706 research outputs found
On the experimental feasibility of continuous-variable optical entanglement distillation
Entanglement distillation aims at preparing highly entangled states out of a
supply of weakly entangled pairs, using local devices and classical
communication only. In this note we discuss the experimentally feasible schemes
for optical continuous-variable entanglement distillation that have been
presented in [D.E. Browne, J. Eisert, S. Scheel, and M.B. Plenio, Phys. Rev. A
67, 062320 (2003)] and [J. Eisert, D.E. Browne, S. Scheel, and M.B. Plenio,
Annals of Physics (NY) 311, 431 (2004)]. We emphasize their versatility in
particular with regards to the detection process and discuss the merits of the
two proposed detection schemes, namely photo-detection and homodyne detection,
in the light of experimental realizations of this idea becoming more and more
feasible.Comment: 5 pages, 5 figures, contribution to conference proceeding
Hot entanglement in a simple dynamical model
How mixed can one component of a bi-partite system be initially and still
become entangled through interaction with a thermalized partner? We address
this question here. In particular, we consider the question of how mixed a
two-level system and a field mode may be such that free entanglement arises in
the course of the time evolution according to a Jaynes-Cummings type
interaction. We investigate the situation for which the two-level system is
initially in mixed state taken from a one-parameter set, whereas the field has
been prepared in an arbitrary thermal state. Depending on the particular choice
for the initial state and the initial temperature of the quantised field mode,
three cases can be distinguished: (i) free entanglement will be created
immediately, (ii) free entanglement will be generated, but only at a later time
different from zero, (iii) the partial transpose of the joint state remains
positive at all times. It will be demonstrated that increasing the initial
temperature of the field mode may cause the joint state to become distillable
during the time evolution, in contrast to a non-distillable state at lower
initial temperatures. We further assess the generated entanglement
quantitatively, by evaluating the logarithmic negativity numerically, and by
providing an analytical upper bound.Comment: 5 pages, 2 figures. Contribution to the proceedings of the
'International Conference on Quantum Information', Oviedo, July 13-18, 2002.
Discusses sudden changes of entanglement properties in a dynamical quantum
mode
Classical Rules in Quantum Games
We consider two aspects of quantum game theory: the extent to which the
quantum solution solves the original classical game, and to what extent the new
solution can be obtained in a classical model.Comment: The previous title, "Quantum games are no fun (yet)", was too
whimsical for Physical Review. This is a comment on most, but not all, papers
on quantum game theor
Optimal entanglement witnesses for continuous-variable systems
This paper is concerned with all tests for continuous-variable entanglement
that arise from linear combinations of second moments or variances of canonical
coordinates, as they are commonly used in experiments to detect entanglement.
All such tests for bi-partite and multi-partite entanglement correspond to
hyperplanes in the set of second moments. It is shown that all optimal tests,
those that are most robust against imperfections with respect to some figure of
merit for a given state, can be constructed from solutions to semi-definite
optimization problems. Moreover, we show that for each such test, referred to
as entanglement witness based on second moments, there is a one-to-one
correspondence between the witness and a stronger product criterion, which
amounts to a non-linear witness, based on the same measurements. This
generalizes the known product criteria. The presented tests are all applicable
also to non-Gaussian states. To provide a service to the community, we present
the documentation of two numerical routines, FULLYWIT and MULTIWIT, which have
been made publicly available.Comment: 14 pages LaTeX, 1 figure, presentation improved, references update
Nash equilibria in quantum games with generalized two-parameter strategies
In the Eisert protocol for 2 X 2 quantum games [Phys. Rev. Lett. 83, 3077], a
number of authors have investigated the features arising from making the
strategic space a two-parameter subset of single qubit unitary operators. We
argue that the new Nash equilibria and the classical-quantum transitions that
occur are simply an artifact of the particular strategy space chosen. By
choosing a different, but equally plausible, two-parameter strategic space we
show that different Nash equilibria with different classical-quantum
transitions can arise. We generalize the two-parameter strategies and also
consider these strategies in a multiplayer setting.Comment: 19 pages, 2 eps figure
3-d tracking of shoes for virtual mirror applications
In this paper, augmented reality techniques are used in order to create a Virtual Mirror for the real-time visualization of customized sports shoes. Similar to looking into a mirror when trying on new shoes in a shop, we create the same impression but for virtual shoes that the customer can design individually. For that purpose, we replace the real mirror by a large display that shows the mirrored input of a camera capturing the legs and shoes of a person. 3-D Tracking of both feet and exchanging the real shoes by computer graphics models gives the impression of actually wearing the virtual shoes. The 3-D motion tracker presented in this paper, exploits mainly silhouette information to achieve robust estimates for both shoes from a single camera view. The use of a hierarchical approach in an image pyramid enables real-time estimation at frame rates of more than 30 frames per second. 1
Exploring local quantum many-body relaxation by atoms in optical superlattices
We establish a setting - atoms in optical superlattices with period 2 - in
which one can experimentally probe signatures of the process of local
relaxation and apparent thermalization in non-equilibrium dynamics without the
need of addressing single sites. This opens up a way to explore the convergence
of subsystems to maximum entropy states in quenched quantum many-body systems
with present technology. Remarkably, the emergence of thermal states does not
follow from a coupling to an environment, but is a result of the complex
non-equilibrium dynamics in closed systems. We explore ways of measuring the
relevant signatures of thermalization in this analogue quantum simulation of a
relaxation process, exploiting the possibilities offered by optical
superlattices.Comment: 4 pages, 3 figures, version to published in Physical Review Letter
General entanglement scaling laws from time evolution
We establish a general scaling law for the entanglement of a large class of
ground states and dynamically evolving states of quantum spin chains: we show
that the geometric entropy of a distinguished block saturates, and hence
follows an entanglement-boundary law. These results apply to any ground state
of a gapped model resulting from dynamics generated by a local hamiltonian, as
well as, dually, to states that are generated via a sudden quench of an
interaction as recently studied in the case of dynamics of quantum phase
transitions. We achieve these results by exploiting ideas from quantum
information theory and making use of the powerful tools provided by
Lieb-Robinson bounds. We also show that there exist noncritical fermionic
systems and equivalent spin chains with rapidly decaying interactions whose
geometric entropy scales logarithmically with block length. Implications for
the classical simulatability are outlined.Comment: 4 pages, 1 figure (see also related work by S. Bravyi, M. Hastings,
and F. Verstraete, quant-ph/0603121); replaced with final versio
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