Archive ouverte UNIGE | last documents for author 'Denis Rosset'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Denis Rosset'engReceiver-device-independent quantum key distribution protocolshttps://archive-ouverte.unige.ch/unige:165554https://archive-ouverte.unige.ch/unige:165554We discuss quantum key distribution protocols and their security analysis, considering a receiver-device-independent (RDI) model. The sender’s (Alice’s) device is partially characterized, in the sense that we assume bounds on the overlaps of the prepared quantum states. The receiver’s (Bob’s) device requires no characterisation and can be represented as a black-box. Our protocols are therefore robust to any attack on Bob, such as blinding attacks. In particular, we show that a secret key can be established even when the quantum channel has arbitrarily low transmission by considering RDI protocols exploiting sufficiently many states. Finally, we discuss how the hypothesis of bounded overlaps can be naturally applied to practical devices.Wed, 07 Dec 2022 16:32:24 +0100Universal bound on the cardinality of local hidden variables in networkshttps://archive-ouverte.unige.ch/unige:131901https://archive-ouverte.unige.ch/unige:131901We present an algebraic description of the sets of local correlations in arbitrary networks, when the parties have finite inputs and outputs. We consider networks generalizing the usual Bell scenarios by the presence of multiple uncorrelated sources. We prove a finite upper bound on the cardinality of the value sets of the local hidden variables. Conse- quently, we fond that the sets of local correlations are connected, closed and semialgebraic, and bounded by tight polynomial Bell-like inequalities.Mon, 09 Mar 2020 13:37:24 +0100Self-testing quantum states and measurements in the prepare-and-measure scenariohttps://archive-ouverte.unige.ch/unige:127273https://archive-ouverte.unige.ch/unige:127273The goal of self-testing is to characterize an a priori unknown quantum system based solely on measurement statistics, i.e. using an uncharacterized measurement device. Here we develop self-testing methods for quantum prepare-and-measure experiments, thus not necessarily relying on entanglement and/or violation of a Bell inequality. We present noise-robust techniques for self-testing sets of quantum states and measurements, assuming an upper bound on the Hilbert space dimension. We discuss in detail the case of a 2→1 random access code with qubits, for which we provide analytically optimal self-tests. The simplicity and noise robustness of our methods should make them directly applicable to experiments.Tue, 03 Dec 2019 14:07:01 +0100On the inequivalence of the CH and CHSH inequalities due to finite statisticshttps://archive-ouverte.unige.ch/unige:94565https://archive-ouverte.unige.ch/unige:94565Different variants of a Bell inequality, such as CHSH and CH, are known to be equivalent when evaluated on nonsignaling outcome probability distributions. However, in experimental setups, the outcome probability distributions are estimated using a finite number of samples. Therefore the nonsignaling conditions are only approximately satisfied and the robustness of the violation depends on the chosen inequality variant. We explain that phenomenon using the decomposition of the space of outcome probability distributions under the action of the symmetry group of the scenario, and propose a method to optimize the statistical robustness of a Bell inequality. In the process, we describe the finite group composed of relabeling of parties, measurement settings and outcomes, and identify correspondences between the irreducible representations of this group and properties of outcome probability distributions such as normalization, signaling or having uniform marginals.Wed, 31 May 2017 14:30:44 +0200Resource-Efficient Measurement-Device-Independent Entanglement Witnesshttps://archive-ouverte.unige.ch/unige:83960https://archive-ouverte.unige.ch/unige:83960Imperfections in experimental measurement schemes can lead to falsely identifying, or over estimating, entanglement in a quantum system. A recent solution to this is to define schemes that are robust to measurement imperfections - measurement device independent entanglement witness (MDI-EW). Here we introduce a novel approach for MDI-EW, which significantly reduces the experimental complexity and is applicable to a wide range of physical systems. The scheme requires no prior description of the state, is detection loop-hole free, robust to classical communication, and works for all entangled qubit states. We focus on photonic entanglement, experimentally generating and testing bipartite Werner states, varying the entanglement from the maximally entangled Bell state, past the bound for nonlocal states and down to the separable bound of 1/3. We witness entanglement down to an entangled state fraction close to 0.4. These results could be of particular interest for device independent quantum random number generation.Mon, 30 May 2016 11:15:37 +0200Characterization of correlations in quantum networkshttps://archive-ouverte.unige.ch/unige:77401https://archive-ouverte.unige.ch/unige:77401Les derniers développements en physique quantique ont permis l'établissement de réseaux quantiques sécurisés composés de plusieurs sources et observateurs. D'étonnantes corrélations sont observées dans ces réseaux; jusqu'à récemment, la description théorique qui en était faite ne tenait pas compte de la topologie particulière des réseaux considérés. Le sujet principal de cette thèse est l'étude de ces corrélations selon des topologies variées. La thèse comporte également d'autres pistes de recherche, telles que le classement des inégalités de Bell et la caractérisation des systèmes quantiques.Wed, 18 Nov 2015 11:55:52 +0100All entangled states display some hidden nonlocalityhttps://archive-ouverte.unige.ch/unige:36550https://archive-ouverte.unige.ch/unige:36550A well-known manifestation of quantum entanglement is that it may lead to correlations that are inexplicable within the framework of a locally causal theory—a fact that is demonstrated by the quantum violation of Bell inequalities. The precise relationship between quantum entanglement and the violation of Bell inequalities is, however, not well understood. While it is known that entanglement is necessary for such a violation, it is not clear whether all entangled states violate a Bell inequality, even in the scenario where one allows joint operations on multiple copies of the state and local filtering operations before the Bell experiment. In this paper we show that all entangled states, or more precisely, all not-fully-separable states of arbitrary Hilbert space dimension and arbitrary number of parties, violate a Bell inequality when combined with another state which on its own cannot violate the same Bell inequality. This result shows that quantum entanglement and quantum nonlocality are in some sense equivalent, thus giving an affirmative answer to the aforementioned open question. It follows from our result that two entangled states that are apparently useless in demonstrating quantum nonlocality via a specific Bell inequality can be combined to give a Bell violation of the same inequality. Explicit examples of such activation phenomenon are provided.Tue, 13 May 2014 11:08:52 +0200Measurement-Device-Independent Entanglement Witnesses for All Entangled Quantum Stateshttps://archive-ouverte.unige.ch/unige:36535https://archive-ouverte.unige.ch/unige:36535The problem of demonstrating entanglement is central to quantum information processing applications. Resorting to standard entanglement witnesses requires one to perfectly trust the implementation of the measurements to be performed on the entangled state, which may be an unjustified assumption. Inspired by the recent work of F. Buscemi [Phys. Rev. Lett. 108, 200401 (2012)], we introduce the concept of measurement-device-independent entanglement witnesses (MDI-EWs), which allow one to demonstrate entanglement of all entangled quantum states with untrusted measurement apparatuses. We show how to systematically obtain such MDI-EWs from standard entanglement witnesses. Our construction leads to MDI-EWs that are loss tolerant and can be implemented with current technology.Mon, 12 May 2014 16:58:45 +0200Imperfect measurement settings: Implications for quantum state tomography and entanglement witnesseshttps://archive-ouverte.unige.ch/unige:36532https://archive-ouverte.unige.ch/unige:36532Reliable and well-characterized quantum resources are indispensable ingredients in quantum information processing. Typically, in a realistic characterization of these resources, apparatuses come with intrinsic uncertainties that can manifest themselves in the form of systematic errors. While systematic errors are generally accounted for through careful calibration, the effect of remaining imperfections on the characterization of quantum resources has been largely overlooked in the literature. In this paper, we investigate the effect of systematic errors that arise from imperfect alignment of measurement bases—an error that can conceivably take place due to the limited controllability of measurement devices. We show that characterization of quantum resources using quantum state tomography or entanglement witnesses can be undermined with an amount of such imprecision that is not uncommon in laboratories. Curiously, for quantum state tomography, we find that having entanglement can help to reduce the susceptibility to this kind of error. We also briefly discuss how a given entanglement witness can be modified to incorporate the effect of such errors.Mon, 12 May 2014 16:57:32 +0200Entangled states cannot be classically simulated in generalized Bell experiments with quantum inputshttps://archive-ouverte.unige.ch/unige:36524https://archive-ouverte.unige.ch/unige:36524Simulation tasks are insightful tools to compare information-theoretic resources. Considering a generalization of usual Bell scenarios where external quantum inputs are provided to the parties, we show that any entangled quantum state exhibits correlations that cannot be simulated using only shared randomness and classical communication, even when the amount and rounds of classical communication involved are unrestricted. We indeed construct explicit Bell-like inequalities that are necessarily satisfied by such classical resources but nevertheless violated by correlations obtainable from entangled quantum states, when measured a single copy at a timeMon, 12 May 2014 16:18:21 +0200Classical Simulation of Entanglement Swapping with Bounded Communicationhttps://archive-ouverte.unige.ch/unige:22850https://archive-ouverte.unige.ch/unige:22850Entanglement appears under two different forms in quantum theory, namely, as a property of states of joint systems and as a property of measurement eigenstates in joint measurements. By combining these two aspects of entanglement, it is possible to generate nonlocality between particles that never interacted, using the protocol of entanglement swapping. We show that even in the more constraining bilocal scenario where distant sources of particles are assumed to be independent, i.e., to share no prior randomness, entanglement swapping can be simulated classically with bounded communication, using only 9 bits in total. Our result thus provides an upper bound on the nonlocality of the entanglement swapping process.Wed, 12 Sep 2012 11:43:12 +0200Bilocal versus nonbilocal correlations in entanglement-swapping experimentshttps://archive-ouverte.unige.ch/unige:22788https://archive-ouverte.unige.ch/unige:22788Entanglement swapping is a process by which two initially independent quantum systems can become entangled and generate nonlocal correlations. To characterize such correlations, we compare them to those predicted by bilocal models, where systems that are initially independent are described by uncorrelated states. We extend in this paper the analysis of bilocal correlations initiated in [ Phys. Rev. Lett. 104 170401 (2010)]. In particular, we derive new Bell-type inequalities based on the bilocality assumption in different scenarios, we study their possible quantum violations, and we analyze their resistance to experimental imperfections. The bilocality assumption, being stronger than Bell's standard local causality assumption, lowers the requirements for the demonstration of quantumness in entanglement-swapping experiments.Mon, 10 Sep 2012 12:26:31 +0200Communication cost of simulating entanglement swappinghttps://archive-ouverte.unige.ch/unige:16050https://archive-ouverte.unige.ch/unige:16050Entanglement appears in two different ways in quantum mechanics, namely as a property of states and as a property of measurement outcomes in joint measurements. By combining these two aspects of entanglement, it is possible to generate nonlocality between particles that never interacted, using the protocol of entanglement swapping. We investigate the communication cost of classically simulating this process. While the communication cost of simulating nonlocal correlations of entangled states appears to be generally quite low, we show here that infinite communication is required to simulate entanglement swapping. This result is derived in the scenario of bilocality, where distant sources of particles are assumed to be independent, and takes advantage of a previous result of Massar et al. [Phys. Rev. A 63, 052305 (2001)]. Our result implies that any classical model simulating entanglement swapping must either assume that (i) infinite shared randomness is available between any two locations in the universe, or that (ii) infinite communication takes place.Fri, 03 Jun 2011 15:06:54 +0200