Archive ouverte UNIGE | last documents for author 'Flavien Hirsch'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Flavien Hirsch'engCovariance Bell inequalitieshttps://archive-ouverte.unige.ch/unige:127285https://archive-ouverte.unige.ch/unige:127285We introduce Bell inequalities based on covariance, one of the most common measures of correlation. Explicit examples are discussed, and violations in quantum theory are demonstrated. A crucial feature of these covariance Bell inequalities is their nonlinearity; this has nontrivial consequences for the derivation of their local bound, which is not reached by deterministic local correlations. For our simplest inequality, we derive analytically tight bounds for both local and quantum correlations. An interesting application of covariance Bell inequalities is that they can act as "shared randomness witnesses": specifically, the value of the Bell expression gives device-independent lower bounds on both the dimension and the entropy of the shared random variable in a local model.Tue, 03 Dec 2019 14:44:28 +0100Unbounded sequence of observers exhibiting Einstein-Podolsky-Rosen steeringhttps://archive-ouverte.unige.ch/unige:127284https://archive-ouverte.unige.ch/unige:127284A sequential steering scenario is investigated, where multiple Bobs aim at demonstrating steering using successively the same half of an entangled quantum state. With isotropic entangled states of local dimension d, the number of Bobs that can steer Alice is found to be NBob∼d/logd, thus leading to an arbitrary large number of successive instances of steering with independently chosen and unbiased inputs. This scaling is achieved when considering a general class of measurements along orthonormal bases, as well as complete sets of mutually unbiased bases. Finally, we show that similar results can be obtained in an anonymous sequential scenario, where none of the Bobs know their position in the sequence.Tue, 03 Dec 2019 14:42:05 +0100Algorithmic construction of local models for entangled quantum states: Optimization for two-qubit stateshttps://archive-ouverte.unige.ch/unige:127275https://archive-ouverte.unige.ch/unige:127275The correlations of certain entangled quantum states can be fully reproduced via a local model. We discuss in detail the practical implementation of an algorithm for constructing local models for entangled states, recently introduced by Hirsch et al. [Phys. Rev. Lett. 117, 190402 (2016)] and Cavalcanti et al. [Phys. Rev. Lett. 117, 190401 (2016)]. The method allows one to construct both local hidden state (LHS) and local hidden variable (LHV) models, and can be applied to arbitrary entangled states in principle. Here we develop an improved implementation of the algorithm, discussing the optimization of the free parameters. For the case of two-qubit states, we design a ready-to-use optimized procedure. This allows us to construct LHS models (for projective measurements) that are almost optimal, as we show for Bell diagonal states, for which the optimal model has recently been derived. Finally, we show how to construct fully analytical local models, based on the output of the convex optimization procedure.Tue, 03 Dec 2019 14:16:51 +0100Quantum measurement incompatibility does not imply Bell nonlocalityhttps://archive-ouverte.unige.ch/unige:127267https://archive-ouverte.unige.ch/unige:127267We discuss the connection between the incompatibility of quantum measurements, as captured by the notion of joint measurability, and the violation of Bell inequalities. Specifically, we present explicitly a given a set of non jointly measurable POVMs MA with the following property. Considering a bipartite Bell test where Alice uses MA, then for any possible shared entangled state ρ and any set of (possibly infinitely many) POVMs NB performed by Bob, the resulting statistics admits a local model, and can thus never violate any Bell inequality. This shows that quantum measurement incompatibility does not imply Bell nonlocality in general.Tue, 03 Dec 2019 13:42:01 +0100Algorithmic Construction of Local Hidden Variable Models for Entangled Quantum Stateshttps://archive-ouverte.unige.ch/unige:90701https://archive-ouverte.unige.ch/unige:90701Constructing local hidden variable (LHV) models for entangled quantum states is a fundamental problem, with implications for the foundations of quantum theory and for quantum information processing. It is, however, a challenging problem, as the model should reproduce quantum predictions for all possible local measurements. Here we present a simple method for building LHV models, applicable to any entangled state and considering continuous sets of measurements. This leads to a sequence of tests which, in the limit, fully captures the set of quantum states admitting a LHV model. Similar methods are developed for local hidden state models. We illustrate the practical relevance of these methods with several examples.Wed, 21 Dec 2016 15:16:37 +0100Entanglement without hidden nonlocalityhttps://archive-ouverte.unige.ch/unige:90700https://archive-ouverte.unige.ch/unige:90700We consider Bell tests in which the distant observers can perform local filtering before testing a Bell inequality. Notably, in this setup, certain entangled states admitting a local hidden variable model in the standard Bell scenario can nevertheless violate a Bell inequality after filtering, displaying so-called hidden nonlocality. Here we ask whether all entangled states can violate a Bell inequality after well-chosen local filtering. We answer this question in the negative by showing that there exist entangled states without hidden nonlocality. Specifically, we prove that some two-qubit Werner states still admit a local hidden variable model after any possible local filtering on a single copy of the state.Wed, 21 Dec 2016 15:15:27 +0100Incompatible quantum measurements admitting a local hidden variable modelhttps://archive-ouverte.unige.ch/unige:84467https://archive-ouverte.unige.ch/unige:84467The observation of quantum nonlocality, i.e. quantum correlations violating a Bell inequality, implies the use of incompatible local quantum measurements. Here we consider the converse question. That is, can any set of incompatible measurements be used in order to demonstrate Bell inequality violation? Our main result is to construct a local hidden variable model for an incompatible set of qubit measurements. Specifically, we show that if Alice uses this set of measurements, then for any possible shared entangled state, and any possible dichotomic measurements performed by Bob, the resulting statistics are local. This represents significant progress towards proving that measurement incompatibility does not imply Bell nonlocality in general.Mon, 13 Jun 2016 11:17:26 +0200Sufficient criterion for guaranteeing that a two-qubit state is unsteerablehttps://archive-ouverte.unige.ch/unige:84465https://archive-ouverte.unige.ch/unige:84465Quantum steering can be detected via the violation of steering inequalities, which provide sufficient conditions for the steerability of quantum states. Here we discuss the converse problem, namely ensuring that a state is unsteerable, and hence Bell local. We present a simple criterion, applicable to any two-qubit state, which guarantees that the state admits a local hidden state model for arbitrary projective measurements. We find new classes of unsteerable entangled states, which can thus not violate any steering or Bell inequality. In turn, this leads to sufficient conditions for a state to be only one-way steerable, and provides the simplest possible example of one-way steering. Finally, by exploiting the connection between steering and measurement incompatibility, we give a sufficient criterion for a continuous set of qubit measurements to be jointly measurable.Mon, 13 Jun 2016 11:14:38 +0200Local hidden variable models for entangled quantum states using finite shared randomnesshttps://archive-ouverte.unige.ch/unige:84461https://archive-ouverte.unige.ch/unige:84461The statistics of local measurements performed on certain entangled states can be reproduced using a local hidden variable (LHV) model. While all known models make use of an infinite amount of shared randomness---the physical relevance of which is questionable---we show that essentially all entangled states admitting a LHV model can be simulated with finite shared randomness. Our most economical model simulates noisy two-qubit Werner states using only 3.58 bits of shared randomness. We also discuss the case of POVMs, and the simulation of nonlocal states with finite shared randomness and finite communication. Our work represents a first step towards quantifying the cost of LHV models for entangled quantum states.Mon, 13 Jun 2016 11:10:49 +0200Genuinely Multipartite Entangled Quantum States with Fully Local Hidden Variable Models and Hidden Multipartite Nonlocalityhttps://archive-ouverte.unige.ch/unige:83549https://archive-ouverte.unige.ch/unige:83549The relation between entanglement and nonlocality is discussed in the case of multipartite quantum systems. We show that, for any number of parties, there exist genuinely multipartite entangled states that admit a fully local hidden variable model, i.e., where all parties are separated. Hence, although these states exhibit the strongest form of multipartite entanglement, they cannot lead to Bell inequality violation considering general nonsequential local measurements. Then, we show that the nonlocality of these states can nevertheless be activated using sequences of local measurements, thus revealing genuine multipartite hidden nonlocality.Mon, 09 May 2016 09:37:58 +0200Genuine Hidden Quantum Nonlocalityhttps://archive-ouverte.unige.ch/unige:36410https://archive-ouverte.unige.ch/unige:36410The nonlocality of certain quantum states can be revealed by using local filters before performing a standard Bell test. This phenomenon, known as hidden nonlocality, has been so far demonstrated only for a restricted class of measurements, namely projective measurements. Here we prove the existence of genuine hidden nonlocality. Specifically, we present a class of two-qubit entangled states, for which we construct a local model for the most general local measurements (POVMs), and show that the states violate a Bell inequality after local filtering. Hence there exist entangled states, the nonlocality of which can be revealed only by using a sequence of measurements. Finally, we show that genuine hidden nonlocality can be maximal. There exist entangled states for which a sequence of measurements can lead to maximal violation of a Bell inequality, while the statistics of non-sequential measurements is always local.Tue, 06 May 2014 16:01:34 +0200