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ABSTRACT: In quantum information, complementarity of quantum mechanical observables
plays a key role. If a system resides in an eigenstate of an observable, the
probability distribution for the values of a complementary observable is flat.
The eigenstates of these two observables form a pair of mutually unbiased bases
(MUBs). More generally, a set of MUBs consists of bases that are all pairwise
unbiased. Except for specific dimensions of the Hilbert space, the maximal sets
of MUBs are unknown in general. Even for a dimension as low as six, the
identification of a maximal set of MUBs remains an open problem, although there
is strong numerical evidence that no more than three simultaneous MUBs do
exist. Here, by exploiting a newly developed holographic technique, we
implement and test different sets of three MUBs for a single photon
six-dimensional quantum state (a qusix), encoded either in a hybrid
polarization-orbital angular momentum or a pure orbital angular momentum
Hilbert space. A close agreement is observed between theory and experiments.
Our results can find applications in state tomography, quantitative
wave-particle duality, quantum key distribution and tests on complementarity
and logical indeterminacy.
04/2013;
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ABSTRACT: The main features of quantum mechanics reside in interference deriving from the superposition of different quantum states. While current quantum optical technology enables two-photon interference both in bulk and integrated systems, simultaneous interference of more than two particles, leading to richer quantum phenomena, is still a challenging task. Here we report the experimental observation of three-photon interference in an integrated three-port directional coupler realized by ultrafast laser writing. By exploiting the capability of this technique to produce three-dimensional structures, we realized and tested in the quantum regime a three-port beam splitter, namely a tritter, which allowed us to observe bosonic coalescence of three photons. These results open new important perspectives in many areas of quantum information, such as fundamental tests of quantum mechanics with increasing number of photons, quantum state engineering, quantum sensing and quantum simulation.
Nature Communications 03/2013; 4:1606. · 7.40 Impact Factor
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Proceedings of the National Academy of Sciences 11/2012; · 9.68 Impact Factor
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ABSTRACT: Photons are the ideal carriers of quantum information for communication. Each
photon can have a single qubit or even multiple qubits encoded in its internal
quantum state, as defined by optical degrees of freedom such as polarization,
wavelength, transverse modes, etc. Here, we propose and experimentally
demonstrate a physical process, named "quantum state fusion", in which the
two-dimensional quantum states (qubits) of two input photons are combined into
a single output photon, within a four-dimensional quantum space. The inverse
process is also proposed, in which the four-dimensional quantum state of a
single photon is split into two photons, each carrying a qubit. Both processes
can be iterated, and hence may be used to bridge multi-particle protocols of
quantum information with the multi-degree-of-freedom ones, with possible
applications in quantum communication networks.
09/2012;
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ABSTRACT: The conflict between classical and quantum physics can be identified through
a series of yes-no tests on quantum systems, without it being necessary that
these systems be in special quantum states. Kochen-Specker (KS) sets of yes-no
tests have this property and provide a quantum-versus-classical advantage that
is free of the initialization problem that affects some quantum computers.
Here, we report the first experimental implementation of a complete KS set that
consists of 18 yes-no tests on four-dimensional quantum systems and show how to
use the KS set to obtain a state-independent quantum advantage. We first
demonstrate the unique power of this KS set for solving a task while avoiding
the problem of state initialization. Such a demonstration is done by showing
that, for 28 different quantum states encoded in the orbital-angular-momentum
and polarization degrees of freedom of single photons, the KS set provides an
impossible-to-beat solution. In a second experiment, we generate maximally
contextual quantum correlations by performing compatible sequential
measurements of the polarization and path of single photons. In this case,
state independence is demonstrated for 15 different initial states. Maximum
contextuality and state independence follow from the fact that the sequences of
measurements project any initial quantum state onto one of the KS set's
eigenstates. Our results show that KS sets can be used for quantum-information
processing and quantum computation and pave the way for future developments.
09/2012;
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ABSTRACT: A loophole-free violation of Bell inequalities is of fundamental importance
for demonstrating quantum nonlocality and long-distance device-independent
secure communication. However, transmission losses represent a fundamental
limitation for photonic loophole-free Bell tests. A local precertification of
the presence of the photons immediately before the local measurements may solve
this problem. We show that local precertification is feasible by integrating
three current technologies: (i) enhanced single-photon down-conversion to
locally create a flag photon, (ii) nanowire-based superconducting single-photon
detectors for a fast flag detection, and (iii) superconducting transition-edge
sensors to close the detection loophole. We carry out a precise space-time
analysis of the proposed scheme, showing its viability and feasibility.
06/2012;
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ABSTRACT: Loophole-free Bell tests for quantum nonlocality and long-distance secure
communication require photodetection efficiencies beyond a threshold eta_{crit}
that depends on the Bell inequality and the noise affecting the entangled state
received by the distant parties. Most calculations of eta_{crit} assume that
the noise is random and can be modeled as white noise. However, most sources
suffer from colored noise. Indeed, since entangled states are usually created
as a superposition of two possible deexcitation paths, a partial
distinguishability between the two processes leads to the appearance of colored
noise in the generated state. Recently, there was a proposal for a
loophole-free Bell test [A. Cabello and F. Sciarrino, Phys. Rev. X 2, 021010
(2012)], where a specific colored noise appears as a consequence of the
precertification of the photon's presence through single-photon spontaneous
parametric down-conversion. Here we obtain eta_{crit}, the optimal quantum
states, and the local settings for a loophole-free Bell test as a function of
the amount of colored noise. We consider three bipartite Bell inequalities with
n dichotomic settings: Clauser-Horne-Shimony-Holt (n=2), I_{3322} (n=3), and
A_5 (n=4), both for the case of symmetric efficiencies, corresponding to
photon-photon Bell tests, and for the totally asymmetric case, corresponding to
atom-photon Bell tests. Remarkably, in all these cases, eta_{crit} is robust
against the colored noise. The present analysis can find application in any
test of Bell inequalities in which the dominant noise is of the colored type.
06/2012;
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ABSTRACT: In this paper we analyze the resilience to decoherence of the Macroscopic Quantum Superpositions (MQS) generated by optimal
phase-covariant quantum cloning according to two coherence criteria, both based on the concept of Bures distance in Hilbert
spaces. We show that all MQS generated by this system are characterized by a high resilience to decoherence processes. This
analysis is supported by the results of recent MQS experiments of N=3.5×104 particles.
KeywordsMacroscopic quantum superposition–Decoherence–Phase-covariant cloning
Foundations of Physics 04/2012; 41(3):492-508. · 1.05 Impact Factor
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03/2012; , ISBN: 978-953-51-0153-6
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ABSTRACT: Quantum resources outperform classical ones for certain communication and computational tasks. Remarkably, in some cases, the quantum advantage cannot be improved using hypothetical postquantum resources. A class of tasks with this property can be singled out using graph theory. Here we report the experimental observation of an impossible-to-beat quantum advantage on a four-dimensional quantum system defined by the polarization and orbital angular momentum of a single photon. The results show pristine evidence of the quantum advantage and are compatible with the maximum advantage allowed using postquantum resources.
Physical Review Letters 03/2012; 108(9):090501. · 7.37 Impact Factor
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ABSTRACT: The present work reports on an extended research endeavor focused on the
theoretical and experimental realization of a macroscopic quantum superposition
(MQS) made up with photons. As it is well known, this intriguing, fundamental
quantum condition is at the core of a famous argument conceived by Erwin
Schroedinger, back in 1935. The main experimental challenge to the actual
realization of this object resides generally on the unavoidable and
uncontrolled interactions with the environment, i.e. the decoherence leading to
the cancellation of any evidence of the quantum features associated with the
macroscopic system. The present scheme is based on a nonlinear process, the
"quantum injected optical parametric amplification", that maps by a linearized
cloning process the quantum coherence of a single - particle state, i.e. a
Micro - qubit, into a Macro - qubit, consisting in a large number M of photons
in quantum superposition. Since the adopted scheme was found resilient to
decoherence, the MQS\ demonstration was carried out experimentally at room
temperature with $M\geq $ $10^{4}$. This result elicited an extended study on
quantum cloning, quantum amplification and quantum decoherence. The related
theory is outlined in the article where several experiments are reviewed such
as the test on the "no-signaling theorem" and the dynamical interaction of the
photon MQS with a Bose-Einstein condensate. In addition, the consideration of
the Micro - Macro entanglement regime is extended into the Macro - Macro
condition. The MQS interference patterns for large M were revealed in the
experiment and the bipartite Micro-Macro entanglement was also demonstrated for
a limited number of generated particles: $M\precsim 12$. At last, the
perspectives opened by this new method are considered in the view of further
studies on quantum foundations and quantum measurement.
02/2012;
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ABSTRACT: In this work we experimentally implement a deterministic transfer of a generic qubit initially encoded in the orbital angular momentum of a single-photon to its polarization. Such a transfer of quantum information, which is completely reversible, has been implemented adopting an electrically tunable q-plate device and a Sagnac interferometer with a Dove prism. The adopted scheme exhibits high fidelity and low losses.
Optics Letters 01/2012; 37(2):172-4. · 3.40 Impact Factor
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ABSTRACT: Quantum walk represents one of the most promising resources for the simulation of physical quantum systems, and has also emerged as an alternative to the standard circuit model for quantum computing. Here we investigate how the particle statistics, either bosonic or fermionic, influences a two-particle discrete quantum walk. Such an experiment has been realized by exploiting polarization entanglement to simulate the bunching-antibunching feature of noninteracting bosons and fermions. To this scope a novel three-dimensional geometry for the waveguide circuit is introduced, which allows accurate polarization independent behavior, maintaining remarkable control on both phase and balancement.
Physical Review Letters 01/2012; 108(1):010502. · 7.37 Impact Factor
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ABSTRACT: Quantum communication employs the counter-intuitive features of quantum physics for tasks that are impossible in the classical world. It is crucial for testing the foundations of quantum theory and promises to revolutionize information and communication technologies. However, to execute even the simplest quantum transmission, one must establish, and maintain, a shared reference frame. This introduces a considerable overhead in resources, particularly if the parties are in motion or rotating relative to each other. Here we experimentally show how to circumvent this problem with the transmission of quantum information encoded in rotationally invariant states of single photons. By developing a complete toolbox for the efficient encoding and decoding of quantum information in such photonic qubits, we demonstrate the feasibility of alignment-free quantum key-distribution, and perform proof-of-principle demonstrations of alignment-free entanglement distribution and Bell-inequality violation. The scheme should find applications in fundamental tests of quantum mechanics and satellite-based quantum communication.
Nature Communications 01/2012; 3:961. · 7.40 Impact Factor
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ABSTRACT: The extraction of information from a quantum system unavoidably implies a modification of the measured system itself. In this framework partial measurements can be carried out in order to extract only a portion of the information encoded in a quantum system, at the cost of inducing a limited amount of disturbance. Here we analyze experimentally the dynamics of sequential partial measurements carried out on a quantum system, focusing on the trade-off between the maximal information extractable and the disturbance. In particular we implement two sequential measurements observing that, by exploiting an adaptive strategy, is possible to find an optimal trade-off between the two quantities.
Scientific Reports 01/2012; 2:443.
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ABSTRACT: Quantum interferometry uses quantum resources to improve phase estimation with respect to classical methods. Here we propose and theoretically investigate a new quantum interferometric scheme based on three-dimensional waveguide devices. These can be implemented by femtosecond laser waveguide writing, recently adopted for quantum applications. In particular, multiarm interferometers include "tritter" and "quarter" as basic elements, corresponding to the generalization of a beam splitter to a 3- and 4-port splitter, respectively. By injecting Fock states in the input ports of such interferometers, fringe patterns characterized by nonclassical visibilities are expected. This enables outperforming the quantum Fisher information obtained with classical fields in phase estimation. We also discuss the possibility of achieving the simultaneous estimation of more than one optical phase. This approach is expected to open new perspectives to quantum enhanced sensing and metrology performed in integrated photonics.
Scientific Reports 01/2012; 2:862.
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ABSTRACT: Quantum cryptographic protocols based on complementarity are nonsecure
against attacks in which complementarity is imitated with classical resources.
The Kochen-Specker (KS) theorem provides protection against these attacks,
without requiring entanglement or spatially separated composite systems. We
analyze the maximum tolerated noise to guarantee the security of a KS-protected
cryptographic scheme against these attacks, and describe a photonic realization
of this scheme using hybrid ququarts defined by the polarization and orbital
angular momentum of single photons.
09/2011;
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ABSTRACT: The sensitivity in optical interferometry is strongly affected by losses
during the signal propagation or at the detection stage. The optimal quantum
states of the probing signals in the presence of loss were recently found.
However, in many cases of practical interest, their associated accuracy is
worse than the one obtainable without employing quantum resources (e.g.
entanglement and squeezing) but neglecting the detector's loss. Here we detail
an experiment that can reach the latter even in the presence of imperfect
detectors: it employs a phase-sensitive amplification of the signals after the
phase sensing, before the detection. We experimentally demonstrated the
feasibility of a phase estimation experiment able to reach its optimal working
regime. Since our method uses coherent states as input signals, it is a
practical technique that can be used for high-sensitivity interferometry and,
in contrast to the optimal strategies, does not require one to have an exact
characterization of the loss beforehand.
07/2011;
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ABSTRACT: Quantum walk represents one of the most promising resources for the
simulation of physical quantum systems, and has also emerged as an alternative
to the standard circuit model for quantum computing. Up to now the experimental
implementations have been restricted to single particle quantum walk, while
very recently the quantum walks of two identical photons have been reported.
Here, for the first time, we investigate how the particle statistics, either
bosonic or fermionic, influences a two-particle discrete quantum walk. Such
experiment has been realized by adopting two-photon entangled states and
integrated photonic circuits. The polarization entanglement was exploited to
simulate the bunching-antibunching feature of non interacting bosons and
fermions. To this scope a novel three-dimensional geometry for the waveguide
circuit is introduced, which allows accurate polarization independent
behaviour, maintaining a remarkable control on both phase and balancement.
06/2011;
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ABSTRACT: The orbital angular momentum of light (OAM) provides a promising approach for
the implementation of multidimensional states (qudits) for quantum information
purposes. In order to characterize the degradation undergone by the information
content of qubits encoded in a bidimensional subspace of the orbital angular
momentum degree of freedom of photons, we study how the state fidelity is
affected by a transverse obstruction placed along the propagation direction of
the light beam. Emphasis is placed on the effects of planar and radial
hard-edged aperture functions on the state fidelity of Laguerre-Gaussian
transverse modes and the entanglement properties of polarization-OAM
hybrid-entangled photon pairs.
05/2011;
