Nonlocality, Entanglement Witnesses and Supra-Correlations
Paul Alsing United States Air Force Research Laboratory
PIRSA:12060076
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Towards Universal Quantum Computation Out of Relativistic Motion of Particle Detectors
David Aasen California Institute of Technology
PIRSA:12060049 -
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Tripartite Entanglement, Svetlichny Inequalities, and Non-inertial Observers
Robert Mann University of Waterloo
PIRSA:12060042 -
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Quantum Teleportation from Alice to Rob in Vacuum
Shih-Yuin Lin National Changhua University of Education
PIRSA:12060047 -
Physical Results with Fermions in RQI
PIRSA:12060046 -
Entanglement of a Relativistic Field in the Vacuum State
Benni Reznik Tel Aviv University
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The Quest for Localization
Juan José León García Instituto de Física Fundamental (IFF - CSIC)
PIRSA:12060044 -
PSI Graduation - 2012
PIRSA:12060081 -
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Nonlocality, Entanglement Witnesses and Supra-Correlations
Paul Alsing United States Air Force Research Laboratory
PIRSA:12060076While entanglement is believed to underlie the power of quantum computation and communication, it is not generally well understood for multipartite systems. Recently, it has been appreciated that there exists proper no-signaling probability distributions derivable from operators that do not represent valid quantum states. Such systems exhibit supra-correlations that are stronger than allowed by quantum mechanics, but less than the algebraically allowed maximum in Bell-inequalities (in the bipartite case). Some of these probability distributions are derivable from an entanglement witness W, which is a non-positive Hermitian operator constructed such that its expectation value with a separable quantum state (positive density matrix) rho_sep is non-negative (so that Tr[W rho]< 0 indicates entanglement in quantum state rho). In the bipartite case, it is known that by a modification of the local no-signaling measurements by spacelike separated parties A and B, the supra-correlations exhibited by any W can be modeled as derivable from a physically realizable quantum state ρ. However, this result does not generalize to the n-partite case for n>2. Supra-correlations can also be exhibited in 2- and 3-qubit systems by explicitly constructing "states" O (not necessarily positive quantum states) that exhibit PR correlations for a fixed, but arbitrary number, of measurements available to each party. In this paper we examine the structure of "states" that exhibit supra-correlations. In addition, we examine the affect upon the distribution of the correlations amongst the parties involved when constraints of positivity and purity are imposed. We investigate circumstances in which such "states" do and do not represent valid quantum states. -
Towards Universal Quantum Computation Out of Relativistic Motion of Particle Detectors
David Aasen California Institute of Technology
PIRSA:12060049 -
Boundary Effects on Quantum Entanglement and its Dynamics in a Detector-Field System
PIRSA:12060074We analyze an exactly solvable model consisting of an inertial Unruh-DeWitt detector which interacts linearly with a massless quantum field in Minkowski spacetime with a perfectly reflecting flat plane boundary. This model is related to proposed mirror-field superposition and relevant experiments in macroscopic quantum phenomena, as well as atomic fluctuation forces near a conducting surface. Firstly a coupled set of equations for the detector’s and the field’s Heisenberg operators are derived. After coarse graining the field, the dynamics of the detector’s internal degreeof freedom is described by a quantum Langevin equation, where the dissipation and noise kernels respectively correspond to the retarded Green’s functions and Hadamard elementary functions of the free quantum field in half space. We use the linear entropy as measures of entanglement between the detector and the quantum field under mirror reflection, then solve the early-time detector-fieldentanglement dynamics. At late times when the combined system is in a stationary state, we obtain exact expressions for the detector’s covariance matrix and show that the detector-field entanglement decreases for smaller separation between the detector and the mirror.We explain the behavior of detector-field entanglement qualitatively with the help of a detector’s mirror image, compare them with the case of two real detectors and explain the differences. -
Tripartite Entanglement, Svetlichny Inequalities, and Non-inertial Observers
Robert Mann University of Waterloo
PIRSA:12060042I discuss the behaviour of bipartite and tripartite non-locality between fermionic entangled states shared by observers, one of whom uniformly accelerates. Although fermionic entanglement persists for arbitrarily large acceleration, the Bell/CHSH inequalities cannot be violated for sufficiently large but finite acceleration. However the Svetlichny inequality, which is a measure of genuine tripartite non-locality, can be violated for any finite value of the acceleration. -
Geometric Discord in Non-Inertial Frames
Eric Brown University of Chicago
PIRSA:12060058I review the recent work performed on computing the geometric discord in non-inertial frames. We consider the well-known case of an inertially maximally entangled state shared by inertial Alice and non-inertial Robb. It is found that for high accelerations the geometric discord decays to a negligible amount; this is in stark contrast to the entropic definition of quantum discord which asymptotes to a finite value in the same limit. Such a result has two different implications: the first being that usable quantum correlations are more limited in this regime than previously thought and the second being that geometric discord may not be a sufficient measure of quantum correlations. I will discuss both of these perspectives. -
Quantum Teleportation from Alice to Rob in Vacuum
Shih-Yuin Lin National Changhua University of Education
PIRSA:12060047We consider quantum teleportation of continuous variables in a relativistic system with the Unruh-DeWitt detectorscoupled to a common quantum field initially in the Minkowski vacuum. An unknown coherent state of an Unruh-DeWitt detector is teleported from one inertial agent (Alice) to an almost uniformly accelerated agent (Rob), using a detector pair initially entangled and shared by these two agents. Results for the averaged physical fidelity of quantum teleportation will be discussed. -
Physical Results with Fermions in RQI
PIRSA:12060046A number of works in the field of relativistic quantum information have been devoted to the study of entanglement on certain simple families of Unruh-mode entangled states in non-inertial frames. In the fermionic case remarkable results such as the survival of entanglement at infinite acceleration have been obtained. In this talk we will present and analyze some issues related to the anticommuting character of fermionic field operators, which have been overlooked in the past, sometimes leading to unphysical results. We provide a simple way of obtaining physical results, yielding interesting consequences such as convergence of field entanglement for different families of Unruh mode-entangled states in the infinite acceleration limit. -
Entanglement of a Relativistic Field in the Vacuum State
Benni Reznik Tel Aviv University
We discuss gedanken experiments for measuring local and non-local observables in QFT that
respect causality, and can by used to test the entanglement between two spatially distant regions in the vacuum. It is shown that the entanglement decays exponentially with the distance between the regions and does not vanish, in contrast to the case of lattice models. We discuss in this respect a possible mechanism which might explain this persistence effect, and a connection between the Reeh-Schlieder theorem and superoscillations. -
The Quest for Localization
Juan José León García Instituto de Física Fundamental (IFF - CSIC)
PIRSA:12060044This talk aims to review the obstacles met in QFT to reach an appropriate definition for such a basic concept as localization. The anti-local character of the square root of the "- Laplace-Beltrami + mass^2" operator prevents the existence of localized states with a finite number of quanta. (Bosonic) quantum fields describe elementary excitations of an extended system whose ground state is the vacuum. No wonder, there is a complicated relationship between the cardinal (quantal) and continuous (spatial) sides of the theory. We will also analyze the roles (if any) played in RQI by the localized excitations of the vacuum. -
PSI Graduation - 2012
PIRSA:12060081 -
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Pyrochlore Material Yb_2Ti_2O_7: A Model Quantum Spin Ice
We discuss the thermodynamic properties of the model exchange quantum spin ice material Yb_2Ti_2O_7. Using exchange parameters recently determined from high-field neutron scattering measurements, we calculate the thermodynamic properties of this model system. We find very good agreement with the heat capacity, entropy and magnetization measurements on the materials. We show that, in the weak quantum regime, quantum fluctuations lead to the selection, within the spin-ice manifold, of a conventional ordered ground state. However, the excitations above the ground state and their dynamics remain highly non-trivial. They consist of weakly bound spinon-antispinon pairs separated by long strings. We present several pieces of indirect evidence that the low temperature phase transition observed in these materials is to this conventional ordered state.