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The Quantum Internet
Abstract
Quantum networks provide opportunities and challenges across a range of intellectual and technical frontiers, including quantum computation, communication and metrology. The realization of quantum networks composed of many nodes and channels requires new scientific capabilities for generating and characterizing quantum coherence and entanglement. Fundamental to this endeavour are quantum interconnects, which convert quantum states from one physical system to those of another in a reversible manner. Such quantum connectivity in networks can be achieved by the optical interactions of single photons and atoms, allowing the distribution of entanglement across the network and the teleportation of quantum states between nodes.
... Quantum entanglement is a key concept in quantum physics and plays an essential role in the field of quantum information processing [1][2][3][4]. Among its various applications, entangled operations, such as the controlled unitary (CU) gate including the controlled-NOT gate, are of particular significance, serving as a cornerstone for universal quantum computing [5][6][7]. ...
... One distinguishing feature of CU gates in photonic systems is their intrinsic ability for gate teleportation, enabling the realization of nonlocal quantum gates [17][18][19]. Hence, the implementation of CU gates in a linear optical system is essential for quantum networks, including distributed quantum computing and the quantum internet [2,20,21]. ...
... Note that this controlled non-unitary operation can be used for the entanglement filter [38]. A remote-controlled quantum gate model can be directly applicable to the superposition of quantum gates [39,40], reducing query complexity [41,42], quantum network [2,43], and Hamiltonian simulations [44,45]. This quantum circuit model can provide insights to secure remote quantum information processing, which is useful for clients concerned about information exposure during quantum information processing. ...
Recently, remote-controlled quantum information processing has been proposed for its applications in secure quantum processing protocols and distributed quantum networks. For remote-controlled quantum gates, the experimental realization of controlled unitary (CU) gates between any quantum gates is an essential task. Here, we propose and experimentally demonstrate a scheme for implementing CU gates between arbitrary pairs of unitary gates using the polarization and time-bin degrees of freedom of single-photons. Then, we experimentally implement remote-controlled single-qubit unitary gates by controlling either the state preparation or measurement of the control qubit with high process fidelities. We believe that the proposed remote-controlled quantum gate model can pave the way for secure and efficient quantum information processing.
... Optical nonreciprocal devices, which can allow the flow of light from one direction but prevent it from the other, play an indispensable role in many important applications, ranging from optical signal processing [1][2][3][4][5][6], invisible sensing [7] and noise-free information processing [1] to quantum networks [3,8]. The conventional approach to obtain optical nonreciprocity depending on the magneto-optical Faraday effect [9][10][11] is difficult because it requires a strong magnetic field and bulky materials [12,13]. ...
... Optical nonreciprocal devices, which can allow the flow of light from one direction but prevent it from the other, play an indispensable role in many important applications, ranging from optical signal processing [1][2][3][4][5][6], invisible sensing [7] and noise-free information processing [1] to quantum networks [3,8]. The conventional approach to obtain optical nonreciprocity depending on the magneto-optical Faraday effect [9][10][11] is difficult because it requires a strong magnetic field and bulky materials [12,13]. ...
... Here the effect of quantum jumps can be neglected [3]), where κ is the optical decay rate. Under the weak driving approximation, we truncate the Hilbert space to 2 and the approximate wave function can be written as |ψ(t)⟩ = C 00 (t)|00⟩ + C 10 (t)|10⟩ + C 01 (t)|01⟩ + C 11 (t)|11⟩ + C 20 (t)|20⟩ + C 02 (t)|02⟩. ...
Optical nonreciprocity, which refers to the direction-dependent emission, scattering and absorption of photons, plays a very important role in quantum engineering and quantum information processing. Here, we propose an all-optical approach to achieve the optical dynamical switchable quantum nonreciprocity by an off-resonant chiral two-photon driving in a single microring cavity, which differs from the conventional nonreciprocal schemes. It is shown that the optical field with time-dependent statistical properties can be generated and the nonreciprocity flips periodically, with switchable photon blockade and photon-induced tunneling effects. We find that the dynamical system is robust and immune to the parameter variations, which loosens the parameter range of system. Meanwhile, the time window for one-way quantum information is sufficiently wide and tunable. Our work opens a new idea for the current quantum nonreciprocal research, which can facilitate a memory functionality and be used for future in-memory superconducting quantum compute. The other nonreciprocal quantum devices, i.e., dynamical switchable nonreciprocal squeezing and entanglement, may be inspired by our method, which is expected to have important applications in future quantum technology.
... A future large-scale quantum network can link different functional nodes together via photonic channels [1][2][3][4][5][6][7], and enable numerous applications such as distributed quantum computing [8], networked quantum sensing [9][10][11], and global quantum communication [4,5,12,13] with unprecedented performances. Recent advances on quantum networks have been achieved on various physical systems [11][12][13][14][15][16][17][18][19][20][21][22]. ...
... |2⟩)|V ⟩].We further combine1 2 |1⟩ + √ |2⟩ into |1⟩ and convert the ion state to the memory qubit subspace via three successive Raman transitions, including (1) a 850nm/854nm Raman converting |0⟩ to | ↑ c ⟩, (2) a 866nm/866nm Raman which con-Crosstalk-free storage of quantum information in memory qubit. (A) We measure the decay probability (orange triangle) and the storage fidelity to the initial state |↑m⟩+|↓m⟩ √ 2 (blue circle) with variable storage time, in the protocol shown inFig. ...
Trapped atomic ions constitute one of the leading physical platforms for building the quantum repeater nodes to realize large-scale quantum networks. In a long-distance trapped-ion quantum network, it is essential to have crosstalk-free dual-type qubits: one type, called the communication qubit, to establish entangling interface with telecom photons; and the other type, called the memory qubit, to store quantum information immune from photon scattering under entangling attempts. Here, we report the first experimental implementation of a telecom-compatible and crosstalk-free quantum network node based on two trapped $^{40}$Ca$^{+}$ ions. The memory qubit is encoded on a long-lived metastable level to avoid crosstalk with the communication qubit encoded in another subspace of the same ion species, and a quantum wavelength conversion module is employed to generate ion-photon entanglement over a $12\,$km fiber in a heralded style. Our work therefore constitutes an important step towards the realization of quantum repeaters and long-distance quantum networks.
... According to the common views [10], TPI stems from the indistinguishability of identical photons and the photon bunching is regarded as a manifestation of the bosonic nature of light. This perspective has guided a series of subsequent experiments to make indistinguishable photons from either independent or dissimilar photon sources [11][12][13][14][15][16] for applications in quantum communications [6] and quantum networks [17][18][19]. ...
... Our work provides new insights into TPI and has great implications in both quantum optics and TPIbased photonic quantum technologies, such as longdistance TPI [15,16], asynchronous Bell-state measurement [38,39], asynchronous measurement-deviceindependent quantum key distribution [40,41], and boson sampling [27,42]. Interfacing weak lasers commonly used in quantum cryptography with single photons required for quantum state transfer, quantum repeaters and quantum memories could be a realistic route towards quantum internet [17][18][19]. ...
Two-photon interference (TPI) lies at the heart of photonic quantum technologies. TPI is generally regarded as quantum interference stemming from the indistinguishability of identical photons, hence a common intuition prevails that TPI would disappear if photons are distinguishable. Here we disprove this perspective and uncover the essence of TPI. We report the first demonstration of TPI between distinguishable photons with their frequency separation up to $10^4$ times larger than their linewidths. We perform time-resolved TPI between an independent laser and single photons with ultralong coherence time ($>10\ \mu$s). We observe a maximum TPI visibility of $72\%\pm 2\%$ well above the $50\%$ classical limit indicating the quantum feature, and simultaneously a broad visibility background and a classical beat visibility of less than $50\%$ reflecting the classical feature. These visibilities are independent of the photon frequency separation and show no difference between distinguishable and indistinguishable photons. Based on a general wave superposition model, we derive the cross-correlation functions which fully reproduce and explain the experiments. Our results reveal that TPI as the fourth-order interference arises from the second-order interference of two photons within the mutual coherence time and TPI is not linked to the photon indistinguishability. This work provides new insights into the nature of TPI with great implications in both quantum optics and photonic quantum technologies.
... Factors such as cultural background, environment, and personal experiences shape behaviours, leading to a spectrum of outcomes, such as belief systems and habits like smoking. Both fields confront the inherent unpredictability present in the phenomena they seek to understand [15]. Central to both disciplines is the role of observation and measurement, which profoundly influences the observed outcomes. ...
... While quantum mechanics delineates energy levels of particles, psychology explores emotional states that could be analogously viewed as varying levels of psychological energy. For instance, a person in a tranquil state may exhibit lower psychological energy, whereas one experiencing motivation or excitement may possess a heightened level [15]. ...
Understanding the complexities of human psychology and addressing mental health challenges require a multidimensional approach that transcends conventional boundaries. This manuscript explores the intersection between quantum mechanics and human science, proposing novel insights into the dynamics of human traits and behaviour. By examining the principles of quantum mechanics, particularly superposition, we hypothesize that human traits may exist in a state of potentiality, coexisting with their respective values. This perspective suggests that individuals possess a spectrum of traits, and deliberate effort plays a crucial role in determining their manifestation. Drawing inspiration from quantum mechanics, we advocate for a proactive approach to nurturing positive traits and addressing destructive tendencies. This involves recognizing the power of choice, fostering self-awareness, and actively engaging in personal growth initiatives. We discuss the implications of trait activation and highlight the importance of voluntary effort in shaping behaviour and character. Additionally, we explore practical strategies for navigating psychological challenges. This manuscript underscores the potential of interdisciplinary inquiry to inform innovative approaches to psychological intervention and therapy. Through further empirical research and theoretical exploration, we can unlock new perspectives and strategies for enhancing human flourishing and addressing the complexities of the human psyche.
... Finally, the quantum computing network stage represents a fully developed quantum internet, where quantum computers are interconnected, facilitating the unrestricted exchange of quantum information and the execution of computationally intensive quantum algorithms. This stage realizes the ultimate goal of quantum networking, enabling a vast array of quantum applications that are unfeasible with classical technologies [162,163]. ...
... Also, end nodes/quantum processors range from simple devices capable of preparing and measuring single qubits to sophisticated large-scale quantum computers. These nodes are vital for various quantum network tasks and must have robust storage for quantum states and high-fidelity quantum-information-processing capabilities, along with compatibility with photonic communication hardware, especially for efficient interfacing with light at telecom bands [163,165,166]. ...
In recent years, satellite communication systems (SCSs) have rapidly developed in terms of their role and capabilities, promoted by advancements in space launch technologies. However, this rapid development has also led to the emergence of significant security vulnerabilities, demonstrated through real-world targeted attacks such as AcidRain and AcidPour that demand immediate attention from the security community. In response, various countermeasures, encompassing both technological and policy-based approaches, have been proposed to mitigate these threats. However, the multitude and diversity of these proposals make their comparison complex, requiring a systemized view of the landscape. In this paper, we systematically categorize and analyze both attacks and defenses within the framework of confidentiality, integrity, and availability, focusing on specific threats that pose substantial risks to SCSs. Furthermore, we evaluate existing countermeasures against potential threats in SCS environments and offer insights into the security policies of different nations, recognizing the strategic importance of satellite communications as a national asset. Finally, we present prospective security challenges and solutions for future SCSs, including full quantum communication, AI-integrated SCSs, and standardized protocols for the next generation of terrestrial–space communication.
... The NV center is the main defect considered as a computational qubit 5,6 with recent advancements in fault-tolerant operation 7 . While more work is needed before it can be realized at large scale 8 , it has already demonstrated promise as a flying qubit by transmitting quantum information over long distance 9 and multinode network capabilities 10,11 , which goes towards the quantum internet 12,13 . It has also been demonstrated as memory for quantum information 14,15 . ...
Color centers in diamond are at the forefront of the second quantum revolution. A handful of defects are in use, and finding ones with all the desired properties for quantum applications is arduous. By using high-throughput calculations, we screen 21,607 defects in diamond and collect the results in the ADAQ database. Upon exploring this database, we find not only the known defects but also several unexplored defects. Specifically, defects containing sodium stand out as particularly relevant because of their high spins and predicted improved optical properties compared to the NV center. Hence, we studied these in detail, employing high-accuracy theoretical calculations. The single sodium substitutional (Na C ) has various charge states with spin ranging from 0.5 to 1.5, ZPL in the near-infrared, and a high Debye-Waller factor, making it ideal for biological quantum applications. The sodium vacancy (NaV) has a ZPL in the visible region and a potential rare spin-2 ground state. Our results show sodium implantation yields many interesting spin defects that are valuable additions to the arsenal of point defects in diamond studied for quantum applications.
... Optical nonreciprocity [52][53][54][55][56][57][58][59][60][61][62][63] is characterized by the asymmetric behavior of optical signals as they travel through an optical system in opposite directions. This phenomenon plays a crucial role in optical information processing and quantum networks [64][65][66]. Notably, a theory of nonreciprocal phase transitions in nonequilibrium systems has been proposed, suggesting that asymmetric couplings of multiple species can give rise to time-dependent phases [67]. ...
We demonstrate the emergence of nonreciprocal superradiant phase transitions and novel multicriticality in a cavity quantum electrodynamics (QED) system, where a two-level atom interacts with two counter-propagating modes of a whispering-gallery-mode (WGM) microcavity. The cavity rotates at a certain angular velocity, and is directionally squeezed by a unidirectional parametric pumping $\chi^{(2)}$ nonlinearity. The combination of cavity rotation and directional squeezing leads to nonreciprocal first- and second-order superradiant phase transitions. These transitions do not require ultrastrong atom-field couplings and can be easily controlled by the external pump field. Through a full quantum description of the system Hamiltonian, we identify two types of multicritical points in the phase diagram, both of which exhibit controllable nonreciprocity. These results open a new door for all-optical manipulation of superradiant transitions and multicritical behaviors in light-matter systems, with potential applications in engineering various integrated nonreciprocal quantum devices
... On the other hand, the parametric down-conversion process can be stimulated by placing a crystal inside an optical cavity, resulting in the so-called optical parametric oscillator (OPO) [27]. This device emits light beams that are correlated in their intensities and phases [28,29] and has also found crucial applications in quantum information science [30]. ...
In this paper, we utilize an analog model of the general relativity and investigate the influence of spatial curvature on quantum properties of stimulated parametric down-conversion process. For this purpose, we use two-mode sphere coherent state as the input beams of the aforementioned process. These states are realization of coherent states of two-dimensional harmonic oscillator, which lies on a two-dimension sphere. We calculate the entanglement of output states of stimulated parametric down conversion process, measured by linear entropy, and show that it depends on the spatial curvature. So, by preparing the suitable two-mode sphere coherent states, it is possible to control the entanglement between the output states in the laboratory. In addition, we consider mean number and Mandel parameter of the output states of the process and also, their cross-correlation function, as the convince measures of non-classical behaviors.
... and eventually, the realization of a quantum internet [8], which would enable not just secure key distribution [9] but also distributed quantum computing. While these advancements present exciting opportunities, they also introduce challenges that cannot be adequately addressed by classical means, necessitating specialized quantum methodologies for effective resolution (see, e.g., [10] and [11]). ...
This paper introduces quantum edge detection, aimed at locating boundaries of quantum domains where all particles share the same pure state. Focusing on the 1D scenario of a string of particles, we develop an optimal protocol for quantum edge detection, efficiently computing its success probability through Schur-Weyl duality and semidefinite programming techniques. We analyze the behavior of the success probability as a function of the string length and local dimension, with emphasis in the limit of long strings. We present a protocol based on square root measurement, which proves asymptotically optimal. Additionally, we explore a mixed quantum change point detection scenario where the state of particles transitions from known to unknown, which may find practical applications in detecting malfunctions in quantum devices
... Scalable spin-photon quantum interfaces require single-photon generation with a level structure that enables optical access to the electronic spins [1][2][3] . These systems are critical for the deployment of applications such as quantum repeaters [4][5][6] and quantum sensors [7][8][9][10] . Ideally, a spin-photon interface should display long-lived spin coherence with efficient and coherent optical transitions in a scalable material platform without requiring stringent operation conditions such as cryogenic temperature or applied magnetic field. ...
Solid-state spin–photon interfaces that combine single-photon generation and long-lived spin coherence with scalable device integration—ideally under ambient conditions—hold great promise for the implementation of quantum networks and sensors. Despite rapid progress reported across several candidate systems, those possessing quantum coherent single spins at room temperature remain extremely rare. Here we report quantum coherent control under ambient conditions of a single-photon-emitting defect spin in a layered van der Waals material, namely, hexagonal boron nitride. We identify that the carbon-related defect has a spin-triplet electronic ground-state manifold. We demonstrate that the spin coherence is predominantly governed by coupling to only a few proximal nuclei and is prolonged by decoupling protocols. Our results serve to introduce a new platform to realize a room-temperature spin qubit coupled to a multiqubit quantum register or quantum sensor with nanoscale sample proximity.
... Quantum networks enable distributed quantum computation by distributing entanglement and by transmitting qubits among a set of nodes [1]- [3]. Quantum networks contribute to largescale quantum computers, such as fault-tolerant quantum computers capable of executing quantum algorithms with quantum advantage [4]- [8], and the quantum Internet to utilize quantum phenomena in wide-area distributed quantum computation, such as secure communication and computation, rapid leader election, byzantine agreement, sensing such as ultra-long baseline interferometry for telescopes, and spatial and temporal reference frame synchronization [9]- [15]. ...
... Quantum networks enable distributed quantum computation by distributing entanglement and by transmitting qubits among a set of nodes [1]- [3]. Quantum networks contribute to largescale quantum computers, such as fault-tolerant quantum computers capable of executing quantum algorithms with quantum advantage [4]- [8], and the quantum Internet to utilize quantum phenomena in wide-area distributed quantum computation, such as secure communication and computation, rapid leader election, byzantine agreement, sensing such as ultra-long baseline interferometry for telescopes, and spatial and temporal reference frame synchronization [9]- [15]. ...
... Quantum applications, such as quantum networks and quantum repeaters, rely on efficient mapping of quantum information between stationary qubits and flying qubits [1][2][3]. While photons are a natural choice for flying qubits, since they interact weakly with their environment [4], numerous candidates for stationary qubits are under investigation with individual strengths and weaknesses. ...
Hybrid quantum photonic systems connect classical photonics to the quantum world and promise to deliver efficient light-matter quantum interfaces while leveraging the advantages of both, the classical and the quantum, subsystems. However, combining efficient, scalable photonics and solid-state quantum systems with desirable optical and spin properties remains a formidable challenge. In particular, the access to individual spin states and coherent mapping to photons remains unsolved for hybrid systems. In this paper, we demonstrate all-optical initialization and readout of the electron spin of a negatively charged silicon-vacancy center in a nanodiamond coupled to a silicon nitride photonic crystal cavity. We characterize relevant parameters of the coupled emitter-cavity system and determine the silicon-vacancy center’s spin-relaxation and spin-decoherence rate. Our results mark a key step towards the realization of a hybrid spin-photon interface based on silicon nitride photonics and the silicon-vacancy center’s electron spin in nanodiamonds with potential use for quantum networks, quantum communication, and distributed quantum computation.
Published by the American Physical Society 2024
... Quantum networks enable distributed quantum computation by distributing entanglement and by transmitting qubits among a set of nodes [1]- [3]. Quantum networks contribute to largescale quantum computers, such as fault-tolerant quantum computers capable of executing quantum algorithms with quantum advantage [4]- [8], and the quantum Internet to utilize quantum phenomena in wide-area distributed quantum computation, such as secure communication and computation, rapid leader election, byzantine agreement, sensing such as ultra-long baseline interferometry for telescopes, and spatial and temporal reference frame synchronization [9]- [15]. ...
The optical Bell State Analyzer (BSA) plays a key role in the optical generation of entanglement in quantum networks. The optical BSA is effective in controlling the timing of arriving photons to achieve interference. It is unclear whether timing synchronization is possible even in multi-hop and complex large-scale networks, and if so, how efficient it is. We investigate the scalability of BSA synchronization mechanisms over multiple hops for quantum networks both with and without memory in each node. We first focus on the exchange of entanglement between two network nodes via a BSA, especially effective methods of optical path coordination in achieving the simultaneous arrival of photons at the BSA. In optical memoryless quantum networks, including repeater graph state networks, we see that the quantum optical path coordination works well, though some possible timing coordination mechanisms have effects that cascade to adjacent links and beyond, some of which was not going to work well of timing coordination. We also discuss the effect of quantum memory, given that end-to-end extension of entangled states through multi-node entanglement exchange is essential for the practical application of quantum networks. Finally, cycles of all-optical links in the network topology are shown to may not be to synchronize, this property should be taken into account when considering synchronization in large networks.
... Although the quantum transduction efficiency of current converters is still far below the requirement for building the quantum internet [344,362], the superconducting quantum processor can already benefit from the development of this technology in the near furture. One potential application is the classical optical control and readout of superconducting (SC) qubit. ...
Recent decades have seen significant advancements in integrated photonics, driven by improvements in nanofabrication technology. This field has developed from integrated semiconductor lasers and low-loss waveguides to optical modulators, enabling the creation of sophisticated optical systems on a chip scale capable of performing complex functions like optical sensing, signal processing, and metrology. The tight confinement of optical modes in photonic waveguides further enhances the optical nonlinearity, leading to a variety of nonlinear optical phenomena such as optical frequency combs, second-harmonic generation, and supercontinuum generation. Active tuning of photonic circuits is crucial not only for offsetting variations caused by fabrication in large-scale integration, but also serves as a fundamental component in programmable photonic circuits. Piezoelectric actuation in photonic devices offers a low-power, high-speed solution and is essential in the design of future photonic circuits due to its compatibility with materials like Si and Si3N4, which do not exhibit electro-optic effects. Here, we provide a detailed review of the latest developments in piezoelectric tuning and modulation, by examining various piezoelectric materials, actuator designs tailored to specific applications, and the capabilities and limitations of current technologies. Additionally, we explore the extensive applications enabled by piezoelectric actuators, including tunable lasers, frequency combs, quantum transducers, and optical isolators. These innovative ways of managing photon propagation and frequency on-chip are expected to be highly sought after in the future advancements of advanced photonic chips for both classical and quantum optical information processing and computing.
... Distributing quantum entanglement between quantum memory nodes separated by extended distances 1,4 is an important element for the realization of quantum networks, enabling potential applications ranging from quantum repeaters 2,5 and long-distance secure communication 6,7 to distributed quantum computing 8,9 and distributed quantum sensing and metrology 10,11 . Proposed architectures require quantum nodes containing multiple long-lived qubits that can collect, store and process information communicated by photonic channels based on telecommunication (telecom) fibres or satellite-based links. ...
A key challenge in realizing practical quantum networks for long-distance quantum communication involves robust entanglement between quantum memory nodes connected by fibre optical infrastructure1–3. Here we demonstrate a two-node quantum network composed of multi-qubit registers based on silicon-vacancy (SiV) centres in nanophotonic diamond cavities integrated with a telecommunication fibre network. Remote entanglement is generated by the cavity-enhanced interactions between the electron spin qubits of the SiVs and optical photons. Serial, heralded spin-photon entangling gate operations with time-bin qubits are used for robust entanglement of separated nodes. Long-lived nuclear spin qubits are used to provide second-long entanglement storage and integrated error detection. By integrating efficient bidirectional quantum frequency conversion of photonic communication qubits to telecommunication frequencies (1,350 nm), we demonstrate the entanglement of two nuclear spin memories through 40 km spools of low-loss fibre and a 35-km long fibre loop deployed in the Boston area urban environment, representing an enabling step towards practical quantum repeaters and large-scale quantum networks.
... Photons are regarded as ideal carriers of quantum information due to their fast propagation speed and low dissipation, which play an important role in quantum networks and other quantum information processing [1][2][3][4][5][6][7][8]. As a consequence, studying the interaction between the single photon and matter, and realizing reliable optical quantum devices for manipulating the photon transmission in a photonic system has become a hot issue in modern quantum information science. ...
Single-photon routing between two one-dimensional waveguides mediated by a single-mode cavity embedded with a time-modulated two-level atom is investigated. Two configurations, where the single photon is incident from an infinite or semi-infinite waveguide, are considered. Using the analytical expressions of the single-photon scattering amplitudes, the transmission behaviors in the two waveguides are discussed. The results show that the time modulation of the atomic frequency enables a dynamically tunable quantum router. A single photon with different frequencies can be routed dynamically from the incident waveguide to the other by properly manipulating the amplitude-to-frequency ratio of the atom. The routing efficiency can be improved to approach 100% by terminating the incident waveguide. In the semi-waveguide configuration, the routing behaviors controlled by the quantum coherent feedback are also investigated. The influence of the phase shifts introduced by the terminated waveguide on the routing capability and the conditions for perfect single-photon routing are discussed in detail. A frequency tunable targeted single-photon router can even be realized with the help of chiral coupling. These results may be beneficial to the photon control in a quantum network based on time-modulated quantum nodes.
... The ability to transmit quantum information reliably between distant parties is a prerequisite for any useful application of a quantum internet [1,2]. The primary challenge to achieve this is the exponential attenuation of optical signals in fiber-based networks. ...
Reliable quantum communication over hundreds of kilometers is a daunting yet necessary requirement for a quantum internet. To overcome photon loss, the deployment of quantum repeater stations between distant network nodes is necessary. A plethora of different quantum hardware is being developed for this purpose, each platform with its own opportunities and challenges. Here, we propose to combine two promising hardware platforms in a hybrid quantum repeater architecture to lower the cost and boost the performance of long-distance quantum communication. We outline how ensemble-based quantum memories combined with single-spin photon transducers, which can transfer quantum information between a photon and a single spin, can facilitate massive multiplexing, efficient photon generation, and quantum logic for amplifying communication rates. As a specific example, we describe how a single Rubidium (Rb) atom coupled to nanophotonic resonators can function as a high-rate, telecom-visible entangled photon source with the visible photon being compatible with storage in a Thulium-doped crystal memory (Tm-memory) and the telecom photon being compatible with low loss fiber propagation. We experimentally verify that Tm and Rb transitions are in resonance with each other. Our analysis shows that by employing up to 9 repeater stations, each equipped with two Tm-memories capable of holding up to 625 storage modes, along with four single Rb atoms, one can reach a quantum communication rate of about 10 secret bits per second across distances of up to 1000 km.
... Like erbium in silicon, vanadium offers direct telecom interfacing between spins and photons, albeit with a much faster optical transition [64,65]. The similarities with the silicon vacancy in diamond suggest that the spin coherence lifetime of vanadium will also be sufficient to enable the storage of quantum information for long-distance communication, providing a clear perspective towards high-performance quantum networks [66][67][68]. ...
Vanadium in silicon carbide (SiC) is emerging as an important candidate system for quantum technology due to its optical transitions in the telecom wavelength range. However, several key characteristics of this defect family including their spin relaxation lifetime (T1), charge state dynamics, and level structure are not fully understood. In this work, we determine the T1 of an ensemble of vanadium defects, demonstrating that it can be greatly enhanced at low temperature. We observe a large spin contrast exceeding 90% and long spin-relaxation times of up to 25 s at 100 mK, and of order 1 s at 1.3 K. These measurements are complemented by a characterization of the ensemble charge state dynamics. The stable electron spin furthermore enables high-resolution characterization of the systems’ hyperfine level structure via two-photon magneto-spectroscopy. The acquired insights point towards high-performance spin-photon interfaces based on vanadium in SiC.
... Light-ensemble interaction has been an important topic drawing continuing attention for recent years thanks to its fundamental interest in quantum many-body physics and practical applications in various areas, such as atomic clocks and metrology [1], sensing and precision measurement [2], quantum simulation [3], quantum interface, memory, and network [4][5][6]. In various proposals, schemes based on atomic ensembles are expected to have enhanced coupling strength for more efficient manipulation by increasing the number and/or density of atoms. ...
We investigate the spectral shift known as the collective Lamb shift in forward scattering for a cold dense atomic cloud. The shift results from resonant dipole–dipole interaction mediated by real and virtual photon exchange, forming many-body states displaying various super- and subradiant behaviour. However, the scattering spectrum reflects the overall contributions from these states but also averages out the radiative details associated with the underlying spin orders, causing ambiguity in determination and raising controversy on the scaling property of this shift. We employ a Monte–Carlo simulation to study how the collective states contribute to emission. We thus distinguish two kinds of collective shift that follow different scaling laws. One results from dominant occupation of the near-resonant collective states. This shift is usually small and insensitive to the density or the number of participating atoms. The other comes from large spatial correlation of dipoles, associated with the states of higher degree of emission. This corresponds to larger collective shift that is approximately linearly dependent on the optical depth. We further demonstrate that the spatial spin order plays an essential role in superradiant emission. Our analysis provides a novel perspective for understanding collective scattering and cooperative effects.
... Furthermore, we have demonstrated that the external classical field can serve as a control signal for adjusting the width of the atomic lineshapes, even in the presence of a non-zero average temperature (see Fig. 5). By precisely tuning the external classical driving field, it becomes feasible to finely manipulate the atomic transitions of the two-level system (qubit), which holds importance across various fields of research, including quantum information theory (for instance, see the final comment in Ref. [46]), quantum communications (see the section regarding Cavity QED in Ref. [47]), quantum optics (refer to Larson's exceptional book [48]), spectroscopy, quantum control (see the description of Problem 7.3 in Nielsen's book [49]), and quantum computing (refer, for instance, to Section 2.6.4 of Haroche and Raimond's book [50]). ...
In the framework of the Jaynes–Cummings model, we investigate how atomic lineshapes are affected by coherently driving the atom–field interaction. We pay particular attention to the two-level atom interaction with a thermal cavity field when both are influenced by external classical fields. Adopting a density matrix formalism, we calculate the average atomic inversion and demonstrate how the corresponding lineshapes vary as a function of the average number of thermal photons and the atom–field classical coupling. Furthermore, we compare these results with those obtained from the standard Jaynes–Cummings model and validate our findings through numerical calculations.
... Collective synchronous behaviour of a many-body quantum system has attracted much attention from many scientific disciplines, particularly from quantum optics and quantum information [3,4,8,15,17,20,23]. In order to provide a mathematically rigorous analysis of such a phenomenon, several mathematical models (even phenomenological ones) have been proposed in literature after Winfree [22], Kuramoto [12] and Vicsek [21]. ...
We introduce a new non-abelian quantum synchronisation model over the unitary group, represented as a gradient flow, where state matrices asymptotically converge to a common one up to phase translation. We provide a sufficient framework leading to quantum synchronisation based on Riccati-type differential inequalities. In addition, uniform time-delayed interaction is considered for modelling realistic communication, and we demonstrate that quantum synchronisation is persistent when a small time delay is allowed. Finally, numerical simulation is performed to visualise qualitative behaviours and support theoretical results.
... T he quantum internet 1) is vital for distributed and cloud quantum computing 2) and possesses potential applications in world clocks 3) and ultra-long baseline interferometry. 4) In the optical-fiber-based quantum internet, quantum repeaters share the entanglement of the quantum states between adjacent repeater nodes to overcome fiber loss and achieve long-distance quantum communication. ...
We propose a scheme to phase lock a quantum-memory control laser, frequency-conversion pump laser, and two-photon source over three octaves in frequency to operate a quantum memory. The absolute frequencies of the laser sources are determined based on a Doppler-free iodine hyperfine transition. The achieved relative frequency instability and uncertainty were ≤1 × 10–12 and 3 × 10–11, respectively, which are below the requirement for operating a Pr:YSO quantum memory. This scheme simplifies the instrumentation of laser sources in a quantum repeater, and increases the reliability of quantum communication systems.
Efficient interconnection between distant semiconductor spin qubits with the help of photonic qubits offers exciting new prospects for future quantum communication applications. In this paper, we optimize the extraction efficiency of a novel interface between a singlet-triplet-spin-qubit and a photonic-qubit. The interface is based on a 220-nm-thick GaAs/(Al,Ga)As heterostructure membrane and consists of a gate-defined double quantum dot (GDQD) supporting a singlet-triplet qubit, an optically active quantum dot (OAQD) consisting of a gate-defined exciton trap, a photonic crystal cavity providing in-plane optical confinement, efficient outcoupling to an ideal free-space Gaussian beam while accommodating the gate wiring of the GDQD and OAQD, and a bottom gold reflector to recycle photons and increase the optical extraction efficiency. All the essential components can be lithographically defined and deterministically fabricated on the GaAs/(Al,Ga)As heterostructure membrane, which greatly increases the scalability of on-chip integration. According to our simulations, the interface provides an overall coupling efficiency of 28.7% into a free-space Gaussian beam, assuming a SiO2 interlayer fills the space between the reflector and the membrane. The performance can be further increased by undercutting this SiO2 interlayer below the photonic crystal. In this case, the overall efficiency is calculated to be 48.5%.
Crossbar networks are a cornerstone of network architectures, capable of operating both as standalone interconnections or as integral switching components in complex, multi-stage systems. The main advantages of crossbar networks are their non-blocking operation and unparalleled minimal latency. With the advent of large scale quantum networks, crossbars might be an important asset towards the Quantum Internet. This study proposes a solution for the problem of distributing entanglement within crossbar quantum networks. Entangled particles are a consumable resource in quantum networks, and are being used by most quantum protocols. By ensuring that nodes within quantum networks are being supplied with entanglement, the reliability and efficiency of the network is maintained. By providing an efficient, scalable framework that can be used to achieve optimal entanglement distribution within crossbar quantum networks, this study offers a theoretical achievement which can be also used for enhancing quantum network performance. An algorithm for selecting an optimal entanglement distribution configuration is proposed and fully tested on realistic possible configurations.
Armed with quantum correlations, quantum sensors in a network have shown the potential to outclass their classical counterparts in distributed sensing tasks such as clock synchronization and reference frame alignment. On the other hand, this analysis was done for simple and idealized networks, whereas the correlation shared within a practical quantum network, captured by the notion of network states, is much more complex. Here, we prove a general bound that limits the performance of using quantum network states to estimate a global parameter, establishing the necessity of genuine multipartite entanglement for achieving a quantum advantage. The bound can also serve as an entanglement witness in networks and can be generalized to states generated by shallow circuits. Moreover, while our bound prohibits local network states from achieving the Heisenberg limit, we design a probabilistic protocol that, once successful, attains this ultimate limit of quantum metrology and preserves the privacy of involved parties. Our work establishes both the limitation and the possibility of quantum metrology within quantum networks.
A quantum network concerns several independent entangled resources and can create strong quantum correlations by performing joint measurements on some observers. In this paper, we discuss an n -partite chain network with each of two neighboring observers sharing an arbitrary Bell state and all intermediate observers performing some positive-operator-valued measurements with parameter λ . The expressions of all post-measurement states between any two observers are obtained, and their quantifications of Bell nonlocality, Einstein–Podolsky–Rosen steering and entanglement with different ranges of λ are respectively detected and analyzed.
Diamond has emerged as a highly promising platform for quantum network applications. Color centers in diamond fulfill the fundamental requirements for quantum nodes: they constitute optically accessible quantum systems with long‐lived spin qubits. Furthermore, they provide access to a quantum register of electronic and nuclear spin qubits and they mediate entanglement between spins and photons. All these operations require coherent control of the color center's spin state. This review provides a comprehensive overview of the state‐of‐the‐art, challenges, and prospects of such schemes, including high‐fidelity initialization, coherent manipulation, and readout of spin states. Established microwave and optical control techniques are reviewed, and moreover, emerging methods such as cavity‐mediated spin–photon interactions and mechanical control based on spin–phonon interactions are summarized. For different types of color centers, namely, nitrogen–vacancy and group‐IV color centers, distinct challenges persist that are subject of ongoing research. Beyond fundamental coherent spin qubit control techniques, advanced demonstrations in quantum network applications are outlined, for example, the integration of individual color centers for accessing (nuclear) multiqubit registers. Finally, the role of diamond spin qubits in the realization of future quantum information applications is described.
We study the propagation of a weak probe field through a dissipative optomechanical system, in which an additional weak mechanical drive is acting directly on a vibrating waveguide. In the presence of a strong coupling field, when the frequency of the mechanical drive precisely matches the frequency difference between the probe and coupling fields, we show that the nearly perfect absorption, complete transmission, and significant amplification of the probe field can be achieved by adjusting the amplitude and phase of the mechanical drive and the power of the coupling field. Moreover, by appropriately choosing the amplitude and phase of the mechanical drive, the group delay of the output probe field is tunable from a large negative value to a large positive value and vice versa. Our study offers an alternative way to control and manipulate the propagation of a weak probe field in a dissipative optomechanical system.
Towards realizing the future quantum internet1,2, a pivotal milestone entails the transition from two-node proof-of-principle experiments conducted in laboratories to comprehensive multi-node set-ups on large scales. Here we report the creation of memory–memory entanglement in a multi-node quantum network over a metropolitan area. We use three independent memory nodes, each of which is equipped with an atomic ensemble quantum memory³ that has telecom conversion, together with a photonic server where detection of a single photon heralds the success of entanglement generation. The memory nodes are maximally separated apart for 12.5 kilometres. We actively stabilize the phase variance owing to fibre links and control lasers. We demonstrate concurrent entanglement generation between any two memory nodes. The memory lifetime is longer than the round-trip communication time. Our work provides a metropolitan-scale testbed for the evaluation and exploration of multi-node quantum network protocols and starts a stage of quantum internet research.
We generate remote entanglement between spatially separate color-center based quantum nodes at rates up to 1 Hz. In addition, we demonstrate remote entanglement across a deployed 35km long fiber loop in the Boston urban area.
A fully homomorphic encryption system enables computation on encrypted data without the necessity for prior decryption. This facilitates the seamless establishment of a secure quantum channel, bridging the server and client components, and thereby providing the client with secure access to the server’s substantial computational capacity for executing quantum operations. However, traditional homomorphic encryption systems lack scalability, programmability, and stability. In this Letter, we experimentally demonstrate a proof-of-concept implementation of a homomorphic encryption scheme on a compact quantum chip, verifying the feasibility of using photonic chips for quantum homomorphic encryption. Our work not only provides a solution for circuit expansion, addressing the longstanding challenge of scalability while significantly reducing the size of quantum network infrastructure, but also lays the groundwork for the development of highly sophisticated quantum fully homomorphic encryption systems.
We introduce a stronger form of quantum nonlocality, termed local subset unidentifiability, that arises from the limitation of spatially separated parties to perfectly identify a subset of mutually orthogonal multipartite quantum states, randomly chosen from a larger known set, using local operations and classical communication (LOCC). We show that this nonlocality is stronger than other existing forms of quantum nonlocality, such as local indistinguishability and local unmarkability. If more than one multipartite states from a locally indistinguishable set are distributed between spatially separated parties in a sequentially ordered fashion, then they may or may not mark which state is which using LOCC. However, we show that even when the parties cannot mark the states, they may still locally identify the particular states given to them, though not their order—i.e., they can identify the elements of the given subset of states. Then we prove the existence of such subsets that are not even locally identifiable, thereby manifesting a stronger nonlocality. We also present the genuine version of this nonlocality—genuine subset unidentifiability—where the provided subset remains unidentifiable unless all the parties come together in a common location and perform global measurements. We anticipate potential applications of this nonlocality for future quantum technologies. We discuss one such application in a certain secret password distribution protocol, where this nonlocality outperforms its predecessors as a resource.
The ability to apply user-chosen large-scale unitary operations with high fidelity to a quantum state is key to realizing future photonic quantum technologies. Here, we realize the implementation of programmable unitary operations on up to 64 frequency-bin modes. To benchmark the performance of our system, we probe different quantum walk unitary operations, in particular, Grover walks on four-dimensional hypercubes with similarities exceeding 95% and quantum walks with 400 steps on circles and finite lines with similarities of 98%. Our results open a path toward implementing high-quality unitary operations, which can form the basis for applications in complex tasks, such as Gaussian boson sampling.
Published by the American Physical Society 2024
Surface acoustic waves (SAW) and associated devices are ideal for sensing, metrology, and hybrid quantum devices. While the advances demonstrated to date are largely based on electromechanical coupling, a robust and customizable coherent optical coupling would unlock mature and powerful cavity optomechanical control techniques and an efficient optical pathway for long-distance quantum links. Here we demonstrate direct and robust coherent optical coupling to Gaussian surface acoustic wave cavities with small mode volumes and high quality factors (>10⁵ measured here) through a Brillouin-like optomechanical interaction. High-frequency SAW cavities designed with curved metallic acoustic reflectors deposited on crystalline substrates are efficiently optically accessed along piezo-active directions, as well as non-piezo-active (electromechanically inaccessible) directions. The precise optical technique uniquely enables controlled analysis of dissipation mechanisms as well as detailed transverse spatial mode spectroscopy. These advantages combined with simple fabrication, large power handling, and strong coupling to quantum systems make SAW optomechanical platforms particularly attractive for sensing, material science, and hybrid quantum systems.
Surface defect-induced photoluminescence blinking is ubiquitous in lead halide perovskite quantum dots (QDs). Despite efforts to passivate the defects on perovskite QDs by chemically engineering ligand binding moieties, blinking accompanied by photodegradation still poses barriers to studying and implementing quantum-confined perovskite QDs in quantum emitters. We posited that the intermolecular interaction between ligands can affect the QD surface passivation. In the solid state, steric repulsions among bulky ligand tails prevent adequate QD surface ligand coverage. Alternatively, attractive π-π stacking between low-steric phenethylammonium (PEA) ligands promotes the formation of a nearly epitaxial surface ligand layer. Here, we demonstrate that single CsPbBr 3 QDs covered by these PEA ligands are nearly non-blinking, with single photon purity reaching 98%. Moreover, these QDs exhibited no spectral shifting and photodegradations, and they remained blinking-free after 12 hours of continuous operation. Free of interferences from blinking and photodegradation, we present size-dependent exciton radiative rates and emission line widths of single CsPbBr 3 QDs ranging from strongly to weakly confined regimes.
We demonstrate the emergence of nonreciprocal superradiant phase transitions and novel multicriticality in a cavity quantum electrodynamics system, where a two-level atom interacts with two counterpropagating modes of a whispering-gallery-mode microcavity. The cavity rotates at a certain angular velocity and is directionally squeezed by a unidirectional parametric pumping χ(2) nonlinearity. The combination of cavity rotation and directional squeezing leads to nonreciprocal first- and second-order superradiant phase transitions. These transitions do not require ultrastrong atom-field couplings and can be easily controlled by the external pump field. Through a full quantum description of the system Hamiltonian, we identify two types of multicritical points in the phase diagram, both of which exhibit controllable nonreciprocity. These results open a new door for all-optical manipulation of superradiant transitions and multicritical behaviors in light-matter systems, with potential applications in engineering various integrated nonreciprocal quantum devices.
A protocol for regulating the distribution of quantum information between multiple parties is put forward. To prohibit the unrestricted distribution of quantum-resource states in a public quantum network, agents can apply a resource-destroying map to each sender's channel. Since resource-destroying maps only exist for affine quantum resource theories, censorship of a nonaffine resource theory is established on an operationally motivated subspace of free states. This is achieved by using what we name a resource-censoring map. The protocol is applied to censoring coherence, reference frames, and entanglement. Because of the local nature of the censorship protocol, it is, in principle, possible for collaborating parties to bypass censorship. Thus, we additionally derive necessary and sufficient conditions under which the censorship protocol is unbreakable.
Quantum information science may lead to technological breakthroughs in computing, cryptography, and sensing. For the implementation of these tasks, however, complex devices with many components are needed and the quantum advantage may easily be spoiled by the failure of only a few parts. A paradigmatic example is quantum networks. There, not only do noise sources such as photon absorption or imperfect quantum memories lead to long waiting times and low fidelity, but also hardware components may break, leading to a dysfunctionality of the entire network. For the successful long-term deployment of quantum networks in the future, it is important to take such deterioration effects into consideration during the design phase. Using methods from reliability theory and the theory of aging, we develop an analytical approach for characterizing the functionality of networks under aging and repair mechanisms, also for nontrivial topologies. Combined with numerical simulations, our results allow us to optimize long-distance entanglement distribution under aging effects.
The quantum routing of single photons takes the central role in an optical quantum network. The matching between the routing probability and the selected frequency of single photons limits the robustness and universality of the quantum router. Here, we investigate how to implement the quantum routing of single photons within bandwidth frequencies by two cavities embedding atoms acting as quantum nodes. We show that the routing capabilities of the single photons and the range of the frequencies can be manipulated by properly designing the chiral coupling strengths and the channel boundaries. Also, we demonstrate that the incident single photons, within bandwidth frequencies, can be completely routed to the targeted output port. This is particularly important to construct optical quantum networks within bandwidth frequencies in the future.
Noise is, in general, inevitable and detrimental to practical and useful quantum communication and computation. Under the resource theory framework, resource distillation serves as a generic tool to overcome the effect of noise. Yet, conventional resource distillation protocols generally require operations on multiple copies of resource states, and strong limitations exist that restrict their practical utilities. Recently, by relaxing the setting of resource distillation to only approximating the measurement statistics instead of the quantum state, a resource-frugal protocol, “virtual resource distillation,” is proposed, which allows more effective distillation of noisy resources. Here, we report its experimental implementation on a photonic quantum system for the distillation of quantum coherence (up to dimension four) and bipartite entanglement. We show the virtual distillation of the maximal superposed state of dimension four from the state of dimension two, an impossible task in conventional coherence distillation. Furthermore, we demonstrate the virtual distillation of entanglement with operations acting only on a single copy of the noisy Einstein-Podolsky-Rosen (EPR) pair and showcase the quantum teleportation task using the virtually distilled EPR pair with a significantly improved fidelity of the teleported state. These results illustrate the feasibility of the virtual resource distillation method and pave the way for accurate manipulation of quantum resources with noisy quantum hardware.
In a fiber-based quantum network, the utilization of the telecom band is crucial for long-distance quantum information (QI) transmission between quantum nodes. However, the near-infrared wavelength is identified as optimal for storing and processing QI through alkaline atoms. Recognizing the challenge of efficiently bridging the frequency gap between atomic quantum devices and telecom fibers while maintaining the QI carried by photons, quantum frequency conversion (QFC) serves as a pivotal quantum interface. In this study, we explore an efficient telecom-band QFC mechanism based on diamond-type four-wave mixing (FWM) with rubidium energy levels. The mechanism enables the conversion of photons between the near-infrared wavelength of 795 nm and the telecom band of 1367 or 1529 nm. Using the Heisenberg-Langevin approach, we optimize conversion efficiency (CE) across varying optical depths while considering quantum noises and present corresponding experimental parameters. Unlike previous works neglecting the applied field absorption loss, our results are more relevant to practical scenarios. Moreover, by employing the reduced-density-operator theory to construct a theoretical framework, we demonstrate that this diamond-type FWM scheme can maintain the quantum characteristics of input photons with high fidelity, such as quadrature variances and photon statistics. Importantly, these properties remain unaffected by vacuum field noise, enabling the system to achieve high-purity QFC. Another significant contribution lies in examining how this scheme impacts QI encoded in photon-number, path, and polarization degrees of freedom. These encoded qubits exhibit remarkable entanglement retention under sufficiently high CE. In the case of perfect CE, the scheme can achieve unity fidelity. This comprehensive exploration establishes a theoretical foundation for the application of the diamond-type QFC scheme based on atomic ensembles in quantum networks, laying essential groundwork for advancing the scheme in distributed quantum computing and long-distance quantum communication.
We demonstrated a highly efficient optical memory operation using an atomic frequency comb (AFC) created from a spectrally flattened optical frequency comb with an isotopically pure ¹⁶⁷ Er ³⁺ :Y 2 SiO 5 crystal under zero magnetic field. Compared with standard AFC creation methods such as pulse train method, our method, called comb transfer method, can create a broadband and high-efficiency AFC at lower power and with less effort. This technique can make a significant improvement in the quality of AFCs in crystals doped with low concentrations of rare-earth ions.
Coherent preparation by laser light of quantum states of atoms and molecules can lead to quantum interference in the amplitudes of optical transitions. In this way the optical properties of a medium can be dramatically modified, leading to electromagnetically induced transparency and related effects, which have placed gas-phase systems at the center of recent advances in the development of media with radically new optical properties. This article reviews these advances and the new possibilities they offer for nonlinear optics and quantum information science. As a basis for the theory of electromagnetically induced transparency the authors consider the atomic dynamics and the optical response of the medium to a continuous-wave laser. They then discuss pulse propagation and the adiabatic evolution of field-coupled states and show how coherently prepared media can be used to improve frequency conversion in nonlinear optical mixing experiments. The extension of these concepts to very weak optical fields in the few-photon limit is then examined. The review concludes with a discussion of future prospects and potential new applications.
Generation of non-classical correlations (or entanglement) between atoms, photons or combinations thereof is at the heart of quantum information science. Of particular interest are material systems serving as quantum memories that can be interconnected optically. An ensemble of atoms can store a quantum state in the form of a magnon-which is a quantized collective spin excitation-that can be mapped onto a photon with high efficiency. Here, we report the phase-coherent transfer of a single magnon from one atomic ensemble to another via an optical resonator serving as a quantum bus that in the ideal case is only virtually populated. Partial transfer deterministically creates an entangled state with one excitation jointly stored in the two ensembles. The entanglement is verified by mapping the magnons onto photons, whose correlations can be directly measured. These results should enable deterministic multipartite entanglement between atomic ensembles.
In the last few years there has been a lot of interest in quantum repeater protocols using only atomic ensembles and linear optics. Here we show that the local generation of high-fidelity entangled pairs of atomic excitations, in combination with the use of two-photon detections for long-distance entanglement generation, permits the implementation of a very attractive quantum repeater protocol. Such a repeater is robust with respect to phase fluctuations in the transmission channels, and at the same time achieves higher entanglement generation rates than other protocols using the same ingredients. We propose an efficient method of generating high-fidelity entangled pairs locally, based on the partial readout of the ensemble-based memories. We also discuss the experimental implementation of the proposed protocol.
Strongly correlated systems are difficult to control or even probe at the level of individual interacting elements. Engineered composites of optical cavities, few-level atoms, and laser light could enable greater insight into their behaviour.
The irreversible evolution of a microscopic system under measurement is a central feature of quantum theory. From an initial state generally exhibiting quantum uncertainty in the measured observable, the system is projected into a state in which this observable becomes precisely known. Its value is random, with a probability determined by the initial system's state. The evolution induced by measurement (known as 'state collapse') can be progressive, accumulating the effects of elementary state changes. Here we report the observation of such a step-by-step collapse by non-destructively measuring the photon number of a field stored in a cavity. Atoms behaving as microscopic clocks cross the cavity successively. By measuring the light-induced alterations of the clock rate, information is progressively extracted, until the initially uncertain photon number converges to an integer. The suppression of the photon number spread is demonstrated by correlations between repeated measurements. The procedure illustrates all the postulates of quantum measurement (state collapse, statistical results and repeatability) and should facilitate studies of non-classical fields trapped in cavities.
Quantum teleportation is an important ingredient in distributed quantum networks, and can also serve as an elementary operation in quantum computers. Teleportation was first demonstrated as a transfer of a quantum state of light onto another light beam; later developments used optical relays and demonstrated entanglement swapping for continuous variables. The teleportation of a quantum state between two single material particles (trapped ions) has now also been achieved. Here we demonstrate teleportation between objects of a different nature-light and matter, which respectively represent 'flying' and 'stationary' media. A quantum state encoded in a light pulse is teleported onto a macroscopic object (an atomic ensemble containing 10 caesium atoms). Deterministic teleportation is achieved for sets of coherent states with mean photon number (n) up to a few hundred. The fidelities are 0.58 ± 0.02 for n = 20 and 0.60 ± 0.02 for n = 5- higher than any classical state transfer can possibly achieve. Besides being of fundamental interest, teleportation using a macroscopic atomic ensemble is relevant for the practical implementation of a quantum repeater. An important factor for the implementation of quantum networks is the teleportation distance between transmitter and receiver; this is 0.5 metres in the present experiment. As our experiment uses propagating light to achieve the entanglement of light and atoms required for teleportation, the present approach should be scalable to longer distances.
Over the past decade, strong interactions of light and matter at the single-photon level have enabled a wide set of scientific advances in quantum optics and quantum information science. This work has been performed principally within the setting of cavity quantum electrodynamics with diverse physical systems, including single atoms in Fabry-Perot resonators, quantum dots coupled to micropillars and photonic bandgap cavities and Cooper pairs interacting with superconducting resonators. Experiments with single, localized atoms have been at the forefront of these advances with the use of optical resonators in high-finesse Fabry-Perot configurations. As a result of the extreme technical challenges involved in further improving the multilayer dielectric mirror coatings of these resonators and in scaling to large numbers of devices, there has been increased interest in the development of alternative microcavity systems. Here we show strong coupling between individual caesium atoms and the fields of a high-quality toroidal microresonator. From observations of transit events for single atoms falling through the resonator's evanescent field, we determine the coherent coupling rate for interactions near the surface of the resonator. We develop a theoretical model to quantify our observations, demonstrating that strong coupling is achieved, with the rate of coherent coupling exceeding the dissipative rates of the atom and the cavity. Our work opens the way for investigations of optical processes with single atoms and photons in lithographically fabricated microresonators. Applications include the implementation of quantum networks, scalable quantum logic with photons, and quantum information processing on atom chips.
Einstein often performed thought experiments with 'photon boxes', storing fields for unlimited times. This is yet but a dream. We can nevertheless store quantum microwave fields in superconducting cavities for billions of periods. Using circular Rydberg atoms, it is possible to probe in a very detailed way the quantum state of these trapped fields. Cavity quantum electrodynamics tools can be used for a direct determination of the Husimi Q and Wigner quasi-probability distributions. They provide a very direct insight into the classical or non-classical nature of the field.
We propose a new method for the generation of single photons. Our scheme will lead to the emission of one photon into a single
mode of the radiation field in response to a trigger event. This photon is emitted from an atom strongly coupled to a high-finesse
optical cavity, and the trigger is a classical light pulse. The device combines cavity-QED with an adiabatic transfer technique.
We simulate this process numerically and show that it is possible to control the temporal behaviour of the photon emission
probability by the shape and the detuning of the trigger pulse. An extension of the scheme with a reloading mechanism will
allow one to emit a bit-stream of photons at a given rate.
We present a rare example of a decay mechanism playing a constructive role in quantum information processing. We show how
the state of an atom trapped in a cavity can be teleported to a second atom trapped in a distant cavity by the joint detection
of photon leakage from the cavities. The scheme, which is probabilistic, requires only a single three level atom in a cavity.
We also show how this scheme can be modified to a teleportation with insurance.
The concept of the photon, central to Einstein's explanation of the
photoelectric effect, is exactly 100 years old. Yet, while photons
have been detected individually for more than 50 years, devices
producing individual photons on demand have only appeared in the last
few years. New concepts for single-photon sources, or 'photon guns',
have originated from recent progress in the optical detection,
characterization and manipulation of single quantum objects. Single
emitters usually deliver photons one at a time. This so-called
antibunching of emitted photons can arise from various mechanisms, but
ensures that the probability of obtaining two or more photons at the
same time remains negligible. We briefly recall basic concepts in
quantum optics and discuss potential applications of single-photon
states to optical processing of quantum information: cryptography,
computing and communication. A photon gun's properties are
significantly improved by coupling it to a resonant cavity mode,
either in the Purcell or strong-coupling regimes. We briefly recall
early production of single photons with atomic beams, and the
operation principles of macroscopic parametric sources, which are used
in an overwhelming majority of quantum-optical experiments. We then
review the photophysical and spectroscopic properties and compare the
advantages and weaknesses of various single nanometre-scale objects
used as single-photon sources: atoms or ions in the gas phase and, in
condensed matter, organic molecules, defect centres, semiconductor
nanocrystals and heterostructures. As new generations of sources are
developed, coupling to cavities and nano-fabrication techniques lead
to improved characteristics, delivery rates and spectral ranges.
Judging from the brisk pace of recent progress, we expect single
photons to soon proceed from demonstrations to applications and to
bring with them the first practical uses of quantum information.
We present a basic building block of a quantum network consisting of a quantum dot coupled to a source cavity, which in turn is coupled to a target cavity via a waveguide. The single photon emission from the high-Q/V source cavity is characterized by twelve-fold spontaneous emission ( SE) rate enhancement, SE coupling efficiency beta similar to 0.98 into the source cavity mode, and mean wavepacket indistinguishability of similar to 67%. Single photons are efficiently transferred into the target cavity via the waveguide, with a target/source field intensity ratio of 0.12 +/- 0.01. This system shows great promise as a building block of future on-chip quantum information processing systems. (c) 2007 Optical Society of America.
The recent development of techniques to produce optical semiconductor cavities of very high quality has prepared the stage for observing cavity quantum-electrodynamic effects in solid-state materials. Among the most promising systems for these studies are semiconductor quantum dots inside photonic crystal, micropillar or microdisk resonators. We review the progress so far in obtaining true quantum-optical strong-coupling effects in semiconductors. We discuss the recent results on vacuum Rabi splitting with a single quantum dot, emphasizing the differences from quantum-well systems. Finally, we propose nonlinear tests for the true quantum limit and speculate about applications in quantum information devices.
This Letter describes the generation of biphotons with a temporal length that can be varied over the range of 50-900 ns, with an estimated subnatural linewidth as small as 0.75 MHz. We make use of electromagnetically induced transparency and slow light in a two-dimensional magneto-optical trap with an optical depth as high as 62. We report a sharp leading edge spike that is a Sommerfeld-Brillouin precursor, as observed at the biphoton level.
We present here a new approach to scalable quantum computing - a 'qubus computer' - which realizes qubit measurement and quantum gates through interacting qubits with a quantum communication bus mode. The qubits could be 'static' matter qubits or 'flying' optical qubits, but the scheme we focus on here is particularly suited to matter qubits. There is no requirement for direct interaction between the qubits. Universal two-qubit quantum gates may be effected by schemes which involve measurement of the bus mode, or by schemes where the bus disentangles automatically and no measurement is needed. In effect, the approach integrates together qubit degrees of freedom for computation with quantum continuous variables for communication and interaction.
We present a protocol for performing entanglement connection between pairs of atomic ensembles in the single excitation regime. Two pairs are prepared in an asynchronous fashion and then connected via a Bell measurement. The resulting state of the two remaining ensembles is mapped to photonic modes and a reduced density matrix is then reconstructed. Our observations confirm for the first time the creation of coherence between atomic systems that never interacted, a first step towards entanglement connection, a critical requirement for quantum networking and long distance quantum communications.
We report measurements of cavity-QED effects for the radiative coupling of atoms in a dilute vapor to the external evanescent field of a whispering-gallery mode (WGM) in a fused silica microsphere. The high $Q$ $(5\ifmmode\times\else\texttimes\fi{}{10}^{7})$, small mode volume (10${}^{$-${}8}$ cm${}^{3})$, and unusual symmetry of the microcavity evanescent field enable velocity-selective interactions between fields with photon number of order unity in the WGM and ${N}_{T}$\sim${}1$ atoms in the surrounding vapor.
We report an experiment in which a light pulse is effectively decelerated and trapped in a vapor of Rb atoms, stored for a controlled period of time, and then released on demand. We accomplish this "storage of light" by dynamically reducing the group velocity of the light pulse to zero, so that the coherent excitation of the light is reversibly mapped into a Zeeman (spin) coherence of the Rb vapor.
Entanglement is considered to be one of the most profound features of quantum mechanics. An entangled state of a system consisting of two subsystems cannot be described as a product of the quantum states of the two subsystems. In this sense, the entangled system is considered inseparable and non-local. It is generally believed that entanglement is usually manifest in systems consisting of a small number of microscopic particles. Here we demonstrate experimentally the entanglement of two macroscopic objects, each consisting of a caesium gas sample containing about 1012 atoms. Entanglement is generated via interaction of the samples with a pulse of light, which performs a non-local Bell measurement on the collective spins of the samples. The entangled spin-state can be maintained for 0.5 milliseconds. Besides being of fundamental interest, we expect the robust and long-lived entanglement of material objects demonstrated here to be useful in quantum information processing, including teleportation of quantum states of matter and quantum memory.
Solid-state cavity quantum electrodynamics (QED) systems offer a robust and scalable platform for quantum optics experiments and the development of quantum information processing devices. In particular, systems based on photonic crystal nanocavities and semiconductor quantum dots have seen rapid progress. Recent experiments have allowed the observation of weak and strong coupling regimes of interaction between the photonic crystal cavity and a single quantum dot in photoluminescence. In the weak coupling regime, the quantum dot radiative lifetime is modified; in the strong coupling regime, the coupled quantum dot also modifies the cavity spectrum. Several proposals for scalable quantum information networks and quantum computation rely on direct probing of the cavity–quantum dot coupling, by means of resonant light scattering from strongly or weakly coupled quantum dots. Such experiments have recently been performed in atomic systems and superconducting circuit QED systems, but not in solid-state quantum dot–cavity QED systems. Here we present experimental evidence that this interaction can be probed in solid-state systems, and show that, as expected from theory, the quantum dot strongly modifies the cavity transmission and reflection spectra. We show that when the quantum dot is coupled to the cavity, photons that are resonant with its transition are prohibited from entering the cavity. We observe this effect as the quantum dot is tuned through the cavity and the coupling strength between them changes. At high intensity of the probe beam, we observe rapid saturation of the transmission dip. These measurements provide both a method for probing the cavity–quantum dot system and a step towards the realization of quantum devices based on coherent light scattering and large optical nonlinearities from quantum dots in photonic crystal cavities.
A general photonic channel for quantum communication is defined. By means of local quantum computing with a few auxiliary
atoms, this channel can be reduced to one with effectively less noise. A scheme based on quantum interference is proposed
that iteratively improves the fidelity of distant entangled particles.
Microcavity physics and design will be reviewed. Following an overview of applications in quantum optics, communications and biosensing, recent advances in ultra-high-Q research will be presented.
We show how the state of an atom trapped in a cavity can be teleported
to a second atom trapped in a distant cavity simply by detecting photon
decays from the cavities. This is a rare example of a decay mechanism
playing a constructive role in quantum information processing. The
scheme is comparatively easy to implement, requiring only the ability to
trap a single three level atom in a cavity.
Electromagnetically induced transparency is a technique for eliminating the effect of a medium on a propagating beam of electromagnetic radiation. EIT may also be used, but under more limited conditions, to eliminate optical self‐focusing and defocusing and to improve the transmission of laser beams through inhomogeneous refracting gases and metal vapors, as figure 1 illustrates. The technique may be used to create large populations of coherently driven uniformly phased atoms, thereby making possible new types of optoelectronic devices. One can make opaque resonant transitions transparent to laser radiation, often with most of the atoms remaining in the ground state.
New aspects of the casimir effect - fluctuations and radiative reaction, G. Barton non-perturbative atom-photon interactions in an optical cavity, H.J. Carmichael et al single atom emission in an optical resonator, J.J. Childs et al one electron in a cavity, G. Gabrielse and J. Tan manipulation of non-classical field states in a cavity by atom interferometry, S. Haroche and J.M. Raimond perturbative cavity quantum electrodynamics, E.A. Hinds structure and dynamics in cavity quantum electrodynamics, H.J. Kimble spontaneous emission by moving atoms, P. Meystre and M. Wilkens quantum optics of driven atoms in coloured vacua, T.W. Mossberg and M. Lewenstein the micromaser - a proving ground for quantum physics, G. Raithel et al.
Quantum information describes the new field which bridges quantum physics and information science. The quantum world allows for completely new architectures and protocols. While originally formulated in continuous quantum variables, the field worked almost exclusively with discrete variables, such as single photons and photon pairs. The renaissance of continuous variables came with European research consortia such as ACQUIRE (Advanced Coherent Quantum Information Research) in the late 1990s, and QUICOV (Quantum Information with Continuous Variables) from 2000–2003. The encouraging research results of QUICOV and the new conference series CVQIP (Continuous Variable Quantum Information Processing) triggered the idea for this book.This book presents the state of the art of quantum information with continuous quantum variables. The individual chapters discuss results achieved in QUICOV and presented at the first five CVQIP conferences from 2002-2006. Many world-leading scientists working on continuous variables outside Europe also contribute to the book.
Quantum information science attempts to exploit capabilities from the quantum realm to accomplish tasks that are otherwise impossible in the classical domain. In this regard, a significant advance is the invention of a protocol by Duan, Lukin, Cirac, and Zoller (DLCZ) for the realization of scalable long distance quantum communication and the distribution of entanglement over quantum networks [1]. Here we report the first enabling step in the realization of the protocol of DLCZ, namely the observation of quantum correlations for photon pairs generated in the collective emission from an atomic ensemble. An optically thick sample of three-level atoms in a lambda-configuration is exploited to produce correlated photons. The atomic sample for our experiment is provided by Cesium atoms in a magneto-optical trap (MOT). We find a significant violation of the Cauchy-Schwarz inequality clearly demonstrating the nonclassical character of the correlations between the two photons generated by sequential (write,read) beams. Moreover, the measured coincidence rates clearly demonstrate the cooperative nature of the emission process. These capabilities should help to enable other advances in the field of quantum information, including the implementation of quantum memory and fully controllable single-photon sources, which, combined together, pave the avenue for realization of universal quantum computation. [1] L.-M. Duan, M. Lukin, J. I. Cirac, and P. Zoller, Nature 414, 413 (2001).
In Heisenberg's famous discussion of the measurement of a particle's position using a microscope, the momentum transferred to the particle by the scattered photon makes the particle's momentum uncertain. It is shown that momentum is also transferred when the lack of a scattered photon is used to discover that the particle is absent from the field of view of the microscope (i.e., located outside the light beam). This apparent paradox, a transfer of momentum and/or energy to a missing particle by a light beam (without the scattering of a photon), is discussed and ''resolved'' using quantum measurement theory.
Citation
S. E. HARRIS, "ELECTROMAGNETICALLY INDUCED TRANSPARENCY," Optics & Photonics News 2(12), 29-30 (1991)
http://www.opticsinfobase.org/opn/abstract.cfm?URI=opn-2-12-29
Modern optical techniques allow one to accurately control light using atoms and to manipulate atoms using light. In this Colloquium the author reviews several ideas indicating how such techniques can be used for accurate manipulation of quantum states of atomic ensembles and photons. First a technique is discussed that allows one to transfer quantum states between light fields and metastable states of matter. The technique is based on trapping quantum states of photons in coherently driven atomic media, in which the group velocity is adiabatically reduced to zero. Next, possible mechanisms are outlined for manipulating quantum states of atomic ensembles. Specifically, a “dipole blockade” technique is considered in which optical excitation of mesoscopic samples into Rydberg states can be used to control the state of ensembles at the level of individual quanta. It is also noted that even simple processes involving atom-photon correlations can be used to effectively manipulate the ensemble states. Potentially these techniques can be used for implementation of important concepts from quantum information science.
The authors discuss the technique of stimulated Raman adiabatic passage (STIRAP), a method of using partially overlapping pulses (from pump and Stokes lasers) to produce complete population transfer between two quantum states of an atom or molecule. The procedure relies on the initial creation of a coherence (a population-trapping state) with subsequent adiabatic evolution. The authors present the basic theory, with some extensions, and then describe examples of experimental utilization. They note some applications of the technique not only to preparation of selected states for reaction studies, but also to quantum optics and atom optics.
Some quantum states are hard to create and maintain, but are a valuable resource for computing. Twenty-first century entrepreneurs could make a fortune selling disposable quantum states.
The adiabatic passage scheme for quantum state synthesis, in which atomic Zeeman coherences are mapped to photon states in an optical cavity, is extended to the general case of two degenerate cavity modes with orthogonal polarization. Analytical calculations of the dressed-state structure and Monte Carlo wave-function simulations of the system dynamics show that, for a suitably chosen cavity detuning, it is possible to generate states of photon multiplets that are maximally entangled in polarization. These states display nonclassical correlations of the type described by Greenberger, Horne, and Zeilinger (GHZ). An experimental scheme to realize a GHZ measurement using coincidence detection of the photons escaping from the cavity is proposed. The correlations are found to originate in the dynamics of the adiabatic passage and persist even if cavity decay and GHZ state synthesis compete on the same time scale. Beyond entangled field states, it is also possible to generate entanglement between photons and the atom by using a different atomic transition and initial Zeeman state.
In quantum communication via noisy channels, the error probability scales exponentially with the length of the channel. We present a scheme of a quantum repeater that overcomes this limitation. The central idea is to connect a string of (imperfect) entangled pairs of particles by using a novel nested purification protocol, thereby creating a single distant pair of high fidelity. Our scheme tolerates general errors on the percent level, it works with a polynomial overhead in time and a logarithmic overhead in the number of particles that need to be controlled locally.
The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory using the one-atom maser or micromaser to produce these fields on demand. In order to achieve this goal the maser is operating under the condition of the so-called trapping states. In this situation the field of the micromaser stabilises to a photon number state. In the second part of the paper a single photon source on the basis of a trapped ion will be described. The strong localization and position control available when an ion trap is combined with an optical cavity is a big step forward in producing a deterministic atom-field coupling necessary for quantum information processing.
We present an excerpt of the document “Quantum
Information Processing and Communication: Strategic report on current
status, visions and goals for research in Europe”, which has been recently
published in electronic form at the website of FET (the Future and Emerging
Technologies Unit of the Directorate General Information Society of the
European Commission, http://www.cordis.lu/ist/fet/qipc-sr.htm). This
document has been elaborated, following a former suggestion by FET, by a
committee of QIPC scientists to provide input towards the European
Commission for the preparation of the Seventh Framework Program. Besides
being a document addressed to policy makers and funding agencies (both at
the European and national level), the document contains a detailed
scientific assessment of the state-of-the-art, main research goals,
challenges, strengths, weaknesses, visions and perspectives of all the most
relevant QIPC sub-fields, that we report here.
Dedicated to the memory of Prof.
Th. Beth, one of the pioneers of QIPC,
whose contributions have had a significant scientific impact on the
development as well as on the visibility of a field that he
enthusiastically helped to shape since its early days.
The properties of optical resonators with quality-factor Q???108, effective volume of e.m. field localization Veff???10???9 cm3 and threshold power of optical bistability Wbist???10???5 W are described. The prospects to reduce Veff and Wbist are discussed. With possible reduction of controlling energy of optical switching down to a single quantum and employment of the monophotonic states of light, the whispering-gallery microresonators can open the way to realize Feynman's quantum-mechanical computer.
Algorithms such as quantum factoring and quantum search illustrate the great theoretical promise of quantum computers; but the practical implementation of such devices will require careful consideration of the minimum resource requirements, together with the development of procedures to overcome inevitable residual imperfections in physical systems. Many designs have been proposed, but none allow a large quantum computer to be built in the near future. Moreover, the known protocols for constructing reliable quantum computers from unreliable components can be complicated, often requiring many operations to produce a desired transformation. Here we show how a single technique-a generalization of quantum teleportation-reduces resource requirements for quantum computers and unifies known protocols for fault-tolerant quantum computation. We show that single quantum bit (qubit) operations, Bell-basis measurements and certain entangled quantum states such as Greenberger-Horne-Zeilinger (GHZ) states-all of which are within the reach of current technology-are sufficient to construct a universal quantum computer. We also present systematic constructions for an infinite class of reliable quantum gates that make the design of fault-tolerant quantum computers much more straightforward and methodical.
We investigate the suitability of toroidal microcavities for strong-coupling cavity quantum electrodynamics (QED). Numerical modeling of the optical modes demonstrate a significant reduction of the modal volume with respect to the whispering gallery modes of dielectric spheres, while retaining the high-quality factors representative of spherical cavities. The extra degree of freedom of toroid microcavities can be used to achieve improved cavity QED characteristics. Numerical results for atom-cavity coupling strength g, critical atom number No, and critical photon number no for cesium are calculated and shown to exceed values currently possible using Fabry-Perot cavities. Modeling predicts coupling rates g/2π exceeding 700 MHz and critical atom numbers approaching 10^(-7) in optimized structures. Furthermore, preliminary experimental measurements of toroidal cavities at a wavelength of 852 nm indicate that quality factors in excess of 108 can be obtained in a 50-µm principal diameter cavity, which would result in strong-coupling values of (g/(2π),n(0),N-0) = (86 MHz, 4.6 x 10^(-4), 1.0 x 10^(-3)).
On the occasion of the hundredth anniversary of Albert Einstein's annus mirabilis, we reflect on the development and current state of research in cavity quantum electrodynamics in the optical domain. Cavity QED is a field which undeniably traces its origins to Einstein's seminal work on the statistical theory of light and the nature of its quantized interaction with matter. In this paper, we emphasize the development of techniques for the confinement of atoms strongly coupled to high-finesse resonators and the experiments which these techniques enable.
We report significant improvements in the retrieval efficiency of a single excitation stored in an atomic ensemble and in the subsequent generation of strongly correlated pairs of photons. A 50% probability of transforming the stored excitation into one photon in a well-defined spatio-temporal mode at the output of the ensemble is demonstrated. These improvements are illustrated by the generation of high-quality heralded single photons with a suppression of the two-photon component below 1% of the value for a coherent state. A broad characterization of our system is performed for different parameters in order to provide input for the future design of realistic quantum networks.
A general photonic channel for quantum communication is defined. By means of local quantum computing with a few auxiliary atoms, this channel can be reduced to one with effectively less noise. A scheme based on quantum interference is proposed that iteratively improves the fidelity of distant entangled particles.
A quantum theory of stimulated Raman scattering is presented that takes into account three-dimensional propagation and collisional dephasing, allowing the study of the spatial and temporal coherence properties of the generated Stokes light. Maxwell-Bloch equations for the Stokes field operator and the collective atomic operators are solved analytically under low-signal-gain conditions, where the laser field and the atomic ground states remain undepleted. The intensity and the space-time autocorrelation function of the Stokes field are calculated. The Stokes field is expanded into a set of statistically independent ``coherence modes,'' which are determined explicitly for the case of a cylindrically shaped pumped volume. The Stokes pulse energy W is found to fluctuate from pulse to pulse. The probability distribution function for pulse energies P(W) is calculated for a range of Fresnel numbers of the excited volume and collisional dephasing rates. For small values of Fresnel number and dephasing rate, P(W) is a negative exponential distribution. For large values of either, P(W) narrows and approaches a Gaussian-shaped distribution. This occurs because many independent modes contribute to the Stokes emission, making it spatially and/or temporally incoherent.
The exchange of photons between single Rydberg atoms and a single mode of a superconducting cavity with a quality factor Q = 8 x 10 to the 8th at 2 K was observed. Signals could still be detected with an average number of only 0.06 atom simultaneously in the cavity. With one Rydberg atom the linewidth of the maser transition at about 21 GHz was power broadened, and at high densities asymmetry of the transition was observed which is ascribed to an ac Stark effect.
A scheme for the preparation of general coherent superpositions of photon-number states is proposed. By strongly coupling an atom to a cavity field, atomic ground-state Zeeman coherence can be transferred by (coherent) adiabatic passage to the cavity mode and a general field state can be generated without atomic projection noise.
Electromagnetically induced transparency is a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium; a 'coupling' laser is used to create the interference necessary to allow the transmission of resonant pulses from a 'probe' laser. This technique has been used to slow and spatially compress light pulses by seven orders of magnitude, resulting in their complete localization and containment within an atomic cloud. Here we use electromagnetically induced transparency to bring laser pulses to a complete stop in a magnetically trapped, cold cloud of sodium atoms. Within the spatially localized pulse region, the atoms are in a superposition state determined by the amplitudes and phases of the coupling and probe laser fields. Upon sudden turn-off of the coupling laser, the compressed probe pulse is effectively stopped; coherent information initially contained in the laser fields is 'frozen' in the atomic medium for up to 1 ms. The coupling laser is turned back on at a later time and the probe pulse is regenerated: the stored coherence is read out and transferred back into the radiation field. We present a theoretical model that reveals that the system is self-adjusting to minimize dissipative loss during the 'read' and 'write' operations. We anticipate applications of this phenomenon for quantum information processing.
Quantum communication holds promise for absolutely secure transmission of secret messages and the faithful transfer of unknown quantum states. Photonic channels appear to be very attractive for the physical implementation of quantum communication. However, owing to losses and decoherence in the channel, the communication fidelity decreases exponentially with the channel length. Here we describe a scheme that allows the implementation of robust quantum communication over long lossy channels. The scheme involves laser manipulation of atomic ensembles, beam splitters, and single-photon detectors with moderate efficiencies, and is therefore compatible with current experimental technology. We show that the communication efficiency scales polynomially with the channel length, and hence the scheme should be operable over very long distances.