ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Fri, 27 May 2022 19:10:26 GMT2022-05-27T19:10:26Z50771- Recovering part of the boundary between quantum and nonquantum correlations from information causalityhttps://scholarbank.nus.edu.sg/handle/10635/97776Title: Recovering part of the boundary between quantum and nonquantum correlations from information causality
Authors: Allcock, J.; Brunner, N.; Pawlowski, M.; Scarani, V.
Abstract: Recently, the principle of information causality has appeared as a good candidate for an information-theoretic principle that would single out quantum correlations among more general nonsignaling models. Here, we present results going in this direction, namely, we show that part of the boundary of quantum correlations actually emerges from information causality. © 2009 The American Physical Society.
Wed, 14 Oct 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/977762009-10-14T00:00:00Z
- The device-independent outlook on quantum physicshttps://scholarbank.nus.edu.sg/handle/10635/98254Title: The device-independent outlook on quantum physics
Authors: Scarani, V.
Abstract: This text is an introduction to an operational outlook on Bell inequalities, which has been very fruitful in the past few years. It has lead to the recognition that Bell tests have their own place in applied quantum technologies, because they quantify non-classicality in a device-independent way, that is, without any need to describe the degrees of freedom under study and the measurements that are performed. At the more fundamental level, the same device-independent outlook has allowed the falsification of several other alternative models that could hope to reproduce the observed statistics while keeping some classical features that quantum theory denies; and it has shed new light on the long-standing quest for deriving quantum theory from physical principles.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/982542013-01-01T00:00:00Z
- How non-Local are n noisy popescu-rohrlich machines?https://scholarbank.nus.edu.sg/handle/10635/98742Title: How non-Local are n noisy popescu-rohrlich machines?
Authors: Fitzi, M.; Hänggi, E.; Scarani, V.; Wolf, S.
Abstract: We show that the local part of n symmetric ε-PRMs is of order θ(ε[n/2]) and that the local part of n maximally biased (asymmetric) δ-PRMs is exactly (3δ)n. © 2009 American Institute of Physics.
Thu, 01 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/987422009-01-01T00:00:00Z
- Security bounds for quantum cryptography with finite resourceshttps://scholarbank.nus.edu.sg/handle/10635/98870Title: Security bounds for quantum cryptography with finite resources
Authors: Scarani, V.; Renner, R.
Abstract: A practical quantum key distribution (QKD) protocol necessarily runs in finite time and, hence, only a finite amount of communication is exchanged. This is in contrast to most of the standard results on the security of QKD, which only hold in the limit where the number of transmitted signals approaches infinity. Here, we analyze the security of QKD under the realistic assumption that the amount of communication is finite. At the level of the general formalism, we present new results that help simplifying the actual implementation of QKD protocols: in particular, we show that symmetrization steps, which are required by certain security proofs (e.g., proofs based on de Finetti's representation theorem), can be omitted in practical implementations. Also, we demonstrate how two-way reconciliation protocols can be taken into account in the security analysis. At the level of numerical estimates, we present the bounds with finite resources for "device-independent security" against collective attacks. © 2008 Springer Berlin Heidelberg.
Tue, 01 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/988702008-01-01T00:00:00Z
- Large violation of Bell inequalities using both particle and wave measurementshttps://scholarbank.nus.edu.sg/handle/10635/116093Title: Large violation of Bell inequalities using both particle and wave measurements
Authors: Cavalcanti, D.; Brunner, N.; Skrzypczyk, P.; Salles, A.; Scarani, V.
Sat, 01 Jan 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1160932011-01-01T00:00:00Z
- The effects of free will on randomness expansionhttps://scholarbank.nus.edu.sg/handle/10635/112560Title: The effects of free will on randomness expansion
Authors: Koh, D.E.; Hall, M.J.W.; Setiawan; Pope, J.E.; Ekert, A.; Kay, A.; Scarani, V.
Abstract: One of the assumptions of Bell's Theorem is the existence of experimental free will, meaning that measurement settings can be chosen perfectly at random. With the advent of quantum information, the violation of a Bell inequality constitutes evidence of the lack of an eavesdropper in cryptographic scenarios such as key distribution and randomness expansion. Relaxing the free will assumption changes the bounds on an eavesdropper. We consider a no-signalling model with reduced free will and bound the eavesdropper's capabilities in the randomness expansion setting. We compare the case where the only allowable probability distributions are ones that are factorizable with the case where any general probability distribution is allowed, explicitly giving optimal no-signalling models for maximal violation. © Springer-Verlag Berlin Heidelberg 2013.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1125602013-01-01T00:00:00Z
- Solving the scattering of N photons on a two-level atom without computationhttps://scholarbank.nus.edu.sg/handle/10635/175248Title: Solving the scattering of N photons on a two-level atom without computation
Authors: Roulet, A; Scarani, V
Abstract: We propose a novel approach for solving the scattering of light onto a two-level atom coupled to a one-dimensional waveguide. First we express the physical quantity of interest in terms of Feynman diagrams and treat the atom as a non-saturable linear beamsplitter. By using the atomic response to our advantage, a relevant substitution is then made that captures the nonlinearity of the atom, and the final result is obtained in terms of simple integrals over the initial incoming wavepackets. The procedure is not limited to post-scattering quantities and allows for instance to derive the atomic excitation during the scattering event. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Fri, 01 Jan 2016 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1752482016-01-01T00:00:00Z
- Excitation of a single atom with a temporally shaped light pulseshttps://scholarbank.nus.edu.sg/handle/10635/114436Title: Excitation of a single atom with a temporally shaped light pulses
Authors: Aljunid, S.A.; Leong, V.; Lan, D.H.; Wang, Y.; Maslennikov, G.; Scarani, V.; Kurtsiefer, C.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1144362013-01-01T00:00:00Z
- Extension of the Alberti-Ulhmann criterion beyond qubit dichotomieshttps://scholarbank.nus.edu.sg/handle/10635/198941Title: Extension of the Alberti-Ulhmann criterion beyond qubit dichotomies
Authors: Dall?Arno, M.; Buscemi, F.; Scarani, V.
Abstract: The Alberti-Ulhmann criterion states that any given qubit dichotomy can be transformed into any other given qubit dichotomy by a quantum channel if and only if the testing region of the former dichotomy includes the testing region of the latter dichotomy. Here, we generalize the Alberti-Ulhmann criterion to the case of arbitrary number of qubit or qutrit states. We also derive an analogous result for the case of qubit or qutrit measurements with arbitrary number of elements. We demonstrate the possibility of applying our criterion in a semi-device independent way. @ 2020 Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften. All right reserved.
Wed, 01 Jan 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1989412020-01-01T00:00:00Z
- Quantum memory with a single two-level atom in a half cavityhttps://scholarbank.nus.edu.sg/handle/10635/112497Title: Quantum memory with a single two-level atom in a half cavity
Authors: Wang, Y.; Minář, J.; Hétet, G.; Scarani, V.
Abstract: We propose a setup for quantum memory based on a single two-level atom in a half cavity with a moving mirror. We show that various temporal shapes of incident photon can be efficiently stored and read out by shaping the time-dependent decay rate γ(t) between the atom and the light. This is achieved uniquely by an appropriate motion of the mirror without the need for additional control laser or atomic level. We present an analytical expression for the efficiency of the process and study its dependence on the ratio between the incident light field bandwidth and the atomic decay rate. We discuss possible implementations and experimental issues, particularly for a single atom or ion in a half cavity quantum optical setup as well as a superconducting qubit in the context of circuit QED. © 2012 American Physical Society.
Wed, 18 Jan 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1124972012-01-18T00:00:00Z
- State-dependent atomic excitation by multiphoton pulses propagating along two spatial modeshttps://scholarbank.nus.edu.sg/handle/10635/112520Title: State-dependent atomic excitation by multiphoton pulses propagating along two spatial modes
Authors: Wang, Y.; Minář, J.; Scarani, V.
Abstract: We investigate the dynamics of a single two-level atom, which interacts with pulses propagating in two spatial modes (right and left) and frequency continuum. Using Heisenberg equations of motion, we present the explicit analytical derivations and general formalisms for atomic excitation with two-spatial-mode multiphoton pulses in both the Fock and coherent states. Based on those formalisms, we show that perfect atomic excitation by a single-photon Fock state pulse can only be realized when it is rising exponentially shaped in the even mode, a balanced superposition of the two spatial modes. A single photon from a single spatial mode can only give half of the maximal atomic excitation probability. We also show that the maximum atomic excitation probability with multiphoton pulses in the even mode is a monotonic function of the average photon number for the coherent state, but not for the Fock states. Furthermore, we demonstrate that the atomic dynamics can be controlled by the initial relative phase between the two counterpropagating coherent state pulses incident on the atom, which is not the case with the two Fock state pulses. © 2012 American Physical Society.
Tue, 07 Aug 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1125202012-08-07T00:00:00Z
- Worst-case quantum hypothesis testing with separable measurementshttps://scholarbank.nus.edu.sg/handle/10635/197484Title: Worst-case quantum hypothesis testing with separable measurements
Authors: Thinh, L.P.; Dall’Arno, M.; Scarani, V.
Abstract: For any pair of quantum states (the hypotheses), the task of binary quantum hypotheses testing is to derive the tradeoff relation between the probability p01 of rejecting the null hypothesis and p10 of accepting the alternative hypothesis. The case when both hypotheses are explicitly given was solved in the pioneering work by Helstrom. Here, instead, for any given null hypothesis as a pure state, we consider the worst-case alternative hypothesis that maximizes p10 under a constraint on the distinguishability of such hypotheses. Additionally, we restrict the optimization to separable measurements, in order to describe tests that are performed locally. The case p01 = 0 has been recently studied under the name of “quantum state verification”. We show that the problem can be cast as a semi-definite program (SDP). Then we study in detail the two-qubit case. A comprehensive study in parameter space is done by solving the SDP numerically. We also obtain analytical solutions in the case of commuting hypotheses, and in the case where the two hypotheses can be orthogonal (in the latter case, we prove that the restriction to separable measurements generically prevents perfect distinguishability). In regards to quantum state verification, our work shows the existence of more efficient strategies for noisy measurement scenarios. © 2020 Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften.
Wed, 01 Jan 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1974842020-01-01T00:00:00Z
- Large violation of Bell inequalities using both particle and wave measurementshttps://scholarbank.nus.edu.sg/handle/10635/117258Title: Large violation of Bell inequalities using both particle and wave measurements
Authors: Cavalcanti, D.; Brunner, N.; Skrzypczyk, P.; Salles, A.; Scarani, V.
Abstract: When separated measurements on entangled quantum systems are performed, the theory predicts correlations that cannot be explained by any classical mechanism: communication is excluded because the signal should travel faster than light; pre-established agreement is excluded because Bell inequalities are violated. All optical demonstrations of such violations [1] have involved discrete degrees of freedom and are plagued by the detection-efficiency loophole. A promising alternative is to use continuous variables combined with highly efficient homodyne measurements. However, all schemes proposed so far use states or measurements that are extremely difficult to achieve [2], or produce very weak violations [3,4]. Here we show that large violations for feasible states can be achieved if both photon counting and homodyne detections are used. Our scheme may lead to the first violation of Bell inequalities using continuous-variable measurements and pave the way for a loophole-free Bell test. © 2011 IEEE.
Sat, 01 Jan 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1172582011-01-01T00:00:00Z
- Quantum absorption refrigerator with trapped ionshttps://scholarbank.nus.edu.sg/handle/10635/174186Title: Quantum absorption refrigerator with trapped ions
Authors: Maslennikov, G; Ding, S; Hablützel, R; Gan, J; Roulet, A; Nimmrichter, S; Dai, J; Scarani, V; Matsukevich, D
Abstract: In recent years substantial efforts have been expended in extending thermodynamics to single quantum systems. Quantum effects have emerged as a resource that can improve the performance of heat machines. However in the fully quantum regime their implementation still remains a challenge. Here, we report an experimental realization of a quantum absorption refrigerator in a system of three trapped ions, with three of its normal modes of motion coupled by a trilinear Hamiltonian such that heat transfer between two modes refrigerates the third. We investigate the dynamics and steady-state properties of the refrigerator and compare its cooling capability when only thermal states are involved to the case when squeezing is employed as a quantum resource. We also study the performance of such a refrigerator in the single shot regime made possible by coherence and demonstrate cooling below both the steady-state energy and a benchmark set by classical thermodynamics. © 2019, The Author(s).
Tue, 01 Jan 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1741862019-01-01T00:00:00Z
- Focus on device independent quantum informationhttps://scholarbank.nus.edu.sg/handle/10635/179612Title: Focus on device independent quantum information
Authors: Pironio, S; Scarani, V; Vidick, T
Abstract: It is possible to devise quantum information protocols whose correctness can be certified even when they are run with untrusted quantum devices, on which no a priori assumptions are made. Hence the name 'device-independence' (DI) to refer to such protocols. It allows for certifiable cryptographic randomness generation (RNG) and key distribution (QKD) with unprecedented security. A promising path to the design of more experimentally friendly protocols is to make stronger assumptions on the devices, keeping the spirit of device independence while acknowledging differences in the level of trust about the quantum devices used that can be justified in the context of a realistic implementations. Practical implementations also reveal the limits of the device-independent model: as any model for security, device-independence makes a set of assumptions, such as perfect isolation of the trusted users' laboratories, that may in practice be compromised. Beyond key distribution, it is interesting to investigate if the device-independent approach to security can be extended to other tasks in multi-party cryptography.
Fri, 01 Jan 2016 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1796122016-01-01T00:00:00Z
- All pure bipartite entangled states can be self-testedhttps://scholarbank.nus.edu.sg/handle/10635/174425Title: All pure bipartite entangled states can be self-tested
Authors: Coladangelo, A; Goh, K.T; Scarani, V
Abstract: Quantum technologies promise advantages over their classical counterparts in the fields of computation, security and sensing. It is thus desirable that classical users are able to obtain guarantees on quantum devices, even without any knowledge of their inner workings. That such classical certification is possible at all is remarkable: It is a consequence of the violation of Bell inequalities by entangled quantum systems. Device-independent self-testing refers to the most complete such certification: It enables a classical user to uniquely identify the quantum state shared by uncharacterized devices by simply inspecting the correlations of measurement outcomes. Self-testing was first demonstrated for the singlet state and a few other examples of self-testable states were reported in recent years. Here, we address the long-standing open question of whether every pure bipartite entangled state is self-testable. We answer it affirmatively by providing explicit self-testing correlations for all such states. © 2017 Japan Antibiotics Research Association All rights reserved.
Sun, 01 Jan 2017 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1744252017-01-01T00:00:00Z
- Work production of quantum rotor engineshttps://scholarbank.nus.edu.sg/handle/10635/175118Title: Work production of quantum rotor engines
Authors: Colombo, M; Lòpez-Perolio, I; Meeks, H.D; Caleca, L; Parsons, M.T; Li, H; De Vecchi, G; Tudini, E; Foglia, C; Mondini, P; Manoukian, S; Behar, R; Garcia, E.B.G; Meindl, A; Montagna, M; Niederacher, D; Schmidt, A.Y; Varesco, L; Wappenschmidt, B; Bolla, M.K; Dennis, J; Michailidou, K; Wang, Q; Aittomäki, K; Andrulis, I.L; Anton-Culver, H; Arndt, V; Beckmann, M.W; Beeghly-Fadel, A; Benitez, J; Boeckx, B; Bogdanova, N.V; Bojesen, S.E; Bonanni, B; Brauch, H; Brenner, H; Burwinkel, B; Chang-Claude, J; Conroy, D.M; Couch, F.J; Cox, A; Cross, S.S; Czene, K; Devilee, P; Dörk, T; Eriksson, M; Fasching, P.A; Figueroa, J; Fletcher, O; Flyger, H; Gabrielson, M; García-Closas, M; Giles, G.G; González-Neira, A; Guénel, P; Haiman, C.A; Hall, P; Hamann, U; Hartman, M; Hauke, J; Hollestelle, A; Hopper, J.L; Jakubowska, A; Jung, A; Kosma, V.-M; Lambrechts, D; Le Marchand, L; Lindblom, A; Lubinski, J; Mannermaa, A; Margolin, S; Miao, H; Milne, R.L; Neuhausen, S.L; Nevanlinna, H; Olson, J.E; Peterlongo, P; Peto, J; Pylkäs, K; Sawyer, E.J; Schmidt, M.K; Schmutzler, R.K; Schneeweiss, A; Schoemaker, M.J; See, M.H; Southey, M.C; Swerdlow, A; Teo, S.H; Toland, A.E; Tomlinson, I; Truong, T; van Asperen, C.J; van den Ouweland, A.M.W; van der Kolk, L.E; Winqvist, R; Yannoukakos, D; Zheng, W; Dunning, A.M; Easton, D.F; Henderson, A; Hogervorst, F.B.L; Izatt, L; Offitt, K; Side, L.E; van Rensburg, E.J; Embrace, S; Hebon, S; McGuffog, L; Antoniou, A.C; Chenevix-Trench, G; Spurdle, A.B; Goldgar, D.E; Hoya, M.D.L; Radice, P; kConFab/AOCS Investigators; Seah, S; Nimmrichter, S; Scarani, V
Abstract: We study the mechanical performance of quantum rotor heat engines in terms of common notions of work using two prototypical models: a mill driven by the heat flow from a hot to a cold mode, and a piston driven by the alternate heating and cooling of a single working mode. We evaluate the extractable work in terms of ergotropy, the kinetic energy associated to net directed rotation, as well as the intrinsic work based on the exerted torque under autonomous operation, and we compare them to the energy output for the case of an external dissipative load and for externally driven engine cycles. Our results connect work definitions from both physical and information-theoretical perspectives. In particular, we find that apart from signatures of angular momentum quantization, the ergotropy is consistent with the intuitive notion of work in the form of net directed motion. It also agrees with the energy output to an external load or agent under optimal conditions. This sets forth a consistent thermodynamical description of rotating quantum motors, flywheels, and clocks. © 2018 The Author(s). Published by IOP Publishing Ltd on behalf of Deutsche Physikalische Gesellschaft.
Mon, 01 Jan 2018 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1751182018-01-01T00:00:00Z
- Randomness in post-selected eventshttps://scholarbank.nus.edu.sg/handle/10635/175260Title: Randomness in post-selected events
Authors: Thinh L.P.; Torre G.D.L.; Bancal J.-D.; Pironio S.; Scarani V.
Abstract: Bell inequality violations can be used to certify private randomness for use in cryptographic applications. In photonic Bell experiments, a large amount of the data that is generated comes from no-detection events and presumably contains little randomness. This raises the question as to whether randomness can be extracted only from the smaller post-selected subset corresponding to proper detection events, instead of from the entire set of data. This could in principle be feasible without opening an analogue of the detection loophole as long as the min-entropy of the post-selected data is evaluated by taking all the information into account, including no-detection events. The possibility of extracting randomness from a short string has a practical advantage, because it reduces the computational time of the extraction. Here, we investigate the above idea in a simple scenario, where the devices and the adversary behave according to i.i.d. strategies. We show that indeed almost all the randomness is present in the pair of outcomes for which at least one detection happened. We further show that in some cases applying a pre-processing on the data can capture features that an analysis based on global frequencies only misses, thus resulting in the certification of more randomness. We then briefly consider non-i.i.d strategies and provide an explicit example of such a strategy that is more powerful than any i.i.d. one even in the asymptotic limit of infinitely many measurement rounds, something that was not reported before in the context of Bell inequalities. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Fri, 01 Jan 2016 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1752602016-01-01T00:00:00Z
- Measurement-device-independent quantification of entanglement for given Hilbert space dimensionhttps://scholarbank.nus.edu.sg/handle/10635/175272Title: Measurement-device-independent quantification of entanglement for given Hilbert space dimension
Authors: Goh, K.T; Bancal, J.-D; Scarani, V
Abstract: We address the question of how much entanglement can be certified from the observed correlations and the knowledge of the Hilbert space dimension of the measured systems. We focus on the case in which both systems are known to be qubits. For several correlations (though not for all), one can certify the same amount of entanglement as with state tomography, but with fewer assumptions, since nothing is assumed about the measurements. We also present security proofs of quantum key distribution (QKD) without any assumption on the measurements. We discuss how both the amount of entanglement and the security of QKD are affected by the inefficiency of detectors in this scenario. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Fri, 01 Jan 2016 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1752722016-01-01T00:00:00Z
- All the self-testings of the singlet for two binary measurementshttps://scholarbank.nus.edu.sg/handle/10635/175263Title: All the self-testings of the singlet for two binary measurements
Authors: Wang, Y; Wu, X; Scarani, V
Abstract: Self-testing refers to the possibility of characterizing uniquely (up to local isometries) the state and measurements contained in quantum devices, based only on the observed input-output statistics. Already in the basic case of the two-qubit singlet, self-testing is not unique: the two known criteria (the maximal violation of the CHSH inequality and the Mayers-Yao correlations) are not equivalent. It is unknown how many criteria there are. In this paper, we find the whole set of criteria for the ideal self-testing of a singlet with two measurements and two outcomes on each side; it coincides with all the extremal points of the quantum set that can be obtained by measuring the singlet. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Fri, 01 Jan 2016 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1752632016-01-01T00:00:00Z
- Excitation of a single atom with exponentially rising light pulseshttps://scholarbank.nus.edu.sg/handle/10635/116334Title: Excitation of a single atom with exponentially rising light pulses
Authors: Aljunid, S.A.; Maslennikov, G.; Wang, Y.; Dao, H.L.; Scarani, V.; Kurtsiefer, C.
Abstract: We investigate the interaction between a single atom and optical pulses in a coherent state with a controlled temporal envelope. In a comparison between a rising exponential and a square envelope, we show that the rising exponential envelope leads to a higher excitation probability for fixed low average photon numbers, in accordance with a time-reversed Weisskopf-Wigner model. We characterize the atomic transition dynamics for a wide range of the average photon numbers and are able to saturate the optical transition of a single atom with ≈50 photons in a pulse by a strong focusing technique. © 2013 American Physical Society.
Tue, 03 Sep 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1163342013-09-03T00:00:00Z
- Device-independent bounds for Hardy's experimenthttps://scholarbank.nus.edu.sg/handle/10635/96211Title: Device-independent bounds for Hardy's experiment
Authors: Rabelo, R.; Zhi, L.Y.; Scarani, V.
Abstract: In this Letter, we compute an analogue of Tsirelson's bound for Hardy's test of nonlocality, that is, the maximum violation of locality constraints allowed by the quantum formalism, irrespective of the dimension of the system. The value is found to be the same as the one achievable already with two-qubit systems, and we show that only a very specific class of states can lead to such maximal value, thus highlighting Hardy's test as a device-independent self-test protocol for such states. By considering realistic constraints in Hardy's test, we also compute device-independent upper bounds on this violation and show that these bounds are saturated by two-qubit systems, thus showing that there is no advantage in using higher-dimensional systems in experimental implementations of such a test. © 2012 American Physical Society.
Wed, 31 Oct 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/962112012-10-31T00:00:00Z
- Phase shift of a weak coherent beam induced by a single atomhttps://scholarbank.nus.edu.sg/handle/10635/53088Title: Phase shift of a weak coherent beam induced by a single atom
Authors: Aljunid, S.A.; Tey, M.K.; Chng, B.; Liew, T.; Maslennikov, G.; Scarani, V.; Kurtsiefer, C.
Abstract: We report on a direct measurement of a phase shift on a weak coherent beam by a single Rb87 atom in a Mach-Zehnder interferometer. By strongly focusing the probe mode to the location of the atom, a maximum phase shift of about 1° is observed experimentally. © 2009 The American Physical Society.
Wed, 07 Oct 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/530882009-10-07T00:00:00Z
- The security of practical quantum key distributionhttps://scholarbank.nus.edu.sg/handle/10635/98316Title: The security of practical quantum key distribution
Authors: Scarani, V.; Bechmann-Pasquinucci, H.; Cerf, N.J.; Dušek, M.; Lütkenhaus, N.; Peev, M.
Abstract: Quantum key distribution (QKD) is the first quantum information task to reach the level of mature technology, already fit for commercialization. It aims at the creation of a secret key between authorized partners connected by a quantum channel and a classical authenticated channel. The security of the key can in principle be guaranteed without putting any restriction on an eavesdropper's power. This article provides a concise up-to-date review of QKD, biased toward the practical side. Essential theoretical tools that have been developed to assess the security of the main experimental platforms are presented (discrete-variable, continuous-variable, and distributed-phase- reference protocols). © 2009 The American Physical Society.
Thu, 01 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/983162009-01-01T00:00:00Z
- Cross time-bin photonic entanglement for quantum key distributionhttps://scholarbank.nus.edu.sg/handle/10635/96129Title: Cross time-bin photonic entanglement for quantum key distribution
Authors: Martin, A.; Kaiser, F.; Vernier, A.; Beveratos, A.; Scarani, V.; Tanzilli, S.
Abstract: We report a fully fibered source emitting cross time-bin-entangled photons at 1540 nm from type-II spontaneous parametric down-conversion. Compared to standard time-bin-entanglement realizations, the preparation interferometer requires no phase stabilization, simplifying its implementation in quantum key distribution experiments. Bell-type tests of such a cross time-bin state are performed in the Franson configuration and lead to two-photon interference raw visibilities greater than 95%, which are only limited by the dark counts in the detectors and imperfections in the analysis system. Just by trusting the randomness of the beam splitters, the correlations generated by the source can be proved of nonclassical origin even in a passive implementation. The obtained results confirm the suitability of this source for time-bin-based quantum key distribution. © 2013 American Physical Society.
Mon, 04 Feb 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/961292013-02-04T00:00:00Z
- Quantum networks reveal quantum nonlocalityhttps://scholarbank.nus.edu.sg/handle/10635/116555Title: Quantum networks reveal quantum nonlocality
Authors: Cavalcanti, D.; Almeida, M.L.; Scarani, V.; Acín, A.
Abstract: The results of local measurements on some composite quantum systems cannot be reproduced classically. This impossibility, known as quantum nonlocality, represents a milestone in the foundations of quantum theory. Quantum nonlocality is also a valuable resource for information-processing tasks, for example, quantum communication, quantum key distribution, quantum state estimation or randomness extraction. Still, deciding whether a quantum state is nonlocal remains a challenging problem. Here, we introduce a novel approach to this question: we study the nonlocal properties of quantum states when distributed and measured in networks. We show, using our framework, how any one-way entanglement distillable state leads to nonlocal correlations and prove that quantum nonlocality is a non-additive resource, which can be activated. There exist states, local at the single-copy level, that become nonlocal when taking several copies of them. Our results imply that the nonlocality of quantum states strongly depends on the measurement context. © 2011 Macmillan Publishers Limited. All rights reserved.
Sat, 01 Jan 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1165552011-01-01T00:00:00Z
- Experimental falsification of leggett's nonlocal variable modelhttps://scholarbank.nus.edu.sg/handle/10635/111400Title: Experimental falsification of leggett's nonlocal variable model
Authors: Branciard, C.; Ling, A.; Gisin, N.; Kurtsiefer, C.; Lamas-Linares, A.; Scarani, V.
Abstract: Bell's theorem guarantees that no model based on local variables can reproduce quantum correlations. Also, some models based on nonlocal variables, if subject to apparently "reasonable" constraints, may fail to reproduce quantum physics. In this Letter, we introduce a family of inequalities, which use a finite number of measurement settings, and which therefore allow testing Leggett's nonlocal model versus quantum physics. Our experimental data falsify Leggett's model and are in agreement with quantum predictions. © 2007 The American Physical Society.
Wed, 21 Nov 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1114002007-11-21T00:00:00Z
- Entanglement and irreversibility in the approach to thermal equilibriumhttps://scholarbank.nus.edu.sg/handle/10635/99005Title: Entanglement and irreversibility in the approach to thermal equilibrium
Authors: Scarani, V.
Abstract: When a physical system is put in contact with a very large thermal bath, it undergoes a dissipative (i.e., an apparently irreversible) process that leads to thermal equilibrium. This dynamical process can be described fully within quantum physics, involving only unitary, therefore reversible, maps. The information, initially present in the system, is not erased, but is diluted in the bath because of entanglement. Irreversibility may arise if, after quantum information has been thus diluted, some classical information is lost. This paper reviews a model for thermalization that displays these features. Two new analytical results are provided for the zero-temperature channels: a new quantitative measure of entanglement, and a study of irreversibility in the case where the lost classical information is the label of the particles in the bath. © EDP Sciences/Societá Italiana di Fisica/Springer-Verlag 2007.
Sat, 01 Dec 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/990052007-12-01T00:00:00Z
- Robust self-testing of the singlethttps://scholarbank.nus.edu.sg/handle/10635/97829Title: Robust self-testing of the singlet
Authors: Mckague, M.; Yang, T.H.; Scarani, V.
Abstract: In this paper, we introduce a general framework to study the concept of robust self-testing which can be used to self-test maximally entangled pairs of qubits (EPR pairs) and local measurement operators. The result is based only on probabilities obtained from the experiment, with tolerance to experimental errors. In particular, we show that if the results of an experiment approach the Cirel'son bound, or approximate the Mayers-Yao-type correlations, then the experiment must contain an approximate EPR pair. More specifically, there exist local bases in which the physical state is close to an EPR pair, possibly encoded in a larger environment or ancilla. Moreover, in these bases the measurements are close to the qubit operators used to achieve the Cirel'son bound or the Mayers-Yao results. © 2012 IOP Publishing Ltd.
Fri, 16 Nov 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/978292012-11-16T00:00:00Z
- Strong Constraints on Models that Explain the Violation of Bell Inequalities with Hidden Superluminal Influenceshttps://scholarbank.nus.edu.sg/handle/10635/125036Title: Strong Constraints on Models that Explain the Violation of Bell Inequalities with Hidden Superluminal Influences
Authors: Scarani, V.; Bancal, J.-D.; Suarez, A.; Gisin, N.
Abstract: We 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. © 2014 Springer Science+Business Media New York.
Wed, 01 Jan 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1250362014-01-01T00:00:00Z
- Loophole-free bell test for continuous variables via wave and particle correlationshttps://scholarbank.nus.edu.sg/handle/10635/112562Title: Loophole-free bell test for continuous variables via wave and particle correlations
Authors: Cavalcanti, D.; Scarani, V.
Fri, 20 May 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1125622011-05-20T00:00:00Z
- Nonlocality tests enhanced by a third observerhttps://scholarbank.nus.edu.sg/handle/10635/117091Title: Nonlocality tests enhanced by a third observer
Authors: Cavalcanti, D.; Rabelo, R.; Scarani, V.
Abstract: We consider Bell tests involving bipartite states shared between three parties. We show that the simple inclusion of a third part may greatly simplify the measurement scenario (in terms of the number of measurement settings per part) and allows the identification of previously unknown nonlocal resources. © 2012 American Physical Society.
Wed, 25 Jan 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1170912012-01-25T00:00:00Z
- Extremal correlations of the tripartite no-signaling polytopehttps://scholarbank.nus.edu.sg/handle/10635/96582Title: Extremal correlations of the tripartite no-signaling polytope
Authors: Pironio, S.; Bancal, J.-D.; Scarani, V.
Abstract: The 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. © 2011 IOP Publishing Ltd.
Fri, 11 Feb 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/965822011-02-11T00:00:00Z
- Bell nonlocalityhttps://scholarbank.nus.edu.sg/handle/10635/116935Title: Bell nonlocality
Authors: Brunner, N.; Cavalcanti, D.; Pironio, S.; Scarani, V.; Wehner, S.
Abstract: Bell'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. © 2014 American Physical Society.
Fri, 18 Apr 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1169352014-04-18T00:00:00Z
- Device-independent certification of entangled measurementshttps://scholarbank.nus.edu.sg/handle/10635/112416Title: Device-independent certification of entangled measurements
Authors: Rabelo, R.; Ho, M.; Cavalcanti, D.; Brunner, N.; Scarani, V.
Abstract: We present a device-independent protocol to test if a given black-box measurement device is entangled, that is, has entangled eigenstates. Our scheme involves three parties and is inspired by entanglement swapping; the test uses the Clauser-Horne-Shimony-Holt Bell inequality, checked between each pair of parties. In the case where all particles are qubits, we characterize quantitatively the deviation of the measurement device from a perfect Bell-state measurement. © 2011 American Physical Society.
Mon, 25 Jul 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1124162011-07-25T00:00:00Z
- Information-causality and extremal tripartite correlationshttps://scholarbank.nus.edu.sg/handle/10635/115152Title: Information-causality and extremal tripartite correlations
Authors: Yang, T.H.; Cavalcanti, D.; Almeida, M.L.; Teo, C.; Scarani, V.
Abstract: We study the principle of information-causality (IC) in the presence of extremal no-signaling correlations on a tripartite scenario. We prove that all, except one, of the non-local correlations lead to violation of IC. The remaining non-quantum correlation is shown to satisfy any bipartite physical principle. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Sun, 01 Jan 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1151522012-01-01T00:00:00Z
- Simulation of partial entanglement with nonsignaling resourceshttps://scholarbank.nus.edu.sg/handle/10635/97932Title: Simulation of partial entanglement with nonsignaling resources
Authors: Brunner, N.; Gisin, N.; Popescu, S.; Scarani, V.
Abstract: With the goal of gaining a deeper understanding of quantum nonlocality, we decompose quantum correlations into more elementary nonlocal correlations. We show that the correlations of all pure entangled states of two qubits can be simulated without communication, hence using only no-signaling resources. Our simulation model works in two steps. First, we decompose the quantum correlations into a local and a nonlocal part. Second, we present a model for simulating the nonlocal part using only no-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. © 2008 The American Physical Society.
Thu, 13 Nov 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/979322008-11-13T00:00:00Z
- Large violation of Bell inequalities using both particle and wave measurementshttps://scholarbank.nus.edu.sg/handle/10635/117061Title: Large violation of Bell inequalities using both particle and wave measurements
Authors: Cavalcanti, D.; Brunner, N.; Skrzypczyk, P.; Salles, A.; Scarani, V.
Abstract: When separated measurements on entangled quantum systems are performed, the theory predicts correlations that cannot be explained by any classical mechanism: communication is excluded because the signal should travel faster than light; preestablished agreement is excluded because Bell inequalities are violated. All optical demonstrations of such violations have involved discrete degrees of freedom and are plagued by the detection-efficiency loophole. A promising alternative is to use continuous variables combined with highly efficient homodyne measurements. However, all the schemes proposed so far use states or measurements that are extremely difficult to achieve, or they produce very weak violations. We present a simple method to generate large violations for feasible states using both photon counting and homodyne detections. The present scheme can also be used to obtain nonlocality from easy-to-prepare Gaussian states (e.g., two-mode squeezed state). © 2011 American Physical Society.
Mon, 08 Aug 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1170612011-08-08T00:00:00Z
- Efficient excitation of a two-level atom by a single photon in a propagating modehttps://scholarbank.nus.edu.sg/handle/10635/116316Title: Efficient excitation of a two-level atom by a single photon in a propagating mode
Authors: Wang, Y.; Minář, J.; Sheridan, L.; Scarani, V.
Abstract: State mapping between atoms and photons, and photon-photon interactions play an important role in scalable quantum information processing. We consider the interaction of a two-level atom with a quantized propagating pulse in free space and study the probability Pe(t) of finding the atom in the excited state at any time t. This probability is expected to depend on (i) the quantum state of the pulse field and (ii) the overlap between the pulse and the dipole pattern of the atomic spontaneous emission. We show that the second effect is captured by a single parameter Λ[0,8π3], obtained by weighting the dipole pattern with the numerical aperture. Then, Pe(t) can be obtained by solving time-dependent Heisenberg-Langevin equations. We provide detailed solutions for both single-photon Fock state and coherent states and for various temporal shapes of the pulses. © 2011 American Physical Society.
Tue, 28 Jun 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1163162011-06-28T00:00:00Z
- Device-independent certification of the teleportation of a qubithttps://scholarbank.nus.edu.sg/handle/10635/96212Title: Device-independent certification of the teleportation of a qubit
Authors: Ho, M.; Bancal, J.-D.; Scarani, V.
Abstract: We want to certify in a black-box scenario that two parties simulating the teleportation of a qubit are really using quantum resources. If active compensation is part of the simulation, perfect teleportation can be faked by purely classical means. If active compensation is not implemented, a classical simulation is necessarily imperfect: In this case, we provide bounds for the certification of quantumness using only the observed statistics. In particular, if a uniform shrinking of the Bloch vector is observed on the teleported side, an average teleportation fidelity of 85% guarantees the use of quantum resources. In general, the criterion is not simply related to the fidelity; in an extreme case, the assessment of quantumness can be positive even for an average fidelity as low as 50%. © 2013 American Physical Society.
Mon, 18 Nov 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/962122013-11-18T00:00:00Z
- Finite-key analysis for practical implementations of quantum key distributionhttps://scholarbank.nus.edu.sg/handle/10635/96638Title: Finite-key analysis for practical implementations of quantum key distribution
Authors: Cai, R.Y.Q.; Scarani, V.
Abstract: The lists of bits processed in quantum key distribution are necessarily of finite length. The need for finite-key unconditional security bounds was recognized long ago, but the theoretical tools have become available only very recently. We provide finite-key unconditional security bounds for two practical implementations of the Bennett-Brassard 1984 coding: prepare- and-measure implementations without decoy states and entanglement-based implementations. A finite-key bound for prepare-and-measure implementations with decoy states is also derived under a simplified treatment of the statistical fluctuations. The presentation is tailored to allow direct application of the bounds in experiments. Finally, the bounds are also evaluated on a priori reasonable expected values of the observed parameters. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Thu, 30 Apr 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/966382009-04-30T00:00:00Z
- Time-bin entanglement of quasiparticles in semiconductor deviceshttps://scholarbank.nus.edu.sg/handle/10635/98410Title: Time-bin entanglement of quasiparticles in semiconductor devices
Authors: Chirolli, L.; Giovannetti, V.; Fazio, R.; Scarani, V.
Abstract: A scheme to produce time-bin entangled pairs of electrons and holes is proposed. It is based on a high-frequency time-resolved single-electron source from a quantum dot coupled to one-dimensional chiral channels. Operating the device in the weak tunneling regime, we show that at the lowest order in the tunneling rate, an electron-hole pair is emitted in a coherent superposition state of different time bins determined by the driving pulse sequence. © 2011 American Physical Society.
Mon, 07 Nov 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984102011-11-07T00:00:00Z
- More randomness from the same datahttps://scholarbank.nus.edu.sg/handle/10635/117078Title: More randomness from the same data
Authors: Bancal, J.-D.; Sheridan, L.; Scarani, V.
Abstract: Correlations that cannot be reproduced with local variables certify the generation of private randomness. Usually, the violation of a Bell inequality is used to quantify the amount of randomness produced. Here, we show how private randomness generated during a Bell test can be directly quantified from the observed correlations, without the need to process these data into an inequality. The frequency with which the different measurement settings are used during the Bell test can also be taken into account. This improved analysis turns out to be very relevant for Bell tests performed with a finite collection efficiency. In particular, applying our technique to the data of a recent experiment (Christensen et al 2013 Phys. Rev. Lett. 111 130406), we show that about twice as much randomness as previously reported can be potentially extracted from this setup. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Wed, 01 Jan 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1170782014-01-01T00:00:00Z
- Quantum Bell inequalities from macroscopic localityhttps://scholarbank.nus.edu.sg/handle/10635/53121Title: Quantum Bell inequalities from macroscopic locality
Authors: Yang, T.H.; Navascués, M.; Sheridan, L.; Scarani, V.
Abstract: We propose a method to generate analytical quantum Bell inequalities based on the principle of macroscopic locality. By imposing locality over binary processings of virtual macroscopic intensities, we establish a correspondence between Bell inequalities and quantum Bell inequalities in bipartite scenarios with dichotomic observables. We discuss how to improve the latter approximation and how to extend our ideas to scenarios with more than two outcomes per setting. © 2011 American Physical Society.
Mon, 14 Feb 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/531212011-02-14T00:00:00Z
- Device-independent state estimation based on Bell's inequalitieshttps://scholarbank.nus.edu.sg/handle/10635/112417Title: Device-independent state estimation based on Bell's inequalities
Authors: Bardyn, C.-E.; Liew, T.C.H.; Massar, S.; McKague, M.; Scarani, V.
Abstract: The only information available about an alleged source of entangled quantum states is the amount S by which the Clauser-Horne-Shimony-Holt inequality is violated: nothing is known about the nature of the system or the measurements that are performed. We discuss how the quality of the source can be assessed in this black-box scenario, as compared to an ideal source that would produce maximally entangled states (more precisely, any state for which S=22). To this end, we present several inequivalent notions of fidelity, each one related to the use one can make of the source after having assessed it, and we derive quantitative bounds for each of them in terms of the violation S. We also derive a lower bound on the entanglement of the source as a function of S only. © 2009 The American Physical Society.
Mon, 14 Dec 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1124172009-12-14T00:00:00Z
- Interfacing light and single atoms with a lenshttps://scholarbank.nus.edu.sg/handle/10635/53000Title: Interfacing light and single atoms with a lens
Authors: Tey, M.K.; Maslennikov, G.; Liew, T.C.H.; Aljunid, S.A.; Huber, F.; Chng, B.; Chen, Z.; Scarani, V.; Kurtsiefer, C.
Abstract: We characterize the interaction between a single atom or similar microscopic system and a light field via the scattering ratio. For this, we first derive the electrical field in a strongly focused Gaussian light beam, and then consider the atomic response. Following the simple scattering model, the fraction of scattered optical power for a weak coherent probe field leads to unphysical scattering ratios above 1 in the strong focusing regime. A refined model considering interference between the exciting and scattered field into finitesized detectors or optical fibers is presented, and compared with experimental extinction measurements for various focusing strengths. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Tue, 07 Apr 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/530002009-04-07T00:00:00Z
- Realistic loophole-free Bell test with atom-photon entanglementhttps://scholarbank.nus.edu.sg/handle/10635/116563Title: Realistic loophole-free Bell test with atom-photon entanglement
Authors: Teo, C.; Araújo, M.; Quintino, M.T.; Minář, J.; Cavalcanti, D.; Scarani, V.; Terra Cunha, M.; França Santos, M.
Abstract: The establishment of nonlocal correlations, guaranteed through the violation of a Bell inequality, is not only important from a fundamental point of view but constitutes the basis for device-independent quantum information technologies. Although several nonlocality tests have been conducted so far, all of them suffered from either locality or detection loopholes. Among the proposals for overcoming these problems are the use of atom-photon entanglement and hybrid photonic measurements (for example, photodetection and homodyning). Recent studies have suggested that the use of atom-photon entanglement can lead to Bell inequality violations with moderate transmission and detection efficiencies. Here we combine these ideas and propose an experimental setup realizing a simple atom-photon entangled state that can be used to obtain nonlocality when considering realistic experimental parameters including detection efficiencies and losses due to required propagation distances. © 2013 Macmillan Publishers Limited. All rights reserved.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1165632013-01-01T00:00:00Z
- Analysis of a proposal for a realistic loophole-free Bell test with atom-light entanglementhttps://scholarbank.nus.edu.sg/handle/10635/116224Title: Analysis of a proposal for a realistic loophole-free Bell test with atom-light entanglement
Authors: Teo, C.; Minář, J.; Cavalcanti, D.; Scarani, V.
Abstract: The violation of Bell inequalities where both detection and locality loopholes are closed is crucial for device-independent assessments of quantum information. While of a technological nature, the simultaneous closing of both loopholes still remains a challenge. In Teo, a realistic setup to produce an atom-photon entangled state that could reach a loophole-free Bell inequality violation within current experimental technology is proposed. Here we improve the analysis of this proposal by giving an analytical treatment that shows that the state proposed in Teo can violate a Bell inequality for arbitrarily low photodectection efficiency, when all other losses are ignored. Moreover, it is also able to violate a Bell inequality considering only atomic and homodyne measurements eliminating the need to consider inefficient photocounting measurements. In this case, the maximum Clauser-Horne-Shimony-Holt inequality violation achievable is 2.29, and the minimum transmission required for violation is about 68%. Finally, we show that by postselecting on an atomic measurement, one can engineer superpositions of coherent states for various coherent state amplitudes. © 2013 American Physical Society.
Wed, 27 Nov 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1162242013-11-27T00:00:00Z
- Oblivious transfer and quantum channels as communication resourceshttps://scholarbank.nus.edu.sg/handle/10635/97396Title: Oblivious transfer and quantum channels as communication resources
Authors: Gisin, N.; Popescu, S.; Scarani, V.; Wolf, S.; Wullschleger, J.
Abstract: We 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. © 2012 Springer Science+Business Media Dordrecht.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/973962013-01-01T00:00:00Z
- Upper bounds for the security of two distributed-phase reference protocols of quantum cryptographyhttps://scholarbank.nus.edu.sg/handle/10635/98538Title: Upper bounds for the security of two distributed-phase reference protocols of quantum cryptography
Authors: Branciard, C.; Gisin, N.; Scarani, V.
Abstract: The 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. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Fri, 25 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985382008-01-25T00:00:00Z