Archive ouverte UNIGE | last documents for author 'Antonio Acin'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Antonio Acin'engThe quantum technologies roadmap: a European community viewhttps://archive-ouverte.unige.ch/unige:107161https://archive-ouverte.unige.ch/unige:107161Within the last two decades, quantum technologies (QT) have made tremendous progress, moving from Nobel Prize award-winning experiments on quantum physics (1997: Chu, Cohen-Tanoudji, Phillips; 2001: Cornell, Ketterle, Wieman; 2005: Hall, Hänsch-, Glauber; 2012: Haroche, Wineland) into a cross-disciplinary field of applied research. Technologies are being developed now that explicitly address individual quantum states and make use of the 'strange' quantum properties, such as superposition and entanglement. The field comprises four domains: quantum communication, where individual or entangled photons are used to transmit data in a provably secure way; quantum simulation, where well-controlled quantum systems are used to reproduce the behaviour of other, less accessible quantum systems; quantum computation, which employs quantum effects to dramatically speed up certain calculations, such as number factoring; and quantum sensing and metrology, where the high sensitivity of coherent quantum systems to external perturbations is exploited to enhance the performance of measurements of physical quantities. In Europe, the QT community has profited from several EC funded coordination projects, which, among other things, have coordinated the creation of a 150-page QT Roadmap (http://qurope.eu/h2020/qtflagship/roadmap2016). This article presents an updated summary of this roadmap.Mon, 20 Aug 2018 11:10:40 +0200Extractable Work from Correlationshttps://archive-ouverte.unige.ch/unige:84460https://archive-ouverte.unige.ch/unige:84460Work and quantum correlations are two fundamental resources in thermodynamics and quantum information theory. In this work we study how to use correlations among quantum systems to optimally store work. We analyse this question for isolated quantum ensembles, where the work can be naturally divided into two contributions: a local contribution from each system, and a global contribution originating from correlations among systems. We focus on the latter and consider quantum systems which are locally thermal, thus from which any extractable work can only come from correlations. We compute the maximum extractable work for general entangled states, separable states, and states with fixed entropy. Our results show that while entanglement gives an advantage for small quantum ensembles, this gain vanishes for a large number of systems.Mon, 13 Jun 2016 11:09:29 +0200Thermodynamic cost of creating correlationshttps://archive-ouverte.unige.ch/unige:84456https://archive-ouverte.unige.ch/unige:84456We investigate the fundamental limitations imposed by thermodynamics for creating correlations. Considering a collection of initially uncorrelated thermal quantum systems, we ask how much classical and quantum correlations can be obtained via a cyclic Hamiltonian process. We derive bounds on both the mutual information and entanglement of formation, as a function of the temperature of the systems and the available energy. While for a finite number of systems there is a maximal temperature allowing for the creation of entanglement, we show that genuine multipartite entanglement---the strongest form of entanglement in multipartite systems---can be created at any temperature when sufficiently many systems are considered. This approach may find applications, e.g. in quantum information processing, for physical platforms in which thermodynamic considerations cannot be ignored.Mon, 13 Jun 2016 11:03:32 +0200Experimental Methods for Detecting Entanglementhttps://archive-ouverte.unige.ch/unige:47356https://archive-ouverte.unige.ch/unige:47356Here we present experimental realizations of two new entanglement detection methods: a three-measurement Bell inequality inequivalent to the Clauser-Horne-Shimony-Holt inequality and a nonlinear Bell-type inequality based on the negativity measure. In addition, we provide an experimental and theoretical comparison between these new methods and several techniques already in use: the traditional Clauser-Horne-Shimony-Holt inequality, the entanglement witness, and complete state tomography.Fri, 27 Feb 2015 14:37:41 +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 +0100Quantum nonlocality in two three-level systemshttps://archive-ouverte.unige.ch/unige:36791https://archive-ouverte.unige.ch/unige:36791Recently a new Bell inequality has been introduced (CGLMP,KKCZO) that is strongly resistant to noise for maximally entangled states of two d-dimensional quantum systems. We prove that a larger violation, or equivalently a stronger resistance to noise, is found for a non-maximally entangled state. It is shown that the resistance to noise is not a good measure of non-locality and we introduce some other possible measures. The non-maximally entangled state turns out to be more robust also for these alternative measures. From these results it follows that two Von Neumann measurements per party may be not optimal for detecting non-locality. For d=3,4, we point out some connections between this inequality and distillability. Indeed, we demonstrate that any state violating it, with the optimal Von Neumann settings, is distillable.Tue, 20 May 2014 15:38:35 +0200General properties of nonsignaling theorieshttps://archive-ouverte.unige.ch/unige:36774https://archive-ouverte.unige.ch/unige:36774This article identifies a series of properties common to all theories that do not allow for superluminal signaling and predict the violation of Bell inequalities. Intrinsic randomness, uncertainty due to the incompatibility of two observables, monogamy of correlations, impossibility of perfect cloning, privacy of correlations, bounds in the shareability of some states; all these phenomena are solely a consequence of the no-signaling principle and nonlocality. In particular, it is shown that for any distribution, the properties of (i) nonlocal, (ii) no arbitrarily shareable, and (iii) positive secrecy content are equivalent.Tue, 20 May 2014 13:45:52 +0200Optimal Bell Tests Do Not Require Maximally Entangled Stateshttps://archive-ouverte.unige.ch/unige:36769https://archive-ouverte.unige.ch/unige:36769Any Bell test consists of a sequence of measurements on a quantum state in spacelike separated regions. Thus, a state is better than others for a Bell test when, for the optimal measurements and the same number of trials, the probability of existence of a local model for the observed outcomes is smaller. The maximization over states and measurements defines the optimal nonlocality proof. Numerical results show that the required optimal state does not have to be maximally entangled.Tue, 20 May 2014 13:37:56 +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 +0200Multipartite asymmetric quantum cloninghttps://archive-ouverte.unige.ch/unige:36766https://archive-ouverte.unige.ch/unige:36766We investigate the optimal distribution of quantum information over multipartite systems in asymmetric settings. We introduce cloning transformations that take N identical replicas of a pure state in any dimension as input and yield a collection of clones with nonidentical fidelities. As an example, if the clones are partitioned into a set of MA clones with fidelity FA and another set of MB clones with fidelity FB, the trade-off between these fidelities is analyzed, and particular cases of optimal N→MA+MB cloning machines are exhibited. We also present an optimal 1→1+1+1 cloning machine, which is an example of a tripartite fully asymmetric cloner. Finally, it is shown how these cloning machines can be optically realized.Tue, 20 May 2014 13:35:56 +0200Quantum Correlations and Secret Bitshttps://archive-ouverte.unige.ch/unige:36746https://archive-ouverte.unige.ch/unige:36746It is shown that (i) all entangled states can be mapped by single-copy measurements into probability distributions containing secret correlations, and (ii) if a probability distribution obtained from a quantum state contains secret correlations, then this state has to be entangled. These results prove the existence of a two-way connection between secret and quantum correlations in the process of preparation. They also imply that either it is possible to map any bound entangled state into a distillable probability distribution or bipartite bound information exists.Tue, 20 May 2014 13:05:28 +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 +0200Bell-Type Test of Energy-Time Entangled Qutritshttps://archive-ouverte.unige.ch/unige:36736https://archive-ouverte.unige.ch/unige:36736We have performed a Bell-type test for energy-time entangled qutrits. A method of inferring the Bell violation in terms of an associated interference visibility is derived. Using this scheme we obtained a Bell value of 2.784+/-0.023, representing a violation of 34sigma above the limit for local variables. The scheme has been developed for use at telecom wavelengths and using proven long distance quantum communication architecture to optimize the utility of this high dimensional entanglement resource.Tue, 20 May 2014 12:57:24 +0200Coincidence Bell Inequality for Three Three-Dimensional Systemshttps://archive-ouverte.unige.ch/unige:36734https://archive-ouverte.unige.ch/unige:36734We construct a Bell inequality for coincidence probabilities on a three three-dimensional (qutrit) system. We show that this inequality is violated when each observer measures two noncommuting observables, defined by the so-called unbiased six-port beam splitter, on a maximally entangled state of two qutrits. The strength of the violation agrees with the numerical results presented by Kaszlikowski et al, quant-ph/0202019. It is proven that the inequality defines facets of the polytope of local variable models.Tue, 20 May 2014 12:53:52 +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 +0200Experimental realization of entangled qutrits for quantum communicationhttps://archive-ouverte.unige.ch/unige:36728https://archive-ouverte.unige.ch/unige:36728We have experimentally realized a technique to generate, control and measure entan- gled qutrits, 3-dimensional quantum systems. This scheme uses spontaneous parametric down converted photons and unbalanced 3-arm fiber optic interferometers in a scheme analogous to the Franson interferometric arrangement for qubits. The results reveal a source capable of generating maximally entangled states with a net state fidelity, F = 0.985 ± 0.018. Further the control over the system reveals a high, net, 2-photon interfer- ence fringe visibility, V = 0.919 ± 0.026. This has all been done at telecom wavelengths thus facilitating the advancement towards long distance higher dimensional quantum communication.Tue, 20 May 2014 12:49:12 +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 +0200Equivalence between Two-Qubit Entanglement and Secure Key Distributionhttps://archive-ouverte.unige.ch/unige:36722https://archive-ouverte.unige.ch/unige:36722We study the problem of secret key distillation from bipartite states in the scenario where Alice and Bob can perform measurements only at the single-copy level and classically process the obtained outcomes. Even with these limitations, secret bits can be asymptotically distilled by the honest parties from any two-qubit entangled state, under any individual attack. Our results point out a complete equivalence between two-qubit entanglement and secure key distribution: a key can be established through a one-qubit channel if and only if it allows one to distribute entanglement. These results can be generalized to a higher dimension for all those states that are one-copy distillable.Tue, 20 May 2014 12:43:04 +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 +0200Grothendieck's constant and local models for noisy entangled quantum stateshttps://archive-ouverte.unige.ch/unige:36688https://archive-ouverte.unige.ch/unige:36688We relate the nonlocal properties of noisy entangled states to Grothendieck's constant, a mathematical constant appearing in Banach space theory. For two-qubit Werner states ρWp=p∣ψ−⟩⟨ψ−∣+(1−p)1/4, we show that there is a local model for projective measurements if and only if p⩽1/KG(3), where KG(3) is Grothendieck's constant of order 3. Known bounds on KG(3) prove the existence of this model at least for p≲0.66, quite close to the current region of Bell violation, p∼0.71. We generalize this result to arbitrary quantum states.Mon, 19 May 2014 11:51:26 +0200Nonlocal multipartite correlations from local marginal probabilitieshttps://archive-ouverte.unige.ch/unige:36548https://archive-ouverte.unige.ch/unige:36548Understanding what can be inferred about a multiparticle quantum system given only the knowledge of its subparts is a highly nontrivial task. Clearly, if a global system does not contain an information resource of some kind, neither do its subparts. For the case of entanglement as an information resource, it is known that the converse of this last statement is not true: Some nonentangled reduced states are only compatible with global states which are entangled. We extend this result to correlations and provide local marginal correlations that are only compatible with global genuinely tripartite nonlocal correlations. Quantum nonlocality can thus be deduced from the mere observation of local marginal correlations.Tue, 13 May 2014 11:07:48 +0200All quantum states useful for teleportation are nonlocal resourceshttps://archive-ouverte.unige.ch/unige:36420https://archive-ouverte.unige.ch/unige:36420Understanding the relation between the different forms of inseparability in quantum mechanics is a longstanding problem in the foundations of quantum theory and has implications for quantum information processing. Here we make progress in this direction by establishing a direct link between quantum teleportation and Bell nonlocality. In particular, we show that all entangled states which are useful for teleportation are nonlocal resources, i.e., lead to deterministic violation of Bell's inequality. Our result also extends the phenomenon of superactivation of quantum nonlocality, recently proven by C. Palazuelos [Phys. Rev. Lett. 109, 190401 (2012)], and suggests that the latter might in fact be more general than initially thought.Tue, 06 May 2014 16:38:49 +0200From Bell's Theorem to Secure Quantum Key Distributionhttps://archive-ouverte.unige.ch/unige:12884https://archive-ouverte.unige.ch/unige:12884Any Quantum Key Distribution (QKD) protocol consists first of sequences of measurements that produce some correlation between classical data. We show that these correlation data must violate some Bell inequality in order to contain distillable secrecy, if not they could be produced by quantum measurements performed on a separable state of larger dimension. We introduce a new QKD protocol and prove its security against any individual attack by an adversary only limited by the no-signaling condition.Mon, 13 Dec 2010 18:24:56 +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 +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 +0100Guess your neighbor's input : a multipartite non-local game with no quantum advantagehttps://archive-ouverte.unige.ch/unige:11752https://archive-ouverte.unige.ch/unige:11752We present a multipartite nonlocal game in which each player must guess the input received by his neighbour. We show that quantum correlations do not perform better than classical ones at this game, for any prior distribution of the inputs. There exist, however, input distributions for which general no-signalling correlations can outperform classical and quantum correlations. Some of the Bell inequalities associated with our construction correspond to facets of the local polytope. Thus our multipartite game identifies parts of the boundary between quantum and post-quantum correlations of maximal dimension. These results suggest that quantum correlations might obey a generalization of the usual no-signalling conditions in a multipartite setting.Thu, 02 Sep 2010 14:44:09 +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 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