircraft. In general, quantum chemi-
cal detection can be used against chemical warfare agents or toxic industrial chemicals
[282,283].
In the mid- to long term, such detectors can be placed on autonomous drones or ground
vehicles that are inspecting an area [284].
5.11 New material design
Key points:
• General research impacts; for example, room-temperature superconducting allowing
the highly precise SQUID magnetometers to operate without cooling can have a
remarkable impact on military quantum technology applications.
• Defence industry research on camouflage, stealth, ultra-hard armour or
high-temperature tolerance material.
Modern science is developing new materials, metamaterials, sometimes called quan-
tum material, by exploiting the quantum mechanical properties (e.g. graphene, topolog-
ical insulator). Material as a quantum system can be simulated by a quantum computer;
for example, the electronic structure of the material. The considered applications can be,
for instance, the room-temperature superconductor, better batteries and improvement of
specific material features.
To explain in greater detail, the room-temperature superconductivity material, for ex-
ample, exploits superconductivity at high temperatures [285]. That would allow building
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Krelina EPJ Quantum Technology (2021) 8:24 Page 41 of 53
Josephson junctions, usually used as the building blocks of SQUIDs or superconducting
qubits. So far, cooling near absolute zero is required. It is expected that a quantum com-
puter with about 70 logical qubits [286] could be sufficient for the basic research on high-
temperature superconductors.
For the defence industry, opportunities for research on new materials such as better
camouflage, stealth(electromagneticabsorption),ultra-hardarmourorhigh-temperature
tolerance material design are considered without any details being revealed.12
5.12 Brain imaging and human-machine interfacing
Key points:
• Quantum enabled magneto-encephalography
• Enhanced human-machine interfacing
MEG (magneto-encephalography) scanner is a medical imaging system that visualises
what the brain is doing by measuring the magnetic fields generated by current flowing
through neuronal assemblies. Quantummagnetometers—based, forinstance,on optically
pumped magnetometers [287]—can enable high-resolution magnetoencephalography for
real-time brain activity imaging. This technology is safe and non-invasive, and is already
laboratory tested. The technology itself is small, and wearable [287].
In the near term, quantum MEG could be a part of a soldier’s helmet for continuous
and remote medical monitoring and diagnosis in case of injury. The long-term expecta-
tions include enhanced human–machine interfacing, i.e. practical non-invasive cognitive
communication with machines and autonomous systems [11].
6 Optimism versus pessimism
Many of the quantum technology military applications mentioned above sound very op-
timistic and can drive exaggerated expectations. Some applications are taken from vari-
ous reports and newspapers or magazine articles, wherein the author could have overes-
timated the quantum technology transfer from the laboratory to the battlefield or been
influenced by general quantum technology hype [288]. It is especially important to avoid
exaggerated expectations when the topic concerns national security or defence. This issue
has been described in [14].
Quantum technology military applications described above are based on public-
domain, state-of-the-art research supplemented by various reports and newspaper or
magazine articles about defence applications. Critical remarks on their feasibility are not
given for several technologies, since there is no public information on the same. In these
cases, the reader should be more careful and critical until more detailed studies are avail-
able.
On the other hand, it is known that big defence corporations and national defence labo-
ratorieshave hadquantumresearch anddevelopmentprogrammes forseveralyears.How-
ever, only some detailed information is publicly communicated. The opposite extreme
seems to include announcements, such as from China [245,246,263,273], where it is dif-
ficult to disentangle the real research advancement from the state’s strategic propaganda
[289].
12Usually mentioned in public news articles or interviews but without details or references.
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Krelina EPJ Quantum Technology (2021) 8:24 Page 42 of 53
For many of the mentioned quantum technologies, only a laboratory proof of concept
hasbeenprovidedsofar.The decisive factorsdeterminingwhether thequantumtechnolo-
gies will be applied outside the laboratory to general use are component miniaturisation
and susceptibility to interference. These improvements must not be made at the expense
of sensitivity, resolution and functionality. Another decisive factor in real deployment is
the price of the technology.
In conclusion, considering the advancements in quantum technology research and in
supportive systems, such as laser and cryogenic cooling miniaturisation in the last few
years, it is reasonable to be optimistic rather than pessimistic about the future quantum
technology military applications (from the perspective of military or governmental ac-
tors). One needs to be careful about the real capabilities in operational deployment, to see
whether they fulfil the requirements and if the price-performance ratio justifies acquisi-
tion and deployment.
7 Quantum warfare consequences and challenges
Thedevelopment, acquisition anddeployment ofquantumtechnologies for military appli-
cation will raise new, related challenges. The concept of quantum warfare will impose new
demands on military strategy, tactics and doctrines, on ethics and disarmament activities
and on technical realisation and deployment. Studies should be conducted to understand
the issues, implications, threats and choices that arise from the development of quantum
technologies, and not only for military application.
7.1 Military consequences and challenges
Quantum technologies in military applications have the potential to sharpen the present
capabilities, such as by providing more precise navigation, ultra-secure communication or
advanced ISTAR and computing capabilities. In general, quantum warfare will require an
update, modification or creation of new military doctrines, military scenarios and plans
to develop and acquire new techniques and weapons for the quantum age.
Before this, the development of technology policies and strategies is needed to respond
to the strategic ambitions of individual actors [290]. National technology policies and
strategies should include, for example, the research of national quantum technology re-
sources (universities, laboratories and corporations) and markets, the state of develop-
mentand feasibility studies and themilitary andsecurity threatandpotential assessments,
such as [261].
The monitoring of quantum technology evolution and adaptation is essential to avoid
technological surprises due to neighbouring or potentially hostile countries. Quantum
warfare monitoring is essential even if the quantum technology is beyond their financial,
research or technological capabilities for some countries. Therefore, all modern armies
should be interested in the possible impacts of quantum warfare.
The national trade and export policies are also important. For example, the European
Union has declared quantum computing as an emerging technology of global strategic
importance and is considering more restricted access to the research programme named
Horizon Europe [291]. Further, China has prohibited the export of cryptography technol-
ogy, including quantum cryptography [292].
Another topic is the careful communication of significant quantum advantages along
with allies, especially in the quantum ISTAR and quantum cyber capabilities, which can
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Krelina EPJ Quantum Technology (2021) 8:24 Page 43 of 53
reveal military secrets, such as classified files, nuclear submarines’ positions or under-
ground facilities. A significant disruption of the balance of power could upset allies as
well as neutral or hostile players [9].
7.2 Peace and ethics consequences and challenges
Todate,the militaryapplicationsof quantum technologies mapped in Sect.5do not intro-
duce new weapons even as they sharpen the existing military technology; for instance, by
developing more precise sensing and navigation, new computing capabilities and stronger
information security. Nevertheless, the question if quantum technologies, especially for
military applications, will be good or bad for world peace is relevant.
Already various calls for ethical guidelines for quantum computing [293–295]haveap-
peared, wherein ethical concerns, such as human DNA manipulation, creation of new
materials for war and intrusive AI are mentioned [294].
Despite the fact that quantum technologies do not generate new weapons, their im-
provement of present military technology will sharpen such capabilities, shortening the
time for an attack, warning and decision making. Consequently, quantum technologies
can make the use of force more likely even while reducing individual risk [296], and thus
makewarmoreprobable[297,298].
The preventive arms control of generic dual-use technologies such as quantum tech-
nologies will be more difficult because they can be used for civilian applications too, such
as in quantum sensing for medicine. An analogy with nanotechnologies has been made
[299]. Export controls to prevent or slow down proliferation and military use by other
countriesor non-state groupsarethe most likelyway toattemptto reduceanythreat posed
by quantum technologies [298].
Specifically, quantum computing research and development is very expensive. However,
the goal is to develop a technology that allows simple and reliable qubit production. This
can lead to cheaper, more widely distributed and accessible technology for actors with
fewer skills, which is a trait of upcoming problematic military technology [298].
7.3 Technical consequences and challenges
The transfer of successful laboratory proofs of concept to real ‘outside’ application faces
many technical and technological challenges, such as miniaturisation and operability that
is not at the expense of laboratory-achieved sensitivity and resolution. Further, there are
other related technical challenges.
A significant problem could be the quantum workforce. The quantum workforce does
not need to comprise physicists or scientists with a doctorate. However, they should be
quantum engineers with knowledge of quantum information science and a quantum tech-
nology overview who can understand and be able to process and evaluate the outgoing
data from quantum sensors, computers and communications. Presently, an existing quan-
tum ecosystem is growing continuously, and this ecosystem will require an increasingly
larger quantum workforce [300]. This requires training and educating new quantum en-
gineers and experts; that is, more universities offering quantum programmes and more
students taking them. Besides, it can be even more difficult to get these people to work in
the army. Therefore, the basic principles of quantum information and quantum technol-
ogy should also appear as part of the curriculum at military colleges of modern armies,
wherequantumtechnologiesareorwillbedeployed.
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Krelina EPJ Quantum Technology (2021) 8:24 Page 44 of 53
Another technical challenge will be the enormous amount of data. Quantum technolo-
gies, through all quantum sensors, quantum imaging, quantum communication and com-
puting, will produce a lot of classical and quantum data that will increase requirements for
data transmission, processing and evaluation. These requirements should be considered
during the planning of C4ISR and quantum infrastructure.
The final challenge will be standardisation. The standardisation process is important
for the interoperability of devices manufactured by different producers. Apart from the
unification interface and communication protocols, the standardisation process can also
include security verification, such as in the post-quantum cryptography standardisation
process [90]. Various connected devices in particular (such as nodes, repeaters, switches,
fibre channels and open-space channels) can be expected in the case of a quantum net-
work, and it is important to develop and implement some standards that will allow the
successful transmission of quantum information.
8Conclusion
Quantumtechnologyisan emerging area oftechnologiesthatutilise the manipulationand
control of individual quanta for multiple applications with the potential to be disruptive.
Many of these applications are dual-use or are directly used for military purposes. How-
ever, individual quantum technologies are at TRLs for military use, from TRL 1 (basic
principles observed) to TRL 6 (technology demonstrated in relevant environment).
Quantumtechnologyformilitaryapplicationswill not onlyofferimprovements andnew
capabilities but will also require the development of new strategies, tactics and policies,
assessment of threats to global peace and security and identification of ethics issues. All
this is covered by the term ‘quantum warfare’.
In this report, various quantum technologies at different TRL have been described, fo-
cusing on possible utilisation or deployment in the defence sector. A precise forecast of
quantum technology deployment is not possible, since the transition from the laboratory
to real-world applications has not been implemented or is in progress. This raises ques-
tions such as whether we will be able to reach a resolution providing a real quantum ad-
vantage over the classical systems that are usually significantly cheaper and often in action
already. Despite the fact that the description of the possible military application of quan-
tum technologies sounds very optimistic, one should be wary of the quantum hype and
draw attention to the challenges that lie ahead of the real deployment of quantum tech-
nologies for military applications.
Quantum technologies can be expected to have strategic and long-term impacts. Nev-
ertheless, the probability of technological surprises affecting military and defence forces
is rather low. The best ways to avoid surprises are cultivating quantum technology knowl-
edge and monitoring quantum technology development and employment. Treating quan-
tum technology with care will play the role of quantum insurance.
Acknowledgements
The author is very grateful for several comments and feedback on the draft, especially by Dr Katarzyna Kubiak, Dr Jürgen
Altmann and others. The author is also grateful for the minor comments to the first preprint and valuable suggestions
and journal reviewers’ comments.
Availability of data and materials
The datasets used and analysed during the current study are available from the corresponding author on reasonable
request.
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Krelina EPJ Quantum Technology (2021) 8:24 Page 45 of 53
Declarations
Competing interests
The author declares that he has no competing interests.
Authors’ contributions
As the sole author of the manuscript, MK conceived, designed and performed the analysis and review, and wrote the
paper. The author read and approved the final manuscript.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Received: 6 April 2021 Accepted: 25 October 2021
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