Publications

Proofs of Network Quantum Nonlocality in Continuous Families of Distributions

The study of nonlocality in scenarios that depart from the bipartite Einstein-Podolsky-Rosen setup is allowing one to uncover many fundamental features of quantum mechanics. Recently, an approach to building network-local models based on machine learning led to the conjecture that the family of quantum triangle distributions of [Renou et al., Phys. Rev. Lett. 123, 140401 (2019)] did not admit triangle-local models in a larger range than the original proof. We prove part of this conjecture in the affirmative. Our approach consists of reducing the family of original, four-outcome distributions to families of binary-outcome ones, and then using the inflation technique to prove that these families of binary-outcome distributions do not admit triangle-local models. This constitutes the first successful use of inflation in a proof of quantum nonlocality in networks for distributions whose nonlocality could not be proved with alternative methods. Moreover, we provide a method to extend proofs of network nonlocality in concrete distributions of a parametrized family to continuous ranges of the parameter. In the process, we produce a large collection of network Bell inequalities for the triangle scenario with binary outcomes, which are of independent interest.Received 9 April 2022Revised 13 October 2022Accepted 18 January 2023DOI:https://doi.org/10.1103/PhysRevLett.130.090201© 2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNonlocalityQuantum correlations in quantum informationQuantum information theoryQuantum networksQuantum Information, Science & TechnologyNetworks

Measurement of the polarization coupling length in telecom fibers using nonlinear polarization rotation

The polarization coupling length, an important parameter of the PMD probability distribution, is obtained from measurements and modeling of the nonlinear polarization rotation in optical fibers. Results for different types of fibers are presented.

Nonlocal boxes for networks

Nonlocal boxes are conceptual tools that capture the essence of the phenomenon of quantum non-locality, central to modern quantum theory and quantum technologies. We introduce network nonlocal boxes tailored for quantum networks under the natural assumption that these networks connect independent sources and do not allow signaling. Hence, these boxes satisfy the No-Signaling and Independence (NSI) principle. For the case of boxes without inputs, connecting pairs of bipartite sources and producing binary outputs, we prove that the sources and boxes producing local random outputs and maximal 2-box correlations, i.e. $E_2=\sqrt{2}-1$, $E_2^o=1$, are essentially unique.

Quantum Communication: Quantum Communication is the art of transferring a quantum state from one location, Alice, to a distant one, Bob

Quantum communication Quantum Communication is the art of transferring a quantum state from one location, Alice, to a distant one, Bob. Ultimately, information is carried by the physical state of some quantum systems

Quantum nonlocality: How does Nature perform the trick?\cite{Bellprize}

Since our early childhood we know in our bones that in order to interact with an object we have either to go to it or to throw something at it. Yet, contrary to all our daily experience, Nature is nonlocal: there are spatially separated systems that exhibit nonlocal correlations. In recent years this led to new experiments, deeper understanding of the tension between quantum physics and relativity and to proposals for disruptive technologies.

Witnessing single-photon entanglement with local homodyne measurements

Single-photon entangled states constitute the simplest form of entanglement, yet they provide a valuable resource in quantum information sciences. Specifically, they lie at the heart of quantum networks, as they can be used for quantum teleportation, swapped and purified with linear optics. The main drawback of such resource is the difficulty in measuring it. Here, we present and experimentally test the first operational witness suited for single-photon entanglement which relies only on local homodyning and does not require post-selection. Our results highlight the potential of hybrid methods, where discrete entanglement is characterized through continuous-variable measurements, in verifying the proper functioning of future quantum networks.

Optimal eavesdropping in quantum cryptography. I. Information bound and optimal strategy

We consider the Bennett-Brassard cryptographic scheme, which uses two conjugate quantum bases. An eavesdropper who attempts to obtain information on qubits sent in one of the bases causes a disturbance to qubits sent in the other basis. We derive an upper bound to the accessible information in one basis, for a given error rate in the conjugate basis. Independently fixing the error rates in the conjugate bases, we show that both bounds can be attained simultaneously by an optimal eavesdropping probe. The probe interaction and its subsequent measurement are described explicitly. These results are combined to give an expression for the optimal information an eavesdropper can obtain for a given average disturbance when her interaction and measurements are performed signal by signal. Finally, the relation between quantum cryptography and violations of Bell's inequalities is discussed.

Quantum cryptography

Advisory Board

No abstract is provided for this article.

Second order polarization mode dispersion

No abstract is provided for this article.

The quantum-state diffusion model applied to open systems

A model of a quantum system interacting with its environment is proposed in which the system is represented by a state vector that satisfies a stochastic differential equation, derived from a density operator equation such as the Bloch equation, and consistent with it. The advantages of the numerical solution of these equations over the direct numerical solution of the density operator equations are described. The method is applied to the nonlinear absorber, cascades of quantum transitions, second-harmonic generation and a measurement reduction process. The model provides graphic illustrations of these processes, with statistical fluctuations that mimic those of experiments. The stochastic differential equations originated from studies of the measurement problem in the foundations of quantum mechanics. The model is compared with the quantum-jump model of Dalibard (1992), Carmichael and others, which originated among experimenters looking for intuitive pictures and rules of computation.

Quantum Chance: Nonlocality, Teleportation and Other Quantum Marvels

Quantum physics, which offers an explanation of the world onthe smallestscale, has fundamental implications that pose a serious challenge to ordinary logic. Particularly counterintuitive is the notion of entanglement, which has been explored for the past 30 years and posits an ubiquitous randomness capable of manifesting itself simultaneously in more than one place. This amazing 'non-locality' is more than just an abstractcuriosity or paradox: it has entirely down-to-earth applications in cryptography, serving for exampleto protect financial information; it also has enabled the demonstration of 'quantum teleportation', whose infinite possibilities even science-fiction writers can scarcely imagine. This delightful and conciseexposition does notavoid thedeep logical difficulties of quantum physics, but gives the reader the insights needed to appreciate them. From 'Bell's Theorem' to experiments in quantum entanglement, the reader will gain asolid understanding of one of the most fascinating areas of contemporary physics.

Potentiality realism: A realistic and indeterministic physics based on propensities

We propose an interpretation of physics named potentiality realism. This view, which can be applied to classical as well as to quantum physics, regards potentialities (i.e. intrinsic, objective propensities for individual events to obtain) as elements of reality, thereby complementing the actual properties taken by physical variables. This allows one to naturally reconcile realism and fundamental indeterminism in any theoretical framework. We discuss our specific interpretation of propensities, that require them to depart from being probabilities at the formal level, though allowing for statistics and the law of large numbers. This view helps reconcile classical and quantum physics by showing that most of the conceptual problems that are customarily taken to be unique issues of the latter -- such as the measurement problem -- are actually in common to all indeterministic physical theories.

Tight finite-key analysis for quantum cryptography

Despite enormous theoretical and experimental progress in quantum cryptography, the security of most current implementations of quantum key distribution is still not rigorously established. One significant problem is that the security of the final key strongly depends on the number, M, of signals exchanged between the legitimate parties. Yet, existing security proofs are often only valid asymptotically, for unrealistically large values of M. Another challenge is that most security proofs are very sensitive to small differences between the physical devices used by the protocol and the theoretical model used to describe them. Here we show that these gaps between theory and experiment can be simultaneously overcome by using a recently developed proof technique based on the uncertainty relation for smooth entropies. Although they offer significant promise, practical implementations of quantum key distribution are often not as rigorous as theory predicts. This study demonstrates how two instances of such discrepancies can be resolved by taking advantage of an enotropic formulation of the uncertainty principle.

Stochastic quantum dynamics and relativity

Our aim is to unify the Schroedinger dynamics and the projection postulate. We first prove that the Schroedinger evolution is the only quantum evolution that is deterministic and compatible with relativity. Next we present a non deterministic generalization of the Schroedinger equation compatible with relativity. This time continuous pure state valued stochastic process covers the whole class of quantum dynamical semigroups, as exemplified by the damped harmonic oscillator and spin relaxation. Finally a symmetry preserving generalization of the Ghirardi-Rimini-Weber model is presented. (orig.).

Quantum communications with optical fibers

No abstract is provided for this article.

Spectral decomposition of Bell's operators for qubits

The spectral decomposition is given for the N-qubit Bell operators with two observables per qubit. It is found that the eigenstates (when non-degenerate) are N-qubit GHZ states even for those operators that do not allow the maximal violation of the corresponding inequality. We present two applications of this analysis. In particular, we discuss the existence of pure entangled states that do not violate the Mermin-Klyshko inequality for N ⩾3.

Creating high dimensional time-bin entanglement using mode-locked lasers

We present a new scheme to generate high dimensional entanglement between two photonic systems. The idea is based on parametric down conversion with a sequence of pump pulses generated by a mode-locked laser. We prove experimentally the feasibility of this scheme by performing a Franson-type Bell test using a 2-way interferometer with path-length difference equal to the distance between 2 pump pulses. With this experiment, we can demonstrate entanglement for a two-photon state of at least dimension D=11. Finally, we propose a feasible experiment to show a Fabry-Perot like effect for a high dimensional two-photon state.