ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Mon, 06 Jul 2020 18:30:28 GMT2020-07-06T18:30:28Z50991- Colloquium: The physics of Maxwell's demon and informationhttps://scholarbank.nus.edu.sg/handle/10635/96017Title: Colloquium: The physics of Maxwell's demon and information
Authors: Maruyama, K.; Nori, F.; Vedral, V.
Abstract: Maxwell's demon was born in 1867 and still thrives in modern physics. He plays important roles in clarifying the connections between two theories: thermodynamics and information. Here the history of the demon and a variety of interesting consequences of the second law of thermodynamics are presented, mainly in quantum mechanics, but also in the theory of gravity. Also highlighted are some of the recent work that explores the role of information, illuminated by Maxwell's demon, in the arena of quantum-information theory. © 2009 The American Physical Society.
Tue, 06 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/960172009-01-06T00:00:00Z
- Majorana transport in superconducting nanowire with Rashba and Dresselhaus spin-orbit couplingshttps://scholarbank.nus.edu.sg/handle/10635/127564Title: Majorana transport in superconducting nanowire with Rashba and Dresselhaus spin-orbit couplings
Authors: You Jia Bin; Shao Xiao Qiang; Tong Qing Jun; Chan Aik Hui; Oh Choo Hiap; Vedral Vlatko
Thu, 01 Jan 2015 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1275642015-01-01T00:00:00Z
- Majorana fermions in s-wave noncentrosymmetric superconductor with Dresselhaus (110) spin-orbit couplinghttps://scholarbank.nus.edu.sg/handle/10635/97144Title: Majorana fermions in s-wave noncentrosymmetric superconductor with Dresselhaus (110) spin-orbit coupling
Authors: You, J.; Oh, C.H.; Vedral, V.
Abstract: The asymmetric spin-orbit interactions play a crucial role in realizing topological phases in a noncentrosymmetric superconductor (NCS). We investigate the edge states and the vortex core states in the s-wave NCS with Dresselhaus (110) spin-orbit coupling by both numerical and analytical methods. In particular, we demonstrate that there exists a semimetal phase characterized by the flat Andreev bound states in the phase diagram of the s-wave Dresselhaus NCS which supports the emergence of Majorana fermions. The flat dispersion implies a peak in the density of states which has a clear experimental signature in the tunneling conductance measurements and the Majorana fermions proposed here should be experimentally detectable. © 2013 American Physical Society.
Mon, 04 Feb 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/971442013-02-04T00:00:00Z
- Detecting entanglement with Jarzynski's equalityhttps://scholarbank.nus.edu.sg/handle/10635/96184Title: Detecting entanglement with Jarzynski's equality
Authors: Hide, J.; Vedral, V.
Abstract: We present a method for detecting the entanglement of a state using nonequilibrium processes. A comparison of relative entropies allows us to construct an entanglement witness. The relative entropy can further be related to the quantum Jarzynski equality, allowing nonequilibrium work to be used in entanglement detection. To exemplify our results, we consider two different spin chains. © 2010 The American Physical Society.
Thu, 03 Jun 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/961842010-06-03T00:00:00Z
- Spatial entanglement from off-diagonal long-range order in a Bose-Einstein condensatehttps://scholarbank.nus.edu.sg/handle/10635/97975Title: Spatial entanglement from off-diagonal long-range order in a Bose-Einstein condensate
Authors: Heaney, L.; Anders, J.; Kaszlikowski, D.; Vedral, V.
Abstract: We investigate spatial entanglement-particle-number entanglement between regions of space-in an ideal bosonic gas. We quantify the amount of spatial entanglement around the transition temperature for condensation (TC) by probing the gas with two localized two-level systems. We show that spatial entanglement in the gas is directly related to filling of the ground-state energy level and therefore to the off-diagonal long-range order of the system and the onset of condensation. © 2007 The American Physical Society.
Wed, 07 Nov 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/979752007-11-07T00:00:00Z
- Inadequacy of von neumann entropy for characterizing extractable workhttps://scholarbank.nus.edu.sg/handle/10635/96900Title: Inadequacy of von neumann entropy for characterizing extractable work
Authors: Dahlsten, O.C.O.; Renner, R.; Rieper, E.; Vedral, V.
Abstract: The lack of knowledge that an observer has about a system limits the amount of work it can extract. This lack of knowledge is normally quantified using the Gibbs/von Neumann entropy. We show that this standard approach is, surprisingly, only correct in very specific circumstances. In general, one should use the recently developed smooth entropy approach. For many common physical situations, including large but internally correlated systems, the resulting values for the extractable work can deviate arbitrarily from those suggested by the standard approach. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Sun, 01 May 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/969002011-05-01T00:00:00Z
- Experimental generation of quantum discord via noisy processeshttps://scholarbank.nus.edu.sg/handle/10635/96557Title: Experimental generation of quantum discord via noisy processes
Authors: Lanyon, B.P.; Jurcevic, P.; Hempel, C.; Gessner, M.; Vedral, V.; Blatt, R.; Roos, C.F.
Abstract: Quantum systems in mixed states can be unentangled and yet still nonclassically correlated. These correlations can be quantified by the quantum discord and might provide a resource for quantum information processing tasks. By precisely controlling the interaction of two ionic qubits with their environment, we investigate the capability of noise to generate discord. Firstly, we show that noise acting on only one quantum system can generate discord between two. States generated in this way are restricted in terms of the rank of their correlation matrix. Secondly, we show that classically correlated noise processes are capable of generating a much broader range of discordant states with correlation matrices of any rank. Our results show that noise processes prevalent in many physical systems can automatically generate nonclassical correlations and highlight fundamental differences between discord and entanglement. © 2013 American Physical Society.
Thu, 05 Sep 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/965572013-09-05T00:00:00Z
- Physical interpretation of the Wigner rotations and its implications for relativistic quantum informationhttps://scholarbank.nus.edu.sg/handle/10635/97546Title: Physical interpretation of the Wigner rotations and its implications for relativistic quantum information
Authors: Saldanha, P.L.; Vedral, V.
Abstract: We present a new treatment for the spin of a massive relativistic particle in the context of quantum information based on a physical interpretation of the Wigner rotations, obtaining different results in relation to previous works. We are led to the conclusion that it is not possible to define a reduced density matrix for the particle spin and that the Pauli-Lubanski (or similar) spin operators are not suitable for describing measurements where the spin couples to an electromagnetic field in the measuring apparatus. These conclusions contradict the assumptions made by most of the previous papers on the subject. We also propose an experimental test of our formulation. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Sun, 01 Jan 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/975462012-01-01T00:00:00Z
- Regional versus global entanglement in resonating-valence-bond stateshttps://scholarbank.nus.edu.sg/handle/10635/97789Title: Regional versus global entanglement in resonating-valence-bond states
Authors: Chandran, A.; Kaszlikowski, D.; Sen, A.; Sen, U.; Vedral, V.
Abstract: We investigate the entanglement properties of resonating-valence-bond states on two and higher dimensional lattices, which play a significant role in our understanding of various many-body systems. We show that these states are genuinely multipartite entangled, while there is only a negligible amount of two-site entanglement. We comment on possible physical implications of our findings. © 2007 The American Physical Society.
Thu, 25 Oct 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/977892007-10-25T00:00:00Z
- Witnessing macroscopic entanglement in a staggered magnetic fieldhttps://scholarbank.nus.edu.sg/handle/10635/98590Title: Witnessing macroscopic entanglement in a staggered magnetic field
Authors: Hide, J.; Son, W.; Lawrie, I.; Vedral, V.
Abstract: We investigate macroscopic entanglement in an infinite XX spin- 1 2 chain with staggered magnetic field, Bl =B+ e-iπl b. Using single-site entropy and by constructing an entanglement witness, we search for the existence of entanglement when the system is at absolute zero, as well as in thermal equilibrium. Although the role of the alternating magnetic field b is, in general, to suppress entanglement as do B and T, we find that when T=0, introducing b allows the existence of entanglement even when the uniform magnetic field B is arbitrarily large. We find that the region and the amount of entanglement in the spin chain can be enhanced by a staggered magnetic field. © 2007 The American Physical Society.
Mon, 20 Aug 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985902007-08-20T00:00:00Z
- The classical-quantum boundary for correlations: Discord and related measureshttps://scholarbank.nus.edu.sg/handle/10635/116627Title: The classical-quantum boundary for correlations: Discord and related measures
Authors: Modi, K.; Brodutch, A.; Cable, H.; Paterek, T.; Vedral, V.
Abstract: One of the best signatures of nonclassicality in a quantum system is the existence of correlations that have no classical counterpart. Different methods for quantifying the quantum and classical parts of correlations are among the more actively studied topics of quantum-information theory over the past decade. Entanglement is the most prominent of these correlations, but in many cases unentangled states exhibit nonclassical behavior too. Thus distinguishing quantum correlations other than entanglement provides a better division between the quantum and classical worlds, especially when considering mixed states. Here different notions of classical and quantum correlations quantified by quantum discord and other related measures are reviewed. In the first half, the mathematical properties of the measures of quantum correlations are reviewed, related to each other, and the classical-quantum division that is common among them is discussed. In the second half, it is shown that the measures identify and quantify the deviation from classicality in various quantum-information- processing tasks, quantum thermodynamics, open-system dynamics, and many-body physics. It is shown that in many cases quantum correlations indicate an advantage of quantum methods over classical ones. © 2012 American Physical Society.
Mon, 26 Nov 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1166272012-11-26T00:00:00Z
- Qantum correlations in biomoleculeshttps://scholarbank.nus.edu.sg/handle/10635/98860Title: Qantum correlations in biomolecules
Authors: Vedral, V.
Abstract: In this paper we identify quantum entanglement, quantum discord and classical correlations as the main classes of correlations present in many body systems, such as biomolecules. We show how to witness the presene of entanglement and quantum discord. All witnesses can be accessed with simple measurements and implemented with current technology. We discuss the relationship between the speed of processing and presence of correlations and illustrate it in the case of photosynthesis and magnetoreception. © 2011 Published by Elsevier Ltd.
Sat, 01 Jan 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/988602011-01-01T00:00:00Z
- Geometric local invariants and pure three-qubit stateshttps://scholarbank.nus.edu.sg/handle/10635/52960Title: Geometric local invariants and pure three-qubit states
Authors: Williamson, M.S.; Ericsson, M.; Johansson, M.; Sjöqvist, E.; Sudbery, A.; Vedral, V.; Wootters, W.K.
Abstract: We explore a geometric approach to generating local SU(2) and SL(2,C) invariants for a collection of qubits inspired by lattice gauge theory. Each local invariant or "gauge" invariant is associated with a distinct closed path (or plaquette) joining some or all of the qubits. In lattice gauge theory, the lattice points are the discrete space-time points, the transformations between the points of the lattice are defined by parallel transporters, and the gauge invariant observable associated with a particular closed path is given by the Wilson loop. In our approach the points of the lattice are qubits, the link transformations between the qubits are defined by the correlations between them, and the gauge invariant observable, the local invariants associated with a particular closed path, are also given by a Wilson looplike construction. The link transformations share many of the properties of parallel transporters, although they are not undone when one retraces one's steps through the lattice. This feature is used to generate many of the invariants. We consider a pure three-qubit state as a test case and find we can generate a complete set of algebraically independent local invariants in this way; however, the framework given here is applicable to generating local unitary invariants for mixed states composed of any number of d-level quantum systems. We give an operational interpretation of these invariants in terms of observables. © 2011 American Physical Society.
Tue, 07 Jun 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/529602011-06-07T00:00:00Z
- Optomechanical to mechanical entanglement transformationhttps://scholarbank.nus.edu.sg/handle/10635/97467Title: Optomechanical to mechanical entanglement transformation
Authors: Vacanti, G.; Paternostro, M.; Palma, G.M.; Vedral, V.
Abstract: We present a scheme for generating entanglement between two mechanical oscillators that have never interacted with each other by using an entanglement-swapping protocol. The system under study consists of a Michelson-Morley interferometer comprising mechanical systems embodied by two cantilevers. Each of them is coupled to a field mode via the radiation pressure mechanism. Entanglement between the two mechanical systems is set by measuring the output modes of the interferometer. We also propose a control mechanism for the amount of entanglement based on path-length difference between the two arms. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Tue, 30 Sep 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/974672008-09-30T00:00:00Z
- Extracting quantum work statistics and fluctuation theorems by single-qubit interferometryhttps://scholarbank.nus.edu.sg/handle/10635/115723Title: Extracting quantum work statistics and fluctuation theorems by single-qubit interferometry
Authors: Dorner, R.; Clark, S.R.; Heaney, L.; Fazio, R.; Goold, J.; Vedral, V.
Abstract: We propose an experimental scheme to verify the quantum nonequilibrium fluctuation relations using current technology. Specifically, we show that the characteristic function of the work distribution for a nonequilibrium quench of a general quantum system can be extracted by Ramsey interferometry of a single probe qubit. Our scheme paves the way for the full characterization of nonequilibrium processes in a variety of quantum systems, ranging from single particles to many-body atomic systems and spin chains. We demonstrate our idea using a time-dependent quench of the motional state of a trapped ion, where the internal pseudospin provides a convenient probe qubit. © 2013 American Physical Society.
Fri, 07 Jun 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1157232013-06-07T00:00:00Z
- When Casimir meets Kibble-Zurekhttps://scholarbank.nus.edu.sg/handle/10635/98581Title: When Casimir meets Kibble-Zurek
Authors: Vacanti, G.; Pugnetti, S.; Didier, N.; Paternostro, M.; Palma, G.M.; Fazio, R.; Vedral, V.
Abstract: Verification of the dynamical Casimir effect (DCE) in optical systems is still elusive due to the very demanding requirements for its experimental implementation. This typically requires very fast changes in the boundary conditions of the problem. We show that an ensemble of two-level atoms collectively coupled to the electromagnetic field of a cavity, driven at low frequencies and close to a quantum phase transition, stimulates the production of photons from the vacuum. This paves the way for an effective simulation of the DCE through a mechanism that has recently found experimental demonstration. The spectral properties of the emitted radiation reflect the critical nature of the system and allow us to link the detection of DCE to the Kibble-Zurek mechanism for the production of defects when crossing a continuous phase transition. © 2012 The Royal Swedish Academy of Sciences.
Thu, 01 Nov 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985812012-11-01T00:00:00Z
- Necessary and sufficient condition for nonzero quantum discordhttps://scholarbank.nus.edu.sg/handle/10635/97304Title: Necessary and sufficient condition for nonzero quantum discord
Authors: Dakić, B.; Vedral, V.; Brukner, C.
Abstract: Quantum discord characterizes "nonclassicality" of correlations in quantum mechanics. It has been proposed as the key resource present in certain quantum communication tasks and quantum computational models without containing much entanglement. We obtain a necessary and sufficient condition for the existence of nonzero quantum discord for any dimensional bipartite states. This condition is easily experimentally implementable. Based on this, we propose a geometrical way of quantifying quantum discord. For two qubits this results in a closed form of expression for discord. We apply our results to the model of deterministic quantum computation with one qubit, showing that quantum discord is unlikely to be the reason behind its speedup. © 2010 The American Physical Society.
Tue, 02 Nov 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/973042010-11-02T00:00:00Z
- How much of one-way computation is just thermodynamics?https://scholarbank.nus.edu.sg/handle/10635/96836Title: How much of one-way computation is just thermodynamics?
Authors: Anders, J.; Hajdušek, M.; Markham, D.; Vedral, V.
Abstract: In this paper we argue that one-way quantum computation can be seen as a form of phase transition with the available information about the solution of the computation being the order parameter. We draw a number of striking analogies between standard thermodynamical quantities such as energy, temperature, work, and corresponding computational quantities such as the amount of entanglement, time, potential capacity for computation, respectively. Aside from being intuitively pleasing, this picture allows us to make novel conjectures, such as an estimate of the necessary critical time to finish a computation and a proposal of suitable architectures for universal one-way computation in 1D. © 2008 Springer Science+Business Media, LLC.
Sun, 01 Jun 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/968362008-06-01T00:00:00Z
- Entanglement production in non-equilibrium thermodynamicshttps://scholarbank.nus.edu.sg/handle/10635/96493Title: Entanglement production in non-equilibrium thermodynamics
Authors: Vedral, V.
Abstract: We define and analyse the concept of entanglement production during the evolution of a general quantum mechanical dissipative system. While it is important to minimise entropy production in order to achieve thermodynamical efficiency, maximising the rate of change of entanglement is important in quantum information processing. Quantitative relations are obtained between entropy and entanglement productions, under specific assumptions detailed in the text. We apply these to the processes of dephasing and decay of correlations between two initially entangled qubits. Both the Master equation treatment as well as the higher Hilbert space analysis are presented. Our formalism is very general and contains as special cases many reported individual instance of entanglement dynamics, such as, for example, the recently discovered notion of the sudden death of entanglement. © 2009 IOP Publishing Ltd.
Thu, 01 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/964932009-01-01T00:00:00Z
- Behavior of entanglement and Cooper pairs under relativistic boostshttps://scholarbank.nus.edu.sg/handle/10635/95861Title: Behavior of entanglement and Cooper pairs under relativistic boosts
Authors: Palge, V.; Vedral, V.; Dunningham, J.A.
Abstract: Recent work has shown how single-particle entangled states are transformed when boosted in relativistic frames for certain restricted geometries. Here we extend that work to consider completely general inertial boosts. We then apply our single-particle results to multiparticle entanglements by focusing on Cooper pairs of electrons. We show that a standard Cooper pair state consisting of a spin-singlet acquires spin-triplet components in a relativistically boosted inertial frame, regardless of the geometry. We also show that, if we start with a spin-triplet pair, two out of the three triplet states acquire a singlet component, the size of which depends on the geometry. This transformation between the different singlet and triplet superconducting pairs may lead to a better understanding of unconventional superconductivity. © 2011 American Physical Society.
Fri, 14 Oct 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/958612011-10-14T00:00:00Z
- Quantum Correlations in Mixed-State Metrologyhttps://scholarbank.nus.edu.sg/handle/10635/116547Title: Quantum Correlations in Mixed-State Metrology
Authors: Modi, K.; Cable, H.; Williamson, M.; Vedral, V.
Abstract: We analyze the effects of quantum correlations, such as entanglement and discord, on the efficiency of phase estimation by studying four quantum circuits that can be readily implemented using NMR techniques. These circuits define a standard strategy of repeated single-qubit measurements, a classical strategy where only classical correlations are allowed, and two quantum strategies where nonclassical correlations are allowed. In addition to counting space (number of qubits) and time (number of gates) requirements, we introduce mixedness as a key constraint of the experiment.We compare the efficiency of the four strategies as a function of the mixedness parameter. We find that the quantum strategy gives ffiffiffiffi N p enhancement over the standard strategy for the same amount of mixedness. This result applies even for highly mixed states that have nonclassical correlations but no entanglement.
Sat, 01 Jan 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1165472011-01-01T00:00:00Z
- Quantum correlation without classical correlationshttps://scholarbank.nus.edu.sg/handle/10635/112493Title: Quantum correlation without classical correlations
Authors: Kaszlikowski, D.; Sen, A.; Sen, U.; Vedral, V.; Winter, A.
Abstract: We show that genuine multiparty quantum correlations can exist on its own, without a supporting background of genuine multiparty classical correlations, even in macroscopic systems. Such possibilities can have important implications in the physics of quantum information and phase transitions. © 2008 The American Physical Society.
Wed, 13 Aug 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1124932008-08-13T00:00:00Z
- Quantum mechanics can reduce the complexity of classical modelshttps://scholarbank.nus.edu.sg/handle/10635/97692Title: Quantum mechanics can reduce the complexity of classical models
Authors: Gu, M.; Wiesner, K.; Rieper, E.; Vedral, V.
Abstract: Mathematical models are an essential component of quantitative science. They generate predictions about the future, based on information available in the present. In the spirit of simpler is better; should two models make identical predictions, the one that requires less input is preferred. Yet, for almost all stochastic processes, even the provably optimal classical models waste information. The amount of input information they demand exceeds the amount of predictive information they output. Here we show how to systematically construct quantum models that break this classical bound, and that the system of minimal entropy that simulates such processes must necessarily feature quantum dynamics. This indicates that many observed phenomena could be significantly simpler than classically possible should quantum effects be involved. © 2012 Macmillan Publishers Limited. All rights reserved.
Sun, 01 Jan 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/976922012-01-01T00:00:00Z
- Entanglement in disordered and non-equilibrium systemshttps://scholarbank.nus.edu.sg/handle/10635/96486Title: Entanglement in disordered and non-equilibrium systems
Authors: Hide, J.; Vedral, V.
Abstract: We calculate an entanglement witness for a disordered spin system using the method of functional many-body perturbation theory, comparing the effect of taking a quenched and an annealed average over the disorder. We find, on considering the example of an XX Heisenberg spin chain with a Dzyaloshinskii-Moriya interaction, that disorder in the Dzyaloshinskii-Moriya interaction increases the region of entanglement detected by the witness. We also discuss a method of detecting entanglement in far from equilibrium systems. © 2009 Elsevier B.V. All rights reserved.
Fri, 01 Jan 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/964862010-01-01T00:00:00Z
- Heat capacity as an indicator of entanglementhttps://scholarbank.nus.edu.sg/handle/10635/117034Title: Heat capacity as an indicator of entanglement
Authors: Wieśniak, M.; Vedral, V.; Brukner, C.
Abstract: We demonstrate that the presence of entanglement in macroscopic bodies (e.g., solids) in thermodynamical equilibrium could be revealed by measuring heat capacity. The idea is that if the system was in a separable state, then for certain Hamiltonians heat capacity would not tend asymptotically to zero as the temperature approaches absolute zero. Since this would contradict the third law of thermodynamics, one concludes that the system must contain entanglement. The separable bounds are obtained by minimalization of the heat capacity over separable states and using its universal low-temperature behavior. Our results open up a possibility to use standard experimental techniques of solid-state physics-namely, heat-capacity measurements-to detect entanglement in macroscopic samples. © 2008 The American Physical Society.
Tue, 12 Aug 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1170342008-08-12T00:00:00Z
- Unifying Typical Entanglement and Coin Tossing: On Randomization in Probabilistic Theorieshttps://scholarbank.nus.edu.sg/handle/10635/98522Title: Unifying Typical Entanglement and Coin Tossing: On Randomization in Probabilistic Theories
Authors: Müller, M.P.; Dahlsten, O.C.O.; Vedral, V.
Abstract: It is well-known that pure quantum states are typically almost maximally entangled, and thus have close to maximally mixed subsystems. We consider whether this is true for probabilistic theories more generally, and not just for quantum theory. We derive a formula for the expected purity of a subsystem in any probabilistic theory for which this quantity is well-defined. It applies to typical entanglement in pure quantum states, coin tossing in classical probability theory, and randomization in post-quantum theories; a simple generalization yields the typical entanglement in (anti)symmetric quantum subspaces. The formula is exact and simple, only containing the number of degrees of freedom and the information capacity of the respective systems. It allows us to generalize statistical physics arguments in a way which depends only on coarse properties of the underlying theory. The proof of the formula generalizes several randomization notions to general probabilistic theories. This includes a generalization of purity, contributing to the recent effort of finding appropriate generalized entropy measures. © 2012 Springer-Verlag Berlin Heidelberg.
Sun, 01 Jan 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985222012-01-01T00:00:00Z
- Positive phase space transformation incompatible with classical physicshttps://scholarbank.nus.edu.sg/handle/10635/117122Title: Positive phase space transformation incompatible with classical physics
Authors: Son, W.; Kofler, J.; Kim, M.S.; Vedral, V.; Brukner, Č.
Abstract: Bell conjectured that a positive Wigner function does not allow violation of the inequalities imposed by local hidden variable theories. A requirement for this conjecture is "when phase space measurements are performed." We introduce the theory-independent concept of "operationally local transformations" which refers to the change of the switch on a local measurement apparatus. We show that two separated parties, performing only phase space measurements on a composite quantum system with a positive Wigner function and performing only operationally local transformations that preserve this positivity, can nonetheless violate Bell's inequality. Such operationally local transformations are realized using entangled ancillae. © 2009 The American Physical Society.
Fri, 20 Mar 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1171222009-03-20T00:00:00Z
- Extreme nonlocality with one photonhttps://scholarbank.nus.edu.sg/handle/10635/96584Title: Extreme nonlocality with one photon
Authors: Heaney, L.; Cabello, A.; Santos, M.F.; Vedral, V.
Abstract: Quantum nonlocality is typically assigned to systems of two or more well-separated particles, but nonlocality can also exist in systems consisting of just a single particle when one considers the subsystems to be distant spatial field modes. Single particle nonlocality has been confirmed experimentally via a bipartite Bell inequality. In this paper, we introduce an N-party Hardy-like proof of the impossibility of local elements of reality and a Bell inequality for local realistic theories in the case of a single particle superposed symmetrically over N spatial field modes (i.e. N qubit W state). We show that, in the limit of large N, the Hardy-like proof effectively becomes an all-versus-nothing (or Greenberger-Horne-Zeilinger (GHZ)-like) proof, and the quantum-classical gap of the Bell inequality tends to be the same as that in a three-particle GHZ experiment. We describe how to test the nonlocality in realistic systems. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Sun, 01 May 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/965842011-05-01T00:00:00Z
- Quantum nonlocality test by spectral joint measurements of qubits in driven cavityhttps://scholarbank.nus.edu.sg/handle/10635/97699Title: Quantum nonlocality test by spectral joint measurements of qubits in driven cavity
Authors: Yuan, H.; Wei, L.F.; Huang, J.S.; Vedral, V.
Abstract: We propose a feasible approach to test quantum nonlocality with two qubits dispersively coupled to a driven cavity. Our proposal is based on spectral joint measurements of two qubits, i.e., their quantum states in the computational basis states {|kl,k,l=0,1} can be measured nondestructively by detecting the steady-state transmission spectra of the driven cavity. With this kind of measurements, the existence of Bell state can be robustly confirmed instead of conventional quantum state tomography. Then this kind of measurements is further utilized to test CHSH-Bell inequality. The advantage and feasibility of our proposal are also discussed. © Copyright EPLA, 2012.
Mon, 01 Oct 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/976992012-10-01T00:00:00Z
- Information-theoretic lower bound on energy cost of stochastic computationhttps://scholarbank.nus.edu.sg/handle/10635/96933Title: Information-theoretic lower bound on energy cost of stochastic computation
Authors: Wiesner, K.; Gu, M.; Rieper, E.; Vedral, V.
Abstract: Physical systems are often simulated using a stochastic computation where different final states result from identical initial states. Here, we derive the minimum energy cost of simulating a data sequence of a general physical system by stochastic computation. We show that the cost is proportional to the difference between two informationtheoretic measures of complexity of the data-the statistical complexity and the predictive information. We derive the difference as the amount of information erased during the computation. Finally, we illustrate the physics of information by implementing the stochastic computation as a Gedanken experiment with a Szilard-type engine. The results create a new link between thermodynamics, information theory and complexity. © 2012 The Royal Society.
Sat, 08 Dec 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/969332012-12-08T00:00:00Z
- The Elusive Source of Quantum Speeduphttps://scholarbank.nus.edu.sg/handle/10635/98267Title: The Elusive Source of Quantum Speedup
Authors: Vedral, V.
Abstract: We discuss two qualities of quantum systems: various correlations existing between their subsystems and the distinguishability of different quantum states. This is then applied to analysing quantum information processing. While quantum correlations, or entanglement, are clearly of paramount importance for efficient pure state manipulations, mixed states present a much richer arena and reveal a more subtle interplay between correlations and distinguishability. The current work explores a number of issues related with identifying the important ingredients needed for quantum information processing. We discuss the Deutsch-Jozsa algorithm, the Shor algorithm, the Grover algorithm and the power of a single qubit class of algorithms. In the latter, a quantity called discord is seen to be more important than entanglement. One section is dedicated to cluster states where entanglement is crucial, but its precise role is highly counter-intuitive. Here we see that the notion of distinguishability becomes a more useful concept. © 2010 Springer Science+Business Media, LLC.
Fri, 01 Jan 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/982672010-01-01T00:00:00Z
- Classical and quantum correlations under decoherencehttps://scholarbank.nus.edu.sg/handle/10635/95991Title: Classical and quantum correlations under decoherence
Authors: Maziero, J.; Céleri, L.C.; Serra, R.M.; Vedral, V.
Abstract: Recently some authors have pointed out that there exist nonclassical correlations which are more general, and possibly more fundamental, than entanglement. For these general quantum correlations and their classical counterparts, under the action of decoherence, we identify three general types of dynamics that include a peculiar sudden change in their decay rates. We show that, under suitable conditions, the classical correlation is unaffected by decoherence. Such dynamic behavior suggests an operational measure of both classical and quantum correlations that can be computed without any extremization procedure. © 2009 The American Physical Society.
Wed, 07 Oct 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/959912009-10-07T00:00:00Z
- Wigner rotations and an apparent paradox in relativistic quantum informationhttps://scholarbank.nus.edu.sg/handle/10635/98587Title: Wigner rotations and an apparent paradox in relativistic quantum information
Authors: Saldanha, P.L.; Vedral, V.
Abstract: It is shown that a general model for particle detection in combination with a linear application of the Wigner rotations, which correspond to momentum-dependent changes of the particle spin under Lorentz transformations, to the state of a massive relativistic particle in a superposition of two counterpropagating momentum states leads to a paradox. The paradox entails the instantaneous transmission of information between two spatially separated parties. A solution to the paradox is given when the physical construction of the corresponding state is taken into account, suggesting that we cannot in general linearly apply the Wigner rotations to a quantum state without considering the appropriate physical interpretation. © 2013 American Physical Society.
Wed, 03 Apr 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985872013-04-03T00:00:00Z
- Comment on "quantum szilard engine"https://scholarbank.nus.edu.sg/handle/10635/96022Title: Comment on "quantum szilard engine"
Authors: Plesch, M.; Dahlsten, O.; Goold, J.; Vedral, V.
Abstract: A Comment on the Letter by S. W. Kim, Phys. Rev. Lett. 106, 070401 (2011).PRLTAO0031-900710.1103/PhysRevLett.106.070401 The authors of the Letter offer a Reply. © 2013 American Physical Society.
Wed, 30 Oct 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/960222013-10-30T00:00:00Z
- Kaszlikowski et al. Reply:https://scholarbank.nus.edu.sg/handle/10635/98981Title: Kaszlikowski et al. Reply:
Authors: Kaszlikowski, D.; Sen, A.; Sen, U.; Vedral, V.
Mon, 08 Dec 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/989812008-12-08T00:00:00Z
- Natural mode entanglement as a resource for quantum communicationhttps://scholarbank.nus.edu.sg/handle/10635/112468Title: Natural mode entanglement as a resource for quantum communication
Authors: Heaney, L.; Vedral, V.
Abstract: Natural particle-number entanglement resides between spatial modes in coherent ultracold atomic gases. However, operations on the modes are restricted by a superselection rule that forbids coherent superpositions of different particle numbers. This seemingly prevents mode entanglement being used as a resource for quantum communication. In this Letter, we demonstrate that mode entanglement of a single massive particle can be used for dense coding and quantum teleportation despite the superselection rule. In particular, we provide schemes where the dense coding linear photonic channel capacity is reached without a shared reservoir and where the full quantum channel capacity is achieved if both parties share a coherent particle reservoir. © 2009 The American Physical Society.
Wed, 11 Nov 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1124682009-11-11T00:00:00Z
- A framework for phase and interference in generalized probabilistic theorieshttps://scholarbank.nus.edu.sg/handle/10635/95621Title: A framework for phase and interference in generalized probabilistic theories
Authors: Garner, A.J.P.; Dahlsten, O.C.O.; Nakata, Y.; Murao, M.; Vedral, V.
Abstract: Phase plays a crucial role in many quantum effects including interference. Here we lay the foundations for the study of phase in probabilistic theories more generally. Phase is normally defined in terms of complex numbers that appear when representing quantum states as complex vectors. Here we give an operational definition whereby phase is instead defined in terms of measurement statistics. Our definition is phrased in terms of the operational framework known as generalized probabilistic theories or the convex framework. The definition makes it possible to ask whether other theories in this framework can also have phase. We apply our definition to investigate phase and interference in several example theories: classical probability theory, a version of Spekkens' toy model, quantum theory and box-world. We find that phase is ubiquitous; any non-classical theory can be said to have non-trivial phase dynamics. © IOP Publishing and Deutsche Physikalische Gesellschaft.
Sun, 01 Sep 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/956212013-09-01T00:00:00Z
- Detection and engineering of spatial mode entanglement with ultracold bosonshttps://scholarbank.nus.edu.sg/handle/10635/115062Title: Detection and engineering of spatial mode entanglement with ultracold bosons
Authors: Goold, J.; Heaney, L.; Busch, T.; Vedral, V.
Abstract: We outline an interferometric scheme for the detection of bimode and multimode spatial entanglement of finite-temperature interacting Bose gases. Whether entanglement is present in the gas depends on the existence of the single-particle reduced density matrix between different regions of space. We apply the scheme to the problem of a harmonically trapped repulsive boson pair and show that while entanglement is rapidly decreasing with temperature, a significant amount remains for all interaction strengths at zero temperature. Thus, by tuning the interaction parameter, the distribution of entanglement between many spatial modes can be modified. © 2009 The American Physical Society.
Fri, 28 Aug 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1150622009-08-28T00:00:00Z
- Classical to quantum in large-number limithttps://scholarbank.nus.edu.sg/handle/10635/116260Title: Classical to quantum in large-number limit
Authors: Modi, K.; Fazio, R.; Pascazio, S.; Vedral, V.; Yuasa, K.
Abstract: We construct a quantumness witness following the work of Alicki & van Ryn (AvR). We reformulate the AvR test by defining it for quantum states rather than for observables. This allows us to identify the necessary quantities and resources to detect quantumness for any given system. The first quantity turns out to be the purity of the system. When applying the witness to a system with even moderate mixedness, the protocol is unable to reveal any quantumness. We then show that having many copies of the system leads the witness to reveal quantumness. This seems contrary to the Bohr correspondence, which asserts that, in the large-number limit, quantum systems become classical, whereas the witness shows quantumness when several non-quantum systems, as determined by the witness, are considered together. However, the resources required to detect the quantumness increase dramatically with the number of systems. We apply the quantumness witness for systems that are highly mixed but in the large-number limit that resembles nuclear magnetic resonance (NMR) systems. We make several conclusions about detecting quantumness in NMR-like systems. © 2012 The Royal Society.
Sat, 13 Oct 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1162602012-10-13T00:00:00Z
- Photon production from the vacuum close to the superradiant transition: Linking the dynamical Casimir effect to the Kibble-Zurek mechanismhttps://scholarbank.nus.edu.sg/handle/10635/97543Title: Photon production from the vacuum close to the superradiant transition: Linking the dynamical Casimir effect to the Kibble-Zurek mechanism
Authors: Vacanti, G.; Pugnetti, S.; Didier, N.; Paternostro, M.; Palma, G.M.; Fazio, R.; Vedral, V.
Abstract: The dynamical Casimir effect (DCE) predicts the generation of photons from the vacuum due to the parametric amplification of the quantum fluctuations of an electromagnetic field. The verification of such an effect is still elusive in optical systems due to the very demanding requirements of its experimental implementation. We show that an ensemble of two-level atoms collectively coupled to the electromagnetic field of a cavity, driven at low frequencies and close to a quantum phase transition, stimulates the production of photons from the vacuum. This paves the way to an effective simulation of the DCE through a mechanism that has recently found experimental demonstration. The spectral properties of the emitted radiation reflect the critical nature of the system and allow us to link the detection of the DCE to the Kibble-Zurek mechanism for the production of defects when crossing a continuous phase transition. © 2012 American Physical Society.
Tue, 28 Feb 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/975432012-02-28T00:00:00Z
- Geometric-phase backaction in a mesoscopic qubit-oscillator systemhttps://scholarbank.nus.edu.sg/handle/10635/96723Title: Geometric-phase backaction in a mesoscopic qubit-oscillator system
Authors: Vacanti, G.; Fazio, R.; Kim, M.S.; Palma, G.M.; Paternostro, M.; Vedral, V.
Abstract: We illustrate a reverse Von Neumann measurement scheme in which a geometric phase induced on a quantum harmonic oscillator is measured using a microscopic qubit as a probe. We show how such a phase, generated by a cyclic evolution in the phase space of the harmonic oscillator, can be kicked back on the qubit, which plays the role of a quantum interferometer. We also extend our study to finite-temperature dissipative Markovian dynamics and discuss potential implementations in micro- and nanomechanical devices coupled to an effective two-level system. © 2012 American Physical Society.
Wed, 29 Feb 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/967232012-02-29T00:00:00Z
- Experimental demonstration of a unified framework for mixed-state geometric phaseshttps://scholarbank.nus.edu.sg/handle/10635/96552Title: Experimental demonstration of a unified framework for mixed-state geometric phases
Authors: Zhu, J.; Shi, M.; Vedral, V.; Peng, X.; Suter, D.; Du, J.
Abstract: Geometric phases have been found in every major branch of physics and play an important role in mathematics and quantum computation. Here, we unify two proposed definitions of the geometric phase in mixed states - Uhlmann's phase and Sjöqvist's phase - in a new formalism based on interferometry and further provide an experimental demonstration in NMR. This is also the first experimental measurement of Uhlmann's geometric phase. © 2011 Europhysics Letters Association.
Fri, 01 Apr 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/965522011-04-01T00:00:00Z
- Detecting entanglement with a thermometerhttps://scholarbank.nus.edu.sg/handle/10635/96183Title: Detecting entanglement with a thermometer
Authors: Anders, J.; Kaszlikowski, D.; Lunkes, C.; Ohshima, T.; Vedral, V.
Abstract: We present a general argument showing that the temperature as well as other thermodynamical state variables can qualify as entanglement witnesses (EWs) for spatial entanglement. This holds for a variety of systems and we exemplify our ideas using a simple free non-interacting bosonic gas. We find that entanglement can exist at arbitrarily high temperatures, provided that we can probe smaller and smaller regions of space. We then discuss the relationship between the occurrence of Bose-Einstein condensation and our conditions for the presence of entanglement and compare the respective critical temperatures. We close with a short discussion of the idea of seeing entanglement as a macroscopic property in thermodynamical systems and its possible relation to phase transitions in general. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Fri, 18 Aug 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/961832006-08-18T00:00:00Z
- Local characterization of one-dimensional topologically ordered stateshttps://scholarbank.nus.edu.sg/handle/10635/97081Title: Local characterization of one-dimensional topologically ordered states
Authors: Cui, J.; Amico, L.; Fan, H.; Gu, M.; Hamma, A.; Vedral, V.
Abstract: We consider one-dimensional Hamiltonian systems whose ground states display symmetry-protected topological order. We show that ground states within the topological phase cannot be connected with each other through local operations and classical communication between a bipartition of the system. Our claim is demonstrated by analyzing the entanglement spectrum and Rényi entropies of different physical systems that provide examples for symmetry-protected topological phases. Specifically, we consider the spin-1/2 cluster-Ising model and a class of spin-1 models undergoing quantum phase transitions to the Haldane phase. Our results provide a probe for symmetry-protected topological order. Since the picture holds even at the system's local scale, our analysis can serve as a local experimental test for topological order. © 2013 American Physical Society.
Wed, 11 Sep 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/970812013-09-11T00:00:00Z
- Sustained quantum coherence and entanglement in the avian compasshttps://scholarbank.nus.edu.sg/handle/10635/112524Title: Sustained quantum coherence and entanglement in the avian compass
Authors: Gauger, E.M.; Rieper, E.; Morton, J.J.L.; Benjamin, S.C.; Vedral, V.
Abstract: In artificial systems, quantum superposition and entanglement typically decay rapidly unless cryogenic temperatures are used. Could life have evolved to exploit such delicate phenomena? Certain migratory birds have the ability to sense very subtle variations in Earth's magnetic field. Here we apply quantum information theory and the widely accepted "radical pair" model to analyze recent experimental observations of the avian compass. We find that superposition and entanglement are sustained in this living system for at least tens of microseconds, exceeding the durations achieved in the best comparable man-made molecular systems. This conclusion is starkly at variance with the view that life is too "warm and wet" for such quantum phenomena to endure. © 2011 American Physical Society.
Tue, 25 Jan 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1125242011-01-25T00:00:00Z
- Nonlocality of a single particlehttps://scholarbank.nus.edu.sg/handle/10635/97354Title: Nonlocality of a single particle
Authors: Dunningham, J.; Vedral, V.
Abstract: There has been a great deal of debate surrounding the issue of whether it is possible for a single photon to exhibit nonlocality. A number of schemes have been proposed that claim to demonstrate this effect, but each has been met with significant opposition. The objections hinge largely on the fact that these schemes use unobservable initial states and so, it is claimed, they do not represent experiments that could actually be performed. Here we show how it is possible to overcome these objections by presenting an experimentally feasible scheme that uses realistic initial states. Furthermore, all the techniques required for photons are equally applicable to atoms. It should, therefore, also be possible to use this scheme to verify the nonlocality of a single massive particle. © 2007 The American Physical Society.
Fri, 02 Nov 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/973542007-11-02T00:00:00Z
- Quantum phases with differing computational powerhttps://scholarbank.nus.edu.sg/handle/10635/112504Title: Quantum phases with differing computational power
Authors: Cui, J.; Gu, M.; Kwek, L.C.; Santos, M.F.; Fan, H.; Vedral, V.
Abstract: The observation that concepts from quantum information has generated many alternative indicators of quantum phase transitions hints that quantum phase transitions possess operational significance with respect to the processing of quantum information. Yet, studies on whether such transitions lead to quantum phases that differ in their capacity to process information remain limited. Here we show that there exist quantum phase transitions that cause a distinct qualitative change in our ability to simulate certain quantum systems under perturbation of an external field by local operations and classical communication. In particular, we show that in certain quantum phases of the XY model, adiabatic perturbations of the external magnetic field can be simulated by local spin operations, whereas the resulting effect within other phases results in coherent non-local interactions. We discuss the potential implications to adiabatic quantum computation, where a computational advantage exists only when adiabatic perturbation results in coherent multi-body interactions. © 2012 Macmillan Publishers Limited. All rights reserved.
Sun, 01 Jan 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1125042012-01-01T00:00:00Z
- Classification of macroscopic quantum effectshttps://scholarbank.nus.edu.sg/handle/10635/127811Title: Classification of macroscopic quantum effects
Authors: Farrow T; Vedral V
Thu, 01 Jan 2015 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1278112015-01-01T00:00:00Z
- Survival of entanglement in thermal stateshttps://scholarbank.nus.edu.sg/handle/10635/98151Title: Survival of entanglement in thermal states
Authors: Markham, D.; Anders, J.; Vedral, V.; Murao, M.; Miyake, A.
Abstract: We present a general sufficiency condition for the presence of multipartite entanglement in thermal states stemming from the ground-state entanglement. The condition is written in terms of the ground-state entanglement and the partition function and it gives transition temperatures below which entanglement is guaranteed to survive. It is flexible and can be easily adapted to consider entanglement for different splittings, as well as be weakened to allow easier calculations by approximations. Examples where the condition is calculated are given. These examples allow us to characterize a minimum gapping behavior for the survival of entanglement in the thermodynamic limit. Further, the same technique can be used to find noise thresholds in the generation of useful resource states for one-way quantum computing. © Europhysics Letters Association.
Fri, 01 Feb 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/981512008-02-01T00:00:00Z
- Global asymmetry of many-qubit correlations: A lattice-gauge-theory approachhttps://scholarbank.nus.edu.sg/handle/10635/112446Title: Global asymmetry of many-qubit correlations: A lattice-gauge-theory approach
Authors: Williamson, M.S.; Ericsson, M.; Johansson, M.; Sjöqvist, E.; Sudbery, A.; Vedral, V.
Abstract: We introduce a bridge between the familiar gauge field theory approaches used in many areas of modern physics such as quantum field theory and the stochastic local operations and classical communication protocols familiar in quantum information. Although the mathematical methods are the same, the meaning of the gauge group is different. The measure we introduce, "twist," is constructed as a Wilson loop from a correlation-induced holonomy. The measure can be understood as the global asymmetry of the bipartite correlations in a loop of three or more qubits; if the holonomy is trivial (the identity matrix), the bipartite correlations can be globally untwisted using general local qubit operations, the gauge group of our theory, which turns out to be the group of Lorentz transformations familiar from special relativity. If it is not possible to globally untwist the bipartite correlations in a state using local operations, the twistedness is given by a nontrivial element of the Lorentz group, the correlation-induced holonomy. We provide several analytical examples of twisted and untwisted states for three qubits, the most elementary nontrivial loop one can imagine. © 2011 American Physical Society.
Fri, 02 Sep 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1124462011-09-02T00:00:00Z