Quantum information theory
http://hdl.handle.net/2117/23728
Fri, 23 Aug 2019 18:10:42 GMT2019-08-23T18:10:42ZSplit, but still attached
http://hdl.handle.net/2117/116836
Split, but still attached
Cavalcanti, Daniel
Recent years have witnessed the beginning of the second quantum revolution, in which an impressive degree of control over quantum systems has led to several applications in quantum communication, computation, and sensing, along with new host materials reaching commercial success. A key driver behind many of these applications is entanglement, a form of correlation that can develop between quantum systems that is stronger than any type of correlation that can exist between the macroscopic systems we deal with in our everyday life. The creation, manipulation, storage, and detection of entanglement have posed some of the biggest challenges to quantum physicists. On pages 409, 413, and 416 of this issue, Fadel et al. (1), Kunkel et al. (2), and Lange et al. (3), respectively, describe three independent experiments in which entanglement is observed in a system composed of thousands of ultracold atoms. More importantly, the entanglement is observed between atoms occupying different spatial regions, which paves the way to new applications of these systems.
Mon, 30 Apr 2018 13:26:47 GMThttp://hdl.handle.net/2117/1168362018-04-30T13:26:47ZCavalcanti, DanielRecent years have witnessed the beginning of the second quantum revolution, in which an impressive degree of control over quantum systems has led to several applications in quantum communication, computation, and sensing, along with new host materials reaching commercial success. A key driver behind many of these applications is entanglement, a form of correlation that can develop between quantum systems that is stronger than any type of correlation that can exist between the macroscopic systems we deal with in our everyday life. The creation, manipulation, storage, and detection of entanglement have posed some of the biggest challenges to quantum physicists. On pages 409, 413, and 416 of this issue, Fadel et al. (1), Kunkel et al. (2), and Lange et al. (3), respectively, describe three independent experiments in which entanglement is observed in a system composed of thousands of ultracold atoms. More importantly, the entanglement is observed between atoms occupying different spatial regions, which paves the way to new applications of these systems.Secrecy in Prepare-and-Measure Clauser-Horne-Shimony-Holt Tests with a Qubit Bound
http://hdl.handle.net/2117/79206
Secrecy in Prepare-and-Measure Clauser-Horne-Shimony-Holt Tests with a Qubit Bound
Woodhead, Erik; Pironio, Stefano
The security of device-independent (DI) quantum key distribution (QKD) protocols relies on the violation of Bell inequalities. As such, their security can be established based on minimal assumptions about the devices, but their implementation necessarily requires the distribution of entangled states. In a setting with fully trusted devices, any entanglement-based protocol is essentially equivalent to a corresponding prepare-and-measure protocol. This correspondence, however, is not generally valid in the DI setting unless one makes extra assumptions about the devices. Here we prove that a known tight lower bound on the min entropy in terms of the Clauser-Horne-Shimony-Holt Bell correlator, which has featured in a number of entanglement-based DI QKD security proofs, also holds in a prepare-and-measure setting, subject only to the assumption that the source is limited to a two-dimensional Hilbert space.
Fri, 13 Nov 2015 09:44:40 GMThttp://hdl.handle.net/2117/792062015-11-13T09:44:40ZWoodhead, ErikPironio, StefanoThe security of device-independent (DI) quantum key distribution (QKD) protocols relies on the violation of Bell inequalities. As such, their security can be established based on minimal assumptions about the devices, but their implementation necessarily requires the distribution of entangled states. In a setting with fully trusted devices, any entanglement-based protocol is essentially equivalent to a corresponding prepare-and-measure protocol. This correspondence, however, is not generally valid in the DI setting unless one makes extra assumptions about the devices. Here we prove that a known tight lower bound on the min entropy in terms of the Clauser-Horne-Shimony-Holt Bell correlator, which has featured in a number of entanglement-based DI QKD security proofs, also holds in a prepare-and-measure setting, subject only to the assumption that the source is limited to a two-dimensional Hilbert space.Nonlocality in many-body quantum systems detected with two-body correlators
http://hdl.handle.net/2117/78958
Nonlocality in many-body quantum systems detected with two-body correlators
Tura, J.; Augusiak, Remigiusz; Sainz, A. B.; Lücke, B.; Klempt, C.; Lewenstein, Maciej; Acin, Antonio
Contemporary understanding of correlations in quantum many-body systems and in quantum phase transitions
is based to a large extent on the recent intensive studies of entanglement in many-body systems. In contrast,
much less is known about the role of quantum nonlocality in these systems, mostly because the available
multipartite Bell inequalities involve high-order correlations among many particles, which are hard to access
theoretically, and even harder experimentally. Standard, ”theorist- and experimentalist-friendly” many-body
observables involve correlations among only few (one, two, rarely three...) particles. Typically, there is no
multipartite Bell inequality for this scenario based on such low-order correlations. Recently, however, we have
succeeded in constructing multipartite Bell inequalities that involve two- and one-body correlations only, and
showed how they revealed the nonlocality in many-body systems relevant for nuclear and atomic physics [Science
344, 1256 (2014)]. With the present contribution we continue our work on this problem. On the one hand,
we present a detailed derivation of the above Bell inequalities, pertaining to permutation symmetry among the
involved parties. On the other hand, we present a couple of new results concerning such Bell inequalities. First,
we characterize their tightness. We then discuss maximal quantum violations of these inequalities in the general
case, and their scaling with the number of parties. Moreover, we provide new classes of two-body Bell inequalities
which reveal nonlocality of the Dicke states—ground states of physically relevant and experimentally
realizable Hamiltonians. Finally, we shortly discuss various scenarios for nonlocality detection in mesoscopic
systems of trapped ions or atoms, and by atoms trapped in the vicinity of designed nanostructures.
Tue, 10 Nov 2015 12:19:08 GMThttp://hdl.handle.net/2117/789582015-11-10T12:19:08ZTura, J.Augusiak, RemigiuszSainz, A. B.Lücke, B.Klempt, C.Lewenstein, MaciejAcin, AntonioContemporary understanding of correlations in quantum many-body systems and in quantum phase transitions
is based to a large extent on the recent intensive studies of entanglement in many-body systems. In contrast,
much less is known about the role of quantum nonlocality in these systems, mostly because the available
multipartite Bell inequalities involve high-order correlations among many particles, which are hard to access
theoretically, and even harder experimentally. Standard, ”theorist- and experimentalist-friendly” many-body
observables involve correlations among only few (one, two, rarely three...) particles. Typically, there is no
multipartite Bell inequality for this scenario based on such low-order correlations. Recently, however, we have
succeeded in constructing multipartite Bell inequalities that involve two- and one-body correlations only, and
showed how they revealed the nonlocality in many-body systems relevant for nuclear and atomic physics [Science
344, 1256 (2014)]. With the present contribution we continue our work on this problem. On the one hand,
we present a detailed derivation of the above Bell inequalities, pertaining to permutation symmetry among the
involved parties. On the other hand, we present a couple of new results concerning such Bell inequalities. First,
we characterize their tightness. We then discuss maximal quantum violations of these inequalities in the general
case, and their scaling with the number of parties. Moreover, we provide new classes of two-body Bell inequalities
which reveal nonlocality of the Dicke states—ground states of physically relevant and experimentally
realizable Hamiltonians. Finally, we shortly discuss various scenarios for nonlocality detection in mesoscopic
systems of trapped ions or atoms, and by atoms trapped in the vicinity of designed nanostructures.Detection of entanglement in asymmetric quantum networks and multipartite quantum steering
http://hdl.handle.net/2117/78931
Detection of entanglement in asymmetric quantum networks and multipartite quantum steering
Cavalcanti, D.; Skrzypczyk, P.; Aguilar, G. H.; Souto Ribeiro, P. H.; Walborn, S. P.
The future of quantum communication relies on quantum networks composed by observers sharing multipartite quantum states. The certification of multipartite entanglement will be crucial to the usefulness of these networks. In many real situations it is natural to assume that some observers are more trusted than others in the sense that they have more knowledge of their measurement apparatuses. Here we propose a general method to certify all kinds of multipartite entanglement in this asymmetric scenario and experimentally demonstrate it in an optical experiment. Our results, which can be seen as a definition of genuine multipartite quantum steering, give a method to detect entanglement in a scenario in between the standard entanglement and fully device-independent scenarios, and provide a basis for semi-device-independent cryptographic applications in quantum networks.
Mon, 09 Nov 2015 09:15:04 GMThttp://hdl.handle.net/2117/789312015-11-09T09:15:04ZCavalcanti, D.Skrzypczyk, P.Aguilar, G. H.Souto Ribeiro, P. H.Walborn, S. P.The future of quantum communication relies on quantum networks composed by observers sharing multipartite quantum states. The certification of multipartite entanglement will be crucial to the usefulness of these networks. In many real situations it is natural to assume that some observers are more trusted than others in the sense that they have more knowledge of their measurement apparatuses. Here we propose a general method to certify all kinds of multipartite entanglement in this asymmetric scenario and experimentally demonstrate it in an optical experiment. Our results, which can be seen as a definition of genuine multipartite quantum steering, give a method to detect entanglement in a scenario in between the standard entanglement and fully device-independent scenarios, and provide a basis for semi-device-independent cryptographic applications in quantum networks.Locality of temperature in spin chains
http://hdl.handle.net/2117/78929
Locality of temperature in spin chains
Hernández-Santana, Senaida; Riera, Arnau; Hovhannisyan, Karen V.; Perarnau-Llobet, Martí; Tagliacozzo, Luca
In traditional thermodynamics, temperature is a local quantity: a subsystem of a large thermal system is in a thermal state at the same temperature as the original system. For strongly interacting systems, however, the locality of temperature breaks down. We study the possibility of associating an effective thermal state to subsystems of infinite chains of interacting spin particles of arbitrary finite dimension. We study the effect of correlations and criticality in the definition of this effective thermal state and discuss the possible implications for the classical simulation of thermal quantum systems.
Mon, 09 Nov 2015 09:02:29 GMThttp://hdl.handle.net/2117/789292015-11-09T09:02:29ZHernández-Santana, SenaidaRiera, ArnauHovhannisyan, Karen V.Perarnau-Llobet, MartíTagliacozzo, LucaIn traditional thermodynamics, temperature is a local quantity: a subsystem of a large thermal system is in a thermal state at the same temperature as the original system. For strongly interacting systems, however, the locality of temperature breaks down. We study the possibility of associating an effective thermal state to subsystems of infinite chains of interacting spin particles of arbitrary finite dimension. We study the effect of correlations and criticality in the definition of this effective thermal state and discuss the possible implications for the classical simulation of thermal quantum systems.Improved Quantum Magnetometry beyond the Standard Quantum Limit
http://hdl.handle.net/2117/78843
Improved Quantum Magnetometry beyond the Standard Quantum Limit
Brask, J. B.; Chaves, R.; Kołodyński, J.
Under ideal conditions, quantum metrology promises a precision gain over classical techniques scaling quadratically with the number of probe particles. At the same time, no-go results have shown that generic, uncorrelated noise limits the quantum advantage to a constant factor. In frequency estimation scenarios, however, there are exceptions to this rule and, in particular, it has been found that transversal dephasing does allow for a scaling quantum advantage. Yet, it has remained unclear whether such exemptions can be exploited in practical scenarios. Here, we argue that the transversal-noise model applies to the setting of recent magnetometry experiments and show that a scaling advantage can be maintained with one-axis-twisted spin-squeezed states and Ramsey-interferometry-like measurements. This is achieved by exploiting the geometry of the setup that, as we demonstrate, has a strong influence on the achievable quantum enhancement for experimentally feasible parameter settings. When, in addition to the dominant transversal noise, other sources of decoherence are present, the quantum advantage is asymptotically bounded by a constant, but this constant may be significantly improved by exploring the geometry.
Thu, 05 Nov 2015 16:02:50 GMThttp://hdl.handle.net/2117/788432015-11-05T16:02:50ZBrask, J. B.Chaves, R.Kołodyński, J.Under ideal conditions, quantum metrology promises a precision gain over classical techniques scaling quadratically with the number of probe particles. At the same time, no-go results have shown that generic, uncorrelated noise limits the quantum advantage to a constant factor. In frequency estimation scenarios, however, there are exceptions to this rule and, in particular, it has been found that transversal dephasing does allow for a scaling quantum advantage. Yet, it has remained unclear whether such exemptions can be exploited in practical scenarios. Here, we argue that the transversal-noise model applies to the setting of recent magnetometry experiments and show that a scaling advantage can be maintained with one-axis-twisted spin-squeezed states and Ramsey-interferometry-like measurements. This is achieved by exploiting the geometry of the setup that, as we demonstrate, has a strong influence on the achievable quantum enhancement for experimentally feasible parameter settings. When, in addition to the dominant transversal noise, other sources of decoherence are present, the quantum advantage is asymptotically bounded by a constant, but this constant may be significantly improved by exploring the geometry.Entanglement and Nonlocality are Inequivalent for Any Number of Parties
http://hdl.handle.net/2117/78836
Entanglement and Nonlocality are Inequivalent for Any Number of Parties
Augusiak, R.; Demianowicz, M.; Tura, J.; Acín, Antonio
Understanding the relation between nonlocality and entanglement is one of the fundamental problems in quantum physics. In the bipartite case, it is known that these two phenomena are inequivalent, as there exist entangled states of two parties that do not violate any Bell inequality. However, except for a single example of an entangled three-qubit state that has a local model, almost nothing is known about such a relation in multipartite systems. We provide a general construction of genuinely multipartite entangled states that do not display genuinely multipartite nonlocality, thus proving that entanglement and nonlocality are inequivalent for any number of parties.
Thu, 05 Nov 2015 15:06:56 GMThttp://hdl.handle.net/2117/788362015-11-05T15:06:56ZAugusiak, R.Demianowicz, M.Tura, J.Acín, AntonioUnderstanding the relation between nonlocality and entanglement is one of the fundamental problems in quantum physics. In the bipartite case, it is known that these two phenomena are inequivalent, as there exist entangled states of two parties that do not violate any Bell inequality. However, except for a single example of an entangled three-qubit state that has a local model, almost nothing is known about such a relation in multipartite systems. We provide a general construction of genuinely multipartite entangled states that do not display genuinely multipartite nonlocality, thus proving that entanglement and nonlocality are inequivalent for any number of parties.Maximally Nonlocal Theories Cannot Be Maximally Random
http://hdl.handle.net/2117/78791
Maximally Nonlocal Theories Cannot Be Maximally Random
Torre, Gonzalo de la; Hoban, Matty J.; Dhara, Chirag; Prettico, Giuseppe; Acín, Antonio
Correlations that violate a Bell inequality are said to be nonlocal; i.e., they do not admit a local and deterministic explanation. Great effort has been devoted to study how the amount of nonlocality (as measured by a Bell inequality violation) serves to quantify the amount of randomness present in observed correlations. In this work we reverse this research program and ask what do the randomness certification capabilities of a theory tell us about the nonlocality of that theory. We find that, contrary to initial intuition, maximal randomness certification cannot occur in maximally nonlocal theories. We go on and show that quantum theory, in contrast, permits certification of maximal randomness in all dichotomic scenarios. We hence pose the question of whether quantum theory is optimal for randomness; i.e., is it the most nonlocal theory that allows maximal randomness certification? We answer this question in the negative by identifying a larger-than-quantum set of correlations capable of this feat. Not only are these results relevant to understanding quantum mechanics’ fundamental features, but also put fundamental restrictions on device-independent protocols based on the no-signaling principle.
Wed, 04 Nov 2015 16:00:15 GMThttp://hdl.handle.net/2117/787912015-11-04T16:00:15ZTorre, Gonzalo de laHoban, Matty J.Dhara, ChiragPrettico, GiuseppeAcín, AntonioCorrelations that violate a Bell inequality are said to be nonlocal; i.e., they do not admit a local and deterministic explanation. Great effort has been devoted to study how the amount of nonlocality (as measured by a Bell inequality violation) serves to quantify the amount of randomness present in observed correlations. In this work we reverse this research program and ask what do the randomness certification capabilities of a theory tell us about the nonlocality of that theory. We find that, contrary to initial intuition, maximal randomness certification cannot occur in maximally nonlocal theories. We go on and show that quantum theory, in contrast, permits certification of maximal randomness in all dichotomic scenarios. We hence pose the question of whether quantum theory is optimal for randomness; i.e., is it the most nonlocal theory that allows maximal randomness certification? We answer this question in the negative by identifying a larger-than-quantum set of correlations capable of this feat. Not only are these results relevant to understanding quantum mechanics’ fundamental features, but also put fundamental restrictions on device-independent protocols based on the no-signaling principle.Thermodynamics of creating correlations: Limitations and optimal protocols
http://hdl.handle.net/2117/78668
Thermodynamics of creating correlations: Limitations and optimal protocols
Bruschi, David Edward; Perarnau-Llobet, MartÍ; Friis, Nicolai; Hovhannisyan, Karen V; Huber, Marcus
We establish a rigorous connection between fundamental resource theories at the quantum scale. Correlations
and entanglement constitute indispensable resources for numerous quantum information tasks. However, their
establishment comes at the cost of energy, the resource of thermodynamics, and is limited by the initial entropy.
Here, the optimal conversion of energy into correlations is investigated. Assuming the presence of a thermal
bath, we establish general bounds for arbitrary systems and construct a protocol saturating them. The amount
of correlations, quantified by the mutual information, can increase at most linearly with the available energy,
and we determine where the linear regime breaks down. We further consider the generation of genuine quantum
correlations, focusing on the fundamental constituents of our universe: fermions and bosons. For fermionic
modes, we find the optimal entangling protocol. For bosonic modes, we show that while Gauss
Mon, 02 Nov 2015 16:12:17 GMThttp://hdl.handle.net/2117/786682015-11-02T16:12:17ZBruschi, David EdwardPerarnau-Llobet, MartÍFriis, NicolaiHovhannisyan, Karen VHuber, MarcusWe establish a rigorous connection between fundamental resource theories at the quantum scale. Correlations
and entanglement constitute indispensable resources for numerous quantum information tasks. However, their
establishment comes at the cost of energy, the resource of thermodynamics, and is limited by the initial entropy.
Here, the optimal conversion of energy into correlations is investigated. Assuming the presence of a thermal
bath, we establish general bounds for arbitrary systems and construct a protocol saturating them. The amount
of correlations, quantified by the mutual information, can increase at most linearly with the available energy,
and we determine where the linear regime breaks down. We further consider the generation of genuine quantum
correlations, focusing on the fundamental constituents of our universe: fermions and bosons. For fermionic
modes, we find the optimal entangling protocol. For bosonic modes, we show that while GaussDetection loophole attacks on semi-device-independent quantum and classical protocols
http://hdl.handle.net/2117/78576
Detection loophole attacks on semi-device-independent quantum and classical protocols
Dall’Arno, Michele; Passaro, Elsa; Gallego, Rodrigo; Pawlowski, Marcin; Acín, Antonio
Semi-device-independent quantum protocols realize information tasks – e.g. secure key
distribution, random access coding, and randomness generation – in a scenario where no
assumption on the internal working of the devices used in the protocol is made, except
their dimension. These protocols offer two main advantages: first, their implementation
is often less demanding than fully-device-independent protocols. Second, they are more
secure than their device-dependent counterparts. Their classical analogous is represented
by random access codes, which provide a general framework for describing one-sided classical
communication tasks. We discuss conditions under which detection inefficiencies can
be exploited by a malicious provider to fake the performance of semi-device-independent
quantum and classical protocols – and how to prevent it.
Fri, 30 Oct 2015 16:56:50 GMThttp://hdl.handle.net/2117/785762015-10-30T16:56:50ZDall’Arno, MichelePassaro, ElsaGallego, RodrigoPawlowski, MarcinAcín, AntonioSemi-device-independent quantum protocols realize information tasks – e.g. secure key
distribution, random access coding, and randomness generation – in a scenario where no
assumption on the internal working of the devices used in the protocol is made, except
their dimension. These protocols offer two main advantages: first, their implementation
is often less demanding than fully-device-independent protocols. Second, they are more
secure than their device-dependent counterparts. Their classical analogous is represented
by random access codes, which provide a general framework for describing one-sided classical
communication tasks. We discuss conditions under which detection inefficiencies can
be exploited by a malicious provider to fake the performance of semi-device-independent
quantum and classical protocols – and how to prevent it.