Archive ouverte UNIGE | last documents for author 'Valerio Scarani'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Valerio Scarani'engExtremal correlations of the tripartite no-signaling polytopehttps://archive-ouverte.unige.ch/unige:47359https://archive-ouverte.unige.ch/unige:47359The no-signaling polytope associated with a Bell scenario with three parties, two inputs, and two outputs, is found to have 53 856 extremal points, belonging to 46 inequivalent classes. We provide a classification of these points according to various definitions of multipartite nonlocality and briefly discuss other issues such as the interconversion between extremal points seen as a resource and the relation of the extremal points to Bell-type inequalities.Fri, 27 Feb 2015 14:38:48 +0100Nonlocality of cluster states of qubitshttps://archive-ouverte.unige.ch/unige:47355https://archive-ouverte.unige.ch/unige:47355We investigate cluster states of qubits with respect to their nonlocal properties. We demonstrate that a Greenberger-Horne-Zeilinger (GHZ) argument holds for any cluster state: more precisely, it holds for any partial, thence mixed, state of a small number of connected qubits (five, in the case of one-dimensional lattices). In addition, we derive a Bell inequality that is maximally violated by the four-qubit cluster state and is not violated by the four-qubit GHZ state.Fri, 27 Feb 2015 14:37:14 +0100Violation of Bell s inequalities and distillability for N qubitshttps://archive-ouverte.unige.ch/unige:47353https://archive-ouverte.unige.ch/unige:47353We consider quantum systems composed of N qubits, and the family of all Bell's correlation inequalities for two two-valued measurements per site. We show that if an N-qubit state ρ violates any of these inequalities, then it is at least bipartite distillable. Indeed there exists a link between the amount of Bell's inequality violation and the degree of distillability. Thus, we strengthen the interpretation of Bell's inequalities as detectors of useful entanglement.Fri, 27 Feb 2015 14:36:21 +0100Bell's inequalities and distillability in N-quantum-bit systemshttps://archive-ouverte.unige.ch/unige:47351https://archive-ouverte.unige.ch/unige:47351The relation between Bell inequalities with two two-outcome measurements per site and distillability is analyzed in systems of an arbitrary number of quantum bits. We observe that the violation of any of these inequalities by a quantum state implies that pure-state entanglement can be distilled from it. The corresponding distillation protocol may require that some of the parties join into several groups. We show that there exists a link between the amount of the Bell inequality violation and the size of the groups they have to form for distillation. Thus, a strong violation is always sufficient for full N-partite distillability. This result also allows for a security proof of multi-partite quantum key distribution (QKD) protocols.Fri, 27 Feb 2015 14:35:27 +0100Strong Constraints on Models that Explain the Violation of Bell Inequalities with Hidden Superluminal Influenceshttps://archive-ouverte.unige.ch/unige:40669https://archive-ouverte.unige.ch/unige:40669We discuss models that attempt to provide an explanation for the violation of Bell inequalities at a distance in terms of hidden influences. These models reproduce the quantum correlations in most situations, but are restricted to produce local correlations in some configurations. The argument presented in (Bancal et al. Nat Phys 8:867, 2012) applies to all of these models, which can thus be proved to allow for faster-than-light communication. In other words, the signalling character of these models cannot remain hidden.Fri, 03 Oct 2014 16:23:09 +0200The speed of quantum information and the preferred frame: analysis of experimental datahttps://archive-ouverte.unige.ch/unige:37030https://archive-ouverte.unige.ch/unige:37030The results of EPR experiments performed in Geneva are analyzed in the frame of the cosmic microwave background radiation, generally considered as a good candidate for playing the role of preferred frame. We set a lower bound for the speed of quantum information in this frame at 1.5 x 10^4 cMon, 02 Jun 2014 11:42:10 +0200Optical tests of quantum nonlocality: from EPR-Bell tests towards experiments with moving observershttps://archive-ouverte.unige.ch/unige:37029https://archive-ouverte.unige.ch/unige:37029Past, present and future experimental tests of quantum nonlocality are discussed. Consequences of assuming that the state-vector collapse is a real physical phenomenon in space-time are developed. These lead to experiments feasible with today's technology.Mon, 02 Jun 2014 11:41:40 +0200Quantum Communication between N Partners and Bell's Inequalitieshttps://archive-ouverte.unige.ch/unige:37021https://archive-ouverte.unige.ch/unige:37021We consider a family of quantum communication protocols involving N partners. We demonstrate the existence of a link between the security of these protocols against individual attacks by the eavesdropper, and the violation of some Bell's inequalities, generalizing the link that was noticed some years ago for two-partners quantum cryptography. The arguments are independent of the local hidden variable debate.Mon, 02 Jun 2014 09:28:08 +0200Spectral decomposition of Bell's operators for qubitshttps://archive-ouverte.unige.ch/unige:37020https://archive-ouverte.unige.ch/unige:37020The 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 ≥3Mon, 02 Jun 2014 09:26:43 +0200Thermalizing Quantum Machines: Dissipation and Entanglementhttps://archive-ouverte.unige.ch/unige:36816https://archive-ouverte.unige.ch/unige:36816We study the relaxation of a quantum system towards the thermal equilibrium using tools developed within the context of quantum information theory. We consider a model in which the system is a qubit, and reaches equilibrium after several successive two-qubit interactions (thermalizing machines) with qubits of a reservoir. We characterize completely the family of thermalizing machines. The model shows a tight link between dissipation, fluctuations, and the maximal entanglement that can be generated by the machines. The interplay of quantum and classical information processes that give rise to practical irreversibility is discussed.Tue, 20 May 2014 16:26:42 +0200Quantum key distribution between N partners: Optimal eavesdropping and Bell's inequalitieshttps://archive-ouverte.unige.ch/unige:36813https://archive-ouverte.unige.ch/unige:36813Quantum secret-sharing protocols involving N partners (NQSS) are key distribution protocols in which Alice encodes her key into N−1 qubits, in such a way that all the other partners must cooperate in order to retrieve the key. On these protocols, several eavesdropping scenarios are possible: some partners may want to reconstruct the key without the help of the other ones, and consequently collaborate with an Eve that eavesdrops on the other partners' channels. For each of these scenarios, we give the optimal individual attack that the Eve can perform. In case of such an optimal attack, the authorized partners have a higher information on the key than the unauthorized ones if and only if they can violate a Bell's inequality.Tue, 20 May 2014 16:24:21 +0200Quantum Cloning with an Optical Fiber Amplifierhttps://archive-ouverte.unige.ch/unige:36795https://archive-ouverte.unige.ch/unige:36795It has been shown theoretically that a light amplifier working on the physical principle of stimulated emission should achieve optimal quantum cloning of the polarization state of light. We demonstrate close-to-optimal universal quantum cloning of polarization in a standard fiber amplifier for telecom wavelengths. For cloning 1→2 we find a fidelity of 0.82, the optimal value being 5/6=0.83.Tue, 20 May 2014 15:40:41 +0200Bell-Type Inequalities to Detect True n-Body Nonseparabilityhttps://archive-ouverte.unige.ch/unige:36788https://archive-ouverte.unige.ch/unige:36788We analyze the structure of correlations among more than two quantum systems. We introduce a classification of correlations based on the concept of nonseparability, which is different a priori from the concept of entanglement. Generalizing a result of Svetlichny [Phys. Rev. D 35, 3066 (1987)] on threeparticle correlations, we find an inequality for n-particle correlations that holds under the most general separability condition and that is violated by some quantum-mechanical statesTue, 20 May 2014 15:35:53 +0200Diluting quantum information: An analysis of information transfer in system-reservoir interactionshttps://archive-ouverte.unige.ch/unige:36787https://archive-ouverte.unige.ch/unige:36787We design a universal quantum homogenizer, which is a quantum machine that takes as an input a system qubit initially in the state rho and a set of N reservoir qubits initially prepared in the same state xi. In the homogenizer the system qubit sequentially interacts with the reservoir qubits via the partial swap transformation. The homogenizer realizes, in the limit sense, the transformation such that at the output each qubit is in an arbitrarily small neighborhood of the state xi irrespective of the initial states of the system and the reservoir qubits. This means that the system qubit undergoes an evolution that has a fixed point, which is the reservoir state xi. We also study approximate homogenization when the reservoir is composed of a finite set of identically prepared qubits. The homogenizer allows us to understand various aspects of the dynamics of open systems interacting with environments in nonequilibrium states. In particular, the reversibility vs irreversibility of the dynamics of the open system is directly linked to specific (classical) information about the order in which the reservoir qubits interacted with the system qubit. This aspect of the homogenizer leads to a model of a quantum safe with a classical combination. We analyze in detail how entanglement between the reservoir and the system is created during the process of quantum homogenization. We show that the information about the initial state of the system qubit is stored in the entanglement between the homogenized qubits.Tue, 20 May 2014 15:33:41 +0200Superluminal influences, hidden variables, and signalinghttps://archive-ouverte.unige.ch/unige:36786https://archive-ouverte.unige.ch/unige:36786We consider alternative models to quantum mechanics, that have been proposed in the recent years in order to explain the EPR correlations between two particles. These models allow in principle local hidden variables produced at the source, and some superluminal “hidden communication” (or “influences”) to reproduce the non-local correlations. Moving to the case of three particles, we show that these alternative models lead to signaling when “hidden communication” alone is considered as the origin of the correlationsTue, 20 May 2014 15:32:13 +0200Superluminal hidden communication as the underlying mechanism for quantum correlations: constraining modelshttps://archive-ouverte.unige.ch/unige:36770https://archive-ouverte.unige.ch/unige:36770Since Bell's theorem, it is known that quantum correlations cannot be described by local variables (LV) alone: if one does not want to abandon classical mechanisms for correlations, a superluminal form of communication among the particles must be postulated. A natural question is whether such a postulate would imply the possibility of superluminal signaling. Here we show that the assumption of finite-speed superluminal communication indeed leads to signaling when no LV are present, and more generally when only LV derivable from quantum statistics are allowed. When the most general LV are allowed, we prove in a specific case that the model can be made again consistent with relativity, but the question remains open in general.Tue, 20 May 2014 13:38:32 +0200Fast and simple one-way quantum key distributionhttps://archive-ouverte.unige.ch/unige:36768https://archive-ouverte.unige.ch/unige:36768We present and demonstrate a new protocol for practical quantum cryptography, tailored for an implementation with weak coherent pulses to obtain a high key generation rate. The key is obtained by a simple time-of-arrival measurement on the dataline; the presence of an eavesdropper is checked by an interferometer on an additional monitoring line. The setup is experimentally simple; moreover, it is tolerant to reduced interference visibility and to photon number splitting attacks, thus featuring a high efficiency in terms of distilled secret bit per qubit.Tue, 20 May 2014 13:37:18 +0200Quantum cloninghttps://archive-ouverte.unige.ch/unige:36767https://archive-ouverte.unige.ch/unige:36767The impossibility of perfectly copying (or cloning) an unknown quantum state is one of the basic rules governing the physics of quantum systems. The processes that perform the optimal approximate cloning have been found in many cases. These “quantum cloning machines” are important tools for studying a wide variety of tasks, e.g., state estimation and eavesdropping on quantum cryptography. This paper provides a comprehensive review of quantum cloning machines both for discrete-dimensional and for continuous-variable quantum systems. In addition, it presents the role of cloning in quantum cryptography, the link between optimal cloning and light amplification via stimulated emission, and the experimental demonstrations of optimal quantum cloning.Tue, 20 May 2014 13:36:32 +0200Security of two quantum cryptography protocols using the same four qubit stateshttps://archive-ouverte.unige.ch/unige:36765https://archive-ouverte.unige.ch/unige:36765The first quantum cryptography protocol, proposed by Bennett and Brassard in 1984 (BB84), has been widely studied in recent years. This protocol uses four states (more precisely, two complementary bases) for the encoding of the classical bit. Recently, it has been noticed that by using the same four states, but a different encoding of information, one can define a protocol which is more robust in practical implementations, specifically when attenuated laser pulses are used instead of single-photon sources [V. Scarani et al., Phys. Rev. Lett. 92, 057901 (2004), referred to as the SARG04 protocol]. We present a detailed study of SARG04 in two different regimes. In the first part, we consider an implementation with a single-photon source: we derive bounds on the error rate Q for security against all possible attacks by the eavesdropper. The lower and the upper bound obtained for SARG04 (Q≲10.95% and Q≳14.9%, respectively) are close to those obtained for BB84 (Q≲12.4% and Q≳14.6%, respectively). In the second part, we consider a realistic source consisting of an attenuated laser and improve on previous analysis by allowing Alice to optimize the mean number of photons as a function of the distance. The SARG04 protocol is found to perform better than BB84, both in secret-key rate and in maximal achievable distance, for a wide class of Eve's attacks.Tue, 20 May 2014 13:35:02 +0200Entanglement and non-locality are different resourceshttps://archive-ouverte.unige.ch/unige:36753https://archive-ouverte.unige.ch/unige:36753Bell's theorem states that, to simulate the correlations created by measurement on pure entangled quantum states, shared randomness is not enough: some `non-local' resources are required. It has been demonstrated recently that all projective measurements on the maximally entangled state of two qubits can be simulated with a single use of a `non-local machine'. We prove that a strictly larger amount of this non-local resource is required for the simulation of pure non-maximally entangled states of two qubits |ψ(α)⟩ = cosα|00⟩ + sinα|11⟩ with . 0<α≤π/7.8 .Tue, 20 May 2014 13:16:06 +0200Photon-number-splitting versus cloning attacks in practical implementations of the Bennett-Brassard 1984 protocol for quantum cryptographyhttps://archive-ouverte.unige.ch/unige:36751https://archive-ouverte.unige.ch/unige:36751In practical quantum cryptography, the source sometimes produces multi-photon pulses, thus enabling the eavesdropper Eve to perform the powerful photon-number-splitting (PNS) attack. Recently, it was shown by Curty and Lutkenhaus [Phys. Rev. A 69, 042321 (2004)] that the PNS attack is not always the optimal attack when two photons are present: if errors are present in the correlations Alice-Bob and if Eve cannot modify Bob's detection efficiency, Eve gains a larger amount of information using another attack based on a 2->3 cloning machine. In this work, we extend this analysis to all distances Alice-Bob. We identify a new incoherent 2->3 cloning attack which performs better than those described before. Using it, we confirm that, in the presence of errors, Eve's better strategy uses 2->3 cloning attacks instead of the PNS. However, this improvement is very small for the implementations of the Bennett-Brassard 1984 (BB84) protocol. Thus, the existence of these new attacks is conceptually interesting but basically does not change the value of the security parameters of BB84. The main results are valid both for Poissonian and sub-Poissonian sources.Tue, 20 May 2014 13:14:40 +0200Direct Measurement of Superluminal Group Velocity and Signal Velocity in an Optical Fiberhttps://archive-ouverte.unige.ch/unige:36743https://archive-ouverte.unige.ch/unige:36743We present an easy way of observing superluminal group velocities using a birefringent optical fiber and other standard devices. In the theoretical analysis, we show that the optical properties of the setup can be described using the notion of "weak value". The experiment shows that the group velocity can indeed exceed c in the fiber; and we report the first direct observation of the so-called "signal velocity", the speed at which information propagates and that cannot exceed c.Tue, 20 May 2014 13:03:11 +0200Four-photon correction in two-photon Bell experimentshttps://archive-ouverte.unige.ch/unige:36740https://archive-ouverte.unige.ch/unige:36740Correlated photons produced by spontaneous parametric down-conversion are an essential tool for quantum communication, especially suited for long-distance connections. To have a reasonable count rate after all the losses in the propagation and the filters needed to improve the coherence, it is convenient to increase the intensity of the laser that pumps the non-linear crystal. By doing so, however, the importance of the four-photon component of the down-converted field increases, thus degrading the quality of two-photon interferences. In this paper, we present an easy derivation of this nuisance valid for any form of entanglement generated by down-conversion, followed by a full study of the problem for time-bin entanglement. We find that the visibility of two-photon interferences decreases as V=1-2ρ, where ρ is, in usual situations, the probability per pulse of creating a detectable photon pair. In particular, the decrease of V is independent of the coherence of the four-photon term. Thanks to the fact that ρ can be measured independently of V, the experimental verification of our prediction is provided for two different configuration of filters.Tue, 20 May 2014 13:00:54 +0200Two independent photon pairs versus four-photon entangled states in parametric down conversionhttps://archive-ouverte.unige.ch/unige:36738https://archive-ouverte.unige.ch/unige:36738We study the physics of four-photon states generated in spontaneous parametric down-conversion with a pulsed pump field. In the limit where the coherence time of the photons t_c is much shorter than the duration of the pump pulse Delta t, the four photons can be described as two independent pairs. In the opposite limit, the four photons are in a four-particle entangled state. Any intermediate case can be characterized by a single parameter chi, which is a function of tphc/∆t. We present a direct measurement of chi through a simple experimental setup. The full theoretical analysis is also providedTue, 20 May 2014 12:59:26 +0200Quantum Cryptography Protocols Robust against Photon Number Splitting Attacks for Weak Laser Pulse Implementationshttps://archive-ouverte.unige.ch/unige:36732https://archive-ouverte.unige.ch/unige:36732We introduce a new class of quantum key distribution protocols, tailored to be robust against photon number splitting (PNS) attacks. We study one of these protocols, which differs from the original protocol by Bennett and Brassard (BB84) only in the classical sifting procedure. This protocol is provably better than BB84 against PNS attacks at zero error.Tue, 20 May 2014 12:52:08 +0200Tailoring photonic entanglement in high-dimensional Hilbert spaceshttps://archive-ouverte.unige.ch/unige:36731https://archive-ouverte.unige.ch/unige:36731We present an experiment where two photonic systems of arbitrary dimensions can be entangled. The method is based on spontaneous parametric down conversion with trains of d pump pulses with a fixed phase relation, generated by a mode-locked laser. This leads to a photon pair created in a coherent superposition of d discrete emission times, given by the successive laser pulses. Entanglement is shown by performing a two-photon interference experiment and by observing the visibility of the interference fringes increasing as a function of the dimension d. Factors limiting the visibility, such as the presence of multiple pairs in one train, are discussed.Tue, 20 May 2014 12:51:19 +0200Proposal for Energy-Time Entanglement of Quasiparticles in a Solid-State Devicehttps://archive-ouverte.unige.ch/unige:36730https://archive-ouverte.unige.ch/unige:36730We present a proposal for the experimental observation of energy-time entanglement of quasiparticles in mesoscopic physics. This type of entanglement arises whenever correlated particles are produced at the same time and this time is uncertain in the sense of quantum uncertainty, as has been largely used in photonics. We discuss its feasibility for electron-hole pairs. In particular, we argue that junctions between materials in which electrons and holes, respectively, propagate ballistically and behave as "entanglers" for energy-time entanglement when irradiated with a continuous laser.Tue, 20 May 2014 12:50:50 +0200Coherent-pulse implementations of quantum cryptography protocols resistant to photon-number-splitting attackshttps://archive-ouverte.unige.ch/unige:36724https://archive-ouverte.unige.ch/unige:36724A new class of quantum cryptography (QC) protocols that are robust against the most general photon number splitting attacks in a weak coherent pulse implementation has been recently proposed. In this article we give a quite exhaustive analysis of several eavesdropping attacks on these schemes. The eavesdropper (Eve) is supposed to have unlimited technological power while the honest parties (Alice and Bob) use present day technology, in particular an attenuated laser as an approximation of a single-photon source. They exploit the nonorthogonality of quantum states for decreasing the information accessible to Eve in the multi-photon pulses accidentally produced by the imperfect source. An implementation of some of these protocols using present day technology allow for a secure key distribution up to distances of ∼ 150 km. We also show that strong-pulse implementations, where a strong pulse is included as a reference, allow for key distribution robust against photon number splitting attacks.Tue, 20 May 2014 12:44:56 +0200Optical Telecom Networks as Weak Quantum Measurements with Postselectionhttps://archive-ouverte.unige.ch/unige:36723https://archive-ouverte.unige.ch/unige:36723We show that weak measurements with post-selection, proposed in the context of the quantum theory of measurement, naturally appear in the everyday physics of fiber optics telecom networks through polarization-mode dispersion (PMD) and polarization-dependent losses (PDL). Specifically, the PMD leads to a time-resolved discrimination of polarization; the post-selection is done in the most natural way: one post-selects those photons that have not been lost because of the PDL. The quantum formalism is shown to simplify the calculation of optical networks in the telecom limit of weak PMD.Tue, 20 May 2014 12:43:58 +0200Security bounds in quantum cryptography using d-level systemshttps://archive-ouverte.unige.ch/unige:36717https://archive-ouverte.unige.ch/unige:36717We analyze the security of quantum cryptography schemes for d-level systems using 2 or d+1 maximally conjugated bases, under individual eavesdropping attacks based on cloning machines and measurement after the basis reconciliation. We consider classical advantage distillation protocols, that allow to extract a key even in situations where the mutual information between the honest parties is smaller than the eavesdropper's information. In this scenario, advantage distillation protocols are shown to be as powerful as quantum distillation: key distillation is possible using classical techniques if and only if the corresponding state in the entanglement based protocol is distillableTue, 20 May 2014 12:38:59 +0200Time-bin entangled qubits for quantum communication created by femtosecond pulseshttps://archive-ouverte.unige.ch/unige:36710https://archive-ouverte.unige.ch/unige:36710We create pairs of non-degenerate time-bin entangled photons at telecom wavelengths with ultra-short pump pulses. Entanglement is shown by performing Bell kind tests of the Franson type with visibilities of up to 91%. As time-bin entanglement can easily be protected from decoherence as encountered in optical fibers, this experiment opens the road for complex quantum communication protocols over long distances. We also investigate the creation of more than one photon pair in a laser pulse and present a simple tool to quantify the probability of such events to happen.Tue, 20 May 2014 12:30:18 +0200Zero-error attacks and detection statistics in the coherent one-way protocol for quantum cryptographyhttps://archive-ouverte.unige.ch/unige:36696https://archive-ouverte.unige.ch/unige:36696This is a study of the security of the Coherent One-Way (COW) protocol for quantum cryptography, proposed recently as a simple and fast experimental scheme. In the zero-error regime, the eavesdropper Eve can only take advantage of the losses in the transmission. We consider new attacks, based on unambiguous state discrimination, which perform better than the basic beam-splitting attack, but which can be detected by a careful analysis of the detection statistics. These results stress the importance of testing several statistical parameters in order to achieve higher rates of secret bits.Mon, 19 May 2014 12:02:13 +0200Upper bounds for the security of two distributed-phase reference protocols of quantum cryptographyhttps://archive-ouverte.unige.ch/unige:36626https://archive-ouverte.unige.ch/unige:36626The differential-phase-shift (DPS) and the coherent-one-way (COW) are among the most practical protocols for quantum cryptography, and are therefore the object of fast-paced experimental developments. The assessment of their security is also a challenge for theorists: the existing tools, that allow to prove security against the most general attacks, do not apply to these two protocols in any straightforward way. We present new upper bounds for their security in the limit of large distances (d≳50 km with typical values in optical fibers) by considering a large class of collective attacks, namely those in which the adversary attaches ancillary quantum systems to each pulse or to each pair of pulses. We introduce also two modified versions of the COW protocol, which may prove more robust than the original one.Tue, 13 May 2014 15:58:41 +0200Oblivious transfer and quantum channels as communication resourceshttps://archive-ouverte.unige.ch/unige:36538https://archive-ouverte.unige.ch/unige:36538We show that from a communication-complexity perspective, the primitive called oblivious transfer—which was introduced in a cryptographic context—can be seen as the classical analogue to a quantum channel in the same sense as non-local boxes are of maximally entangled qubits. More explicitly, one realization of non-cryptographic oblivious transfer allows for the perfect simulation of sending one qubit and measuring it in an orthogonal basis. On the other hand, a qubit channel allows for realizing non-cryptographic oblivious transfer with probability roughly 85 %, whereas 75 % is the classical limit.Mon, 12 May 2014 17:00:57 +0200Bell nonlocalityhttps://archive-ouverte.unige.ch/unige:36406https://archive-ouverte.unige.ch/unige:36406Bell's 1964 theorem, which states that the predictions of quantum theory cannot be accounted for by any local theory, represents one of the most profound developments in the foundations of physics. In the last two decades, Bell's theorem has been a central theme of research from a variety of perspectives, mainly motivated by quantum information science, where the nonlocality of quantum theory underpins many of the advantages afforded by a quantum processing of information. The focus of this review is to a large extent oriented by these later developments. The main concepts and tools which have been developed to describe and study the nonlocality of quantum theory and which have raised this topic to the status of a full subfield of quantum information science are reviewed.Tue, 06 May 2014 15:56:03 +0200Quantum physics : a first encounter : interference, entanglement, and realityhttps://archive-ouverte.unige.ch/unige:12891https://archive-ouverte.unige.ch/unige:12891abstract not availableMon, 13 Dec 2010 18:28:12 +0100Secrecy extraction from no-signalling correlationshttps://archive-ouverte.unige.ch/unige:12883https://archive-ouverte.unige.ch/unige:12883Quantum cryptography shows that one can guarantee the secrecy of correlation on the sole basis of the laws of physics, that is without limiting the computational power of the eavesdropper. The usual security proofs suppose that the authorized partners, Alice and Bob, have a perfect knowledge and control of their quantum systems and devices; for instance, they must be sure that the logical bits have been encoded in true qubits, and not in higher-dimensional systems. In this paper, we present an approach that circumvents this strong assumption. We define protocols, both for the case of bits and for generic $d$-dimensional outcomes, in which the security is guaranteed by the very structure of the Alice-Bob correlations, under the no-signalling condition. The idea is that, if the correlations cannot be produced by shared randomness, then Eve has poor knowledge of Alice's and Bob's symbols. The present study assumes, on the one hand that the eavesdropper Eve performs only individual attacks (this is a limitation to be removed in further work), on the other hand that Eve can distribute any correlation compatible with the no-signalling condition (in this sense her power is greater than what quantum physics allows). Under these assumptions, we prove that the protocols defined here allow extracting secrecy from noisy correlations, when these correlations violate a Bell-type inequality by a sufficiently large amount. The region, in which secrecy extraction is possible, extends within the region of correlations achievable by measurements on entangled quantum states.Mon, 13 Dec 2010 18:23:56 +0100Pseudo-telepathy: input cardinality and Bell-type inequalitieshttps://archive-ouverte.unige.ch/unige:12877https://archive-ouverte.unige.ch/unige:12877Pseudo-telepathy is the most recent form of rejection of locality. Many of its properties have already been discovered: for instance, the minimal entanglement, as well as the minimal cardinality of the output sets, have been characterized. This paper contains two main results. First, we prove that no bipartite pseudo-telepathy game exists, in which one of the partners receives only two questions; as a corollary, we show that the minimal "input cardinality", that is, the minimal number of questions required in a bipartite pseudo-telepathy game, is 3x3. Second, we study the Bell-type inequality derived from the pseudo-telepathy game known as the Magic Square game: we demonstrate that it is a tight inequality for 3 inputs and 4 outputs on each side and discuss its weak resistance to noise.Mon, 13 Dec 2010 15:03:59 +0100Zero-Error Attacks and Detection Statistics in the Coherent One-Way Protocol for Quantum Cryptographyhttps://archive-ouverte.unige.ch/unige:12872https://archive-ouverte.unige.ch/unige:12872This is a study of the security of the Coherent One-Way (COW) protocol for quantum cryptography, proposed recently as a simple and fast experimental scheme. In the zero-error regime, the eavesdropper Eve can only take advantage of the losses in the transmission. We consider new attacks, based on unambiguous state discrimination, which perform better than the basic beam-splitting attack, but which can be detected by a careful analysis of the detection statistics. These results stress the importance of testing several statistical parameters in order to achieve higher rates of secret bits.Mon, 13 Dec 2010 14:57:19 +0100Bell-type inequalities for non-local resourceshttps://archive-ouverte.unige.ch/unige:12871https://archive-ouverte.unige.ch/unige:12871We present bipartite Bell-type inequalities which allow the two partners to use some non-local resource. Such inequality can only be violated if the parties use a resource which is more non-local than the one permitted by the inequality. We introduce a family of N-inputs non-local machines, which are generalizations of the well-known PR-box. Then we construct Bell-type inequalities that cannot be violated by strategies that use one these new machines. Finally we discuss implications for the simulation of quantum states.Mon, 13 Dec 2010 14:56:34 +0100Initiation à la physique quantique : la matière et ses phénomèneshttps://archive-ouverte.unige.ch/unige:12858https://archive-ouverte.unige.ch/unige:12858abstract not availableMon, 13 Dec 2010 09:36:47 +0100Detection loophole in asymmetric Bell experimentshttps://archive-ouverte.unige.ch/unige:12852https://archive-ouverte.unige.ch/unige:12852The problem of closing the detection loophole with asymmetric systems, such as entangled atom-photon pairs, is addressed. We show that, for the Bell inequality I_3322, a minimal detection efficiency of 43% can be tolerated for one of the particles, if the other one is always detected. We also study the influence of noise and discuss the prospects of experimental implementation.Fri, 10 Dec 2010 15:25:42 +0100Device-independent security of quantum cryptography against collective attackshttps://archive-ouverte.unige.ch/unige:12851https://archive-ouverte.unige.ch/unige:12851We present the optimal collective attack on a quantum key distribution protocol in the "device-independent" security scenario, where no assumptions are made about the way the quantum key distribution devices work or on what quantum system they operate. Our main result is a tight bound on the Holevo information between one of the authorized parties and the eavesdropper, as a function of the amount of violation of a Bell-type inequality.Fri, 10 Dec 2010 15:23:09 +0100Entangling independent photons by time measurementhttps://archive-ouverte.unige.ch/unige:12850https://archive-ouverte.unige.ch/unige:12850Entanglement is at the heart of quantum physics, both for its conceptual foundations and for applications in quantum communication. Remarkably, entanglement can be ‘swapped': if we prepare two independent entangled pairs A1–A2 and B1–B2, a joint measurement on A1 and B1 (called a ‘Bellstate measurement', BSM) has the effect of projecting A2 and B2 onto an entangled state, although these two particles have never interacted nor share any common past1,2. Entanglement swapping with photon pairs has already been experimentally demonstrated3–6 using pulsed sources—where the challenge was to achieve sufficiently sharp synchronization of the photons in the BSM—but never with continuous-wave sources, as originally proposed2. Here, we present an experiment where the coherence time of the photons exceeds the temporal resolution of the detectors.Hence, photon timing can be obtained by the detection times, and pulsed sources can be replaced by continuous-wave sources, which do not require any synchronization6,7. This allows for the first time the use of completely autonomous sources, an important step towards real-world quantum networks with truly independent and distant nodes.Fri, 10 Dec 2010 15:20:14 +0100Experimental Falsification of Leggett's Non-Local Variable Modelhttps://archive-ouverte.unige.ch/unige:12849https://archive-ouverte.unige.ch/unige:12849Bell's theorem guarantees that no model based on local variables can reproduce quantum correlations. Also some models based on non-local variables, if subject to apparently "reasonable" constraints, may fail to reproduce quantum physics. In this paper, we introduce a family of inequalities, which allow testing Leggett's non-local model versus quantum physics, and which can be tested in an experiment without additional assumptions. Our experimental data falsify Leggett's model and are in agreement with quantum predictions.Fri, 10 Dec 2010 15:18:37 +0100Fidelity of an optical memory based on stimulated photon echoeshttps://archive-ouverte.unige.ch/unige:12848https://archive-ouverte.unige.ch/unige:12848We investigated the preservation of information encoded into the relative phase and amplitudes of optical pulses during storage and retrieval in an optical memory based on stimulated photon echo. By interfering photon echoes produced in a Ti-indiffused single-mode Er-doped LiNbO$_{3}$ waveguiding structure at telecom wavelength, we found that decoherence in the atomic medium translates only as losses (and not as degradation) of information, as long as the data pulse series is short compared to the atomic decoherence time. The experimentally measured value of the visibility for interfering echoes is close to 100 %. In addition to the expected three-pulse photon-echo interferences we also observed interference due to a four-pulse photon echo. Our findings are of particular interest for future long-distance quantum communication protocols, which rely on the reversible transfer of quantum states between light and atoms with high fidelity.Fri, 10 Dec 2010 14:30:56 +0100Local content of bipartite qubit correlationshttps://archive-ouverte.unige.ch/unige:11763https://archive-ouverte.unige.ch/unige:11763One of the last open problems concerning two qubits in a pure state is to find the exact local content of their correlation, in the sense of Elitzur, Popescu, and Rohrlich (EPR2) [A. C. Elitzur, S. Popescu, and D. Rohrlich, Phys. Lett. A162, 25 (1992)]. We propose an EPR2 decomposition that allows us to prove, for a wide range of states |ψ(θ)〉=cosθ|00〉+sinθ|11〉, that their local content is pL̅ (θ)=cos2θ. We also share reflections on how to possibly extend this result to all two-qubit pure states.Fri, 03 Sep 2010 10:59:42 +0200Device-independant quantum key distribution secure against collective attackshttps://archive-ouverte.unige.ch/unige:2182https://archive-ouverte.unige.ch/unige:2182Device-independent quantum key distribution (DIQKD) represents a relaxation of the security assumptions made in usual quantum key distribution (QKD). As in usual QKD, the security of DIQKD follows from the laws of quantum physics, but contrary to usual QKD, it does not rely on any assumptions about the internal working of the quantum devices used in the protocol. In this paper, we present in detail the security proof for a DIQKD protocol introduced in Acín et al (2008 Phys. Rev. Lett. 98 230501). This proof exploits the full structure of quantum theory (as opposed to other proofs that exploit only the no-signaling principle), but only holds against collective attacks, where the eavesdropper is assumed to act on the quantum systems of the honest parties independently and identically in each round of the protocol (although she can act coherently on her systems at any time). The security of any DIQKD protocol necessarily relies on the violation of a Bell inequality. We discuss the issue of loopholes in Bell experiments in this context.Mon, 29 Jun 2009 09:35:34 +0200Testing quantum correlations versus single-particle properties within leggett's model and beyondhttps://archive-ouverte.unige.ch/unige:878https://archive-ouverte.unige.ch/unige:878Quantum theory predicts and experiments confirm that nature can produce correlations between distant events that are non-local in the sense of violating a Bell inequality1. Nevertheless, Bell's strong sentence 'Correlations cry out for explanations' (ref. 2) remains relevant. The maturing of quantum information science and the discovery of the power of non-local correlations, for example for cryptographic key distribution beyond the standard quantum key distribution schemes3, 4, 5, strengthen Bell's wish and make it even more timely. In 2003, Leggett proposed an alternative model for non-local correlations6 that he proved to be incompatible with quantum predictions. We present here a new approach to this model, along with new inequalities for testing it. These inequalities can be derived in a very simple way, assuming only the non-negativity of probability distributions; they are also stronger than previously published and experimentally tested Leggett-type inequalities6, 7, 8, 9. The simplest of the new inequalities is experimentally violated. Then we go beyond Leggett's model, and show that we cannot ascribe even partially defined individual properties to the components of a maximally entangled pair.Fri, 20 Feb 2009 11:42:34 +0100Simulation of partial entanglement with nonsignaling resourceshttps://archive-ouverte.unige.ch/unige:788https://archive-ouverte.unige.ch/unige:788With the goal of gaining a deeper understanding of quantum non-locality, we decompose quantum correlations into more elementary non-local correlations. We show that the correlations of all pure entangled states of two qubits can be simulated without communication, hence using only non-signaling resources. Our simulation model works in two steps. First, we decompose the quantum correlations into a local and a non-local part. Second, we present a model for simulating the nonlocal part using only non-signaling resources. In our model partially entangled states require more nonlocal resources than maximally entangled states, but the less the state is entangled, the less frequently must the nonlocal resources be used.Fri, 13 Feb 2009 10:37:16 +0100Testing the dimension of Hilbert spaceshttps://archive-ouverte.unige.ch/unige:23https://archive-ouverte.unige.ch/unige:23Given a set of correlations originating from measurements on a quantum state of unknown Hilbert space dimension, what is the minimal dimension d necessary to describe such correlations? We introduce the concept of dimension witness to put lower bounds on d. This work represents a first step in a broader research program aiming to characterize Hilbert space dimension in various contexts related to fundamental questions and quantum information applications.Wed, 29 Oct 2008 12:37:33 +0100