ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Wed, 11 Dec 2019 21:10:04 GMT2019-12-11T21:10:04Z5081- Violation of multipartite Bell inequalities with classical subsystems via operationally local transformationshttps://scholarbank.nus.edu.sg/handle/10635/112543Title: Violation of multipartite Bell inequalities with classical subsystems via operationally local transformations
Authors: Williamson, M.S.; Heaney, L.; Son, W.
Abstract: Recently, it was demonstrated by Son, Phys. Rev. Lett.PRLTAO0031-900710. 1103/PhysRevLett.102.110404 102, 110404 (2009), that a separable bipartite continuous-variable quantum system can violate the Clauser-Horne-Shimony-Holt (CHSH) inequality via operationally local transformations. Operationally local transformations are parametrized only by local variables; however, in order to allow violation of the CHSH inequality, a maximally entangled ancilla was necessary. The use of the entangled ancilla in this scheme caused the state under test to become dependent on the measurement choice one uses to calculate the CHSH inequality, thus violating one of the assumptions used in deriving a Bell inequality, namely, the free will or statistical independence assumption. The novelty in this scheme however is that the measurement settings can be external free parameters. In this paper, we generalize these operationally local transformations for multipartite Bell inequalities (with dichotomic observables) and provide necessary and sufficient conditions for violation within this scheme. Namely, a violation of a multipartite Bell inequality in this setting is contingent on whether an ancillary system admits any realistic local hidden variable model (i.e., whether the ancilla violates the given Bell inequality). These results indicate that violation of a Bell inequality performed on a system does not necessarily imply that the system is nonlocal. In fact, the system under test may be completely classical. However, nonlocality must have resided somewhere, this may have been in the environment, the physical variables used to manipulate the system or the detectors themselves provided the measurement settings are external free variables. © 2010 The American Physical Society.
Thu, 09 Sep 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1125432010-09-09T00:00:00Z
- 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
- Quantum phase transition between cluster and antiferromagnetic stateshttps://scholarbank.nus.edu.sg/handle/10635/53123Title: Quantum phase transition between cluster and antiferromagnetic states
Authors: Son, W.; Amico, L.; Fazio, R.; Hamma, A.; Pascazio, S.; Vedral, V.
Abstract: We study a Hamiltonian system describing a three-spin-1/2 cluster-like interaction competing with an Ising-like exchange. We show that the ground state in the cluster phase possesses symmetry protected topological order. A continuous quantum phase transition occurs as result of the competition between the cluster and Ising terms. At the critical point the Hamiltonian is self-dual. The geometric entanglement is also studied and used to investigate the quantum phase transition. Our findings in one dimension corroborate the analysis of the two-dimensional generalization of the system, indicating, at a mean-field level, the presence of a direct transition between an antiferromagnetic and a valence bond solid ground state. © 2011 EPLA.
Thu, 01 Sep 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/531232011-09-01T00:00:00Z
- Enhancing the detection of natural thermal entanglement with disorderhttps://scholarbank.nus.edu.sg/handle/10635/52915Title: Enhancing the detection of natural thermal entanglement with disorder
Authors: Hide, J.; Son, W.; Vedral, V.
Abstract: Physical systems have some degree of disorder present in them. We discuss how to treat natural, thermal entanglement in any random macroscopic system from which a thermodynamic witness bounded by a constant can be found. We propose that functional many-body perturbation theory be applied to allow either a quenched or an annealed average over the disorder to be taken. We find, when considering the example of an XX Heisenberg spin chain with a random coupling strength, that the region of natural entanglement detected by both witnesses can be enhanced by the disorder. © 2009 The American Physical Society.
Mon, 09 Mar 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/529152009-03-09T00:00:00Z
- Quantum instability and edge entanglement in the quasi-long-range orderhttps://scholarbank.nus.edu.sg/handle/10635/53122Title: Quantum instability and edge entanglement in the quasi-long-range order
Authors: Son, W.; Amico, L.; Plastina, F.; Vedral, V.
Abstract: We investigate the buildup of quasi-long-range order in the XX chain with a transverse magnetic field at finite size. As the field is varied, the ground state of the system displays multiple level crossings producing a sequence of entanglement jumps. Using the partial fidelity and susceptibility, we study the transition to the thermodynamic limit and argue that the topological order can be described in terms of kink-antikink pairs and marked by edge spin entanglement. © 2009 The American Physical Society.
Mon, 02 Feb 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/531222009-02-02T00:00:00Z
- Unified view of quantum and classical correlationshttps://scholarbank.nus.edu.sg/handle/10635/116020Title: Unified view of quantum and classical correlations
Authors: Modi, K.; Paterek, T.; Son, W.; Vedral, V.; Williamson, M.
Abstract: We discuss the problem of the separation of total correlations in a given quantum state into entanglement, dissonance, and classical correlations using the concept of relative entropy as a distance measure of correlations. This allows us to put all correlations on an equal footing. Entanglement and dissonance, whose definition is introduced here, jointly belong to what is known as quantum discord. Our methods are completely applicable for multipartite systems of arbitrary dimensions. We investigate additivity relations between different correlations and show that dissonance may be present in pure multipartite states. © 2010 The American Physical Society.
Mon, 22 Feb 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1160202010-02-22T00:00:00Z
- Generating topological order from a two-dimensional cluster state using a duality mappinghttps://scholarbank.nus.edu.sg/handle/10635/117026Title: Generating topological order from a two-dimensional cluster state using a duality mapping
Authors: Brown, B.J.; Son, W.; Kraus, C.V.; Fazio, R.; Vedral, V.
Abstract: In this paper, we prove, extend and review possible mappings between the two-dimensional (2D) cluster state, Wen's model, the 2D Ising chain and Kitaev's toric code model. We introduce a 2D duality transformation to map the 2D lattice cluster state into the topologically ordered Wen model. Then, we investigate how this mapping could be achieved physically, which allows us to discuss the rate at which a topologically ordered system can be achieved. Next, using a lattice fermionization method, Wen's model is mapped into a series of 1D Ising interactions. Considering the boundary terms with this mapping then reveals how the Ising chains interact with one another. The duality of these models can be taken as a starting point to address questions as to how their gate operations in different quantum computational models can be related to each other. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Wed, 01 Jun 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1170262011-06-01T00:00:00Z
- Quantum criticality of ground and thermal states in XX modelhttps://scholarbank.nus.edu.sg/handle/10635/53313Title: Quantum criticality of ground and thermal states in XX model
Authors: Son, W.; Vedral, V.
Abstract: Here, we compare the critical behaviour of the ground state and the thermal state of the XX model. We analyze full energy spectra and the exact form of eigenstates to reconstruct the ground state and the thermally excited state. With the solutions, we discuss the properties of quantum phase transition at zero temperature as well as the state in a thermal equilibrium. © 2009 World Scientific Publishing Company.
Tue, 01 Sep 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/533132009-09-01T00:00:00Z