145 research outputs found
Entanglement scaling at first order phase transitions
First order quantum phase transitions (1QPTs) are signaled, in the
thermodynamic limit, by discontinuous changes in the ground state properties.
These discontinuities affect expectation values of observables, including
spatial correlations. When a 1QPT is crossed in the vicinity of a second order
one (2QPT), due to the correlation length divergence of the latter, the
corresponding ground state is modified and it becomes increasingly difficult to
determine the order of the transition when the size of the system is finite.
Here we show that, in such situations, it is possible to apply finite size
scaling to entanglement measures, as it has recently been done for the order
parameters and the energy gap, in order to recover the correct thermodynamic
limit. Such a finite size scaling can unambigously discriminate between first
and second order phase transitions in the vicinity of multricritical points
even when the singularities displayed by entanglement measures lead to
controversial results
Double barrier potentials for matter-wave gap solitons
We investigate collisions of solitons of the gap type, supported by a lattice
potential in repulsive Bose-Einstein condensates, with an effective
double-barrier potential that resembles a Fabry-Perot cavity. We identify
conditions under which the trapping of the entire incident soliton in the
cavity is possible. Collisions of the incident soliton with an earlier trapped
one are considered too. In the latter case, many outcomes of the collisions are
identified, including merging, release of the trapped soliton with or without
being replaced by the incoming one, and trapping of both solitons.Comment: 5 pages, 4 figure
Manipulating mesoscopic multipartite entanglement with atom-light interfaces
Entanglement between two macroscopic atomic ensembles induced by measurement
on an ancillary light system has proven to be a powerful method for engineering
quantum memories and quantum state transfer. Here we investigate the
feasibility of such methods for generation, manipulation and detection of
genuine multipartite entanglement between mesoscopic atomic ensembles. Our
results extend in a non trivial way the EPR entanglement between two
macroscopic gas samples reported experimentally in [B. Julsgaard, A. Kozhekin,
and E. Polzik, Nature {\bf 413}, 400 (2001)]. We find that under realistic
conditions, a second orthogonal light pulse interacting with the atomic
samples, can modify and even reverse the entangling action of the first one
leaving the samples in a separable state.Comment: 8 pages, 6 figure
Efficiency in Quantum Key Distribution Protocols with Entangled Gaussian States
Quantum key distribution (QKD) refers to specific quantum strategies which
permit the secure distribution of a secret key between two parties that wish to
communicate secretly. Quantum cryptography has proven unconditionally secure in
ideal scenarios and has been successfully implemented using quantum states with
finite (discrete) as well as infinite (continuous) degrees of freedom. Here, we
analyze the efficiency of QKD protocols that use as a resource entangled
gaussian states and gaussian operations only. In this framework, it has already
been shown that QKD is possible (M. Navascu\'es et al. Phys. Rev. Lett. 94,
010502 (2005)) but the issue of its efficiency has not been considered. We
propose a figure of merit (the efficiency ) to quantify the number of
classical correlated bits that can be used to distill a key from a sample of
entangled states. We relate the efficiency of the protocol to the
entanglement and purity of the states shared between the parties.Comment: 13 pages, 2 figures, OSID style, published versio
Multipartite Continuous Variable Solution for the Byzantine Agreement Problem
We demonstrate that the Byzantine Agreement (detectable broadcast) is also
solvable in the continuous-variable scenario with multipartite entangled
Gaussian states and Gaussian operations (homodyne detection). Within this
scheme we find that Byzantine Agreement requires a minimum amount of
entanglement in the multipartite states used in order to achieve a solution. We
discuss realistic implementations of the protocol, which consider the
possibility of having inefficient homodyne detectors, not perfectly correlated
outcomes, and noise in the preparation of the resource states. The proposed
protocol is proven to be robust and efficiently applicable under such non-ideal
conditions.Comment: This paper supersedes and extends arXiv:quant-ph/0507249, title
changed to match the published version, 11 pages, 3 figures, published
versio
Spin-driven spatial symmetry breaking of spinor condensates in a double-well
The properties of an F=1 spinor Bose-Einstein condensate trapped in a
double-well potential are discussed using both a mean-field two-mode approach
and a simplified two-site Bose-Hubbard Hamiltonian. We focus in the region of
phase space in which spin effects lead to a symmetry breaking of the system,
favoring the spatial localization of the condensate in one well. To model this
transition we derive, using perturbation theory, an effective Hamiltonian that
describes N/2 spin singlets confined in a double-well potential.Comment: 12 pages, 5 figure
Coherence Properties of Guided-Atom Interferometers
We present a detailed investigation of the coherence properties of beam
splitters and Mach-Zehnder interferometers for guided atoms. It is demonstrated
that such a setup permits coherent wave packet splitting and leads to the
appearance of interference fringes. We study single-mode and thermal input
states and show that even for thermal input states interference fringes can be
clearly observed, thus demonstrating the multimode operation and the robustness
of the interferometer.Comment: 4 pages, 4 figure
Transport and Entanglement Generation in the Bose-Hubbard Model
We study entanglement generation via particle transport across a
one-dimensional system described by the Bose-Hubbard Hamiltonian. We analyze
how the competition between interactions and tunneling affects transport
properties and the creation of entanglement in the occupation number basis.
Alternatively, we propose to use spatially delocalized quantum bits, where a
quantum bit is defined by the presence of a particle either in a site or in the
adjacent one. Our results can serve as a guidance for future experiments to
characterize entanglement of ultracold gases in one-dimensional optical
lattices.Comment: 14 pages, 6 figure
On the volume of the set of mixed entangled states
A natural measure in the space of density matrices describing N-dimensional
quantum systems is proposed. We study the probability P that a quantum state
chosen randomly with respect to the natural measure is not entangled (is
separable). We find analytical lower and upper bounds for this quantity.
Numerical calculations give P = 0.632 for N=4 and P=0.384 for N=6, and indicate
that P decreases exponentially with N. Analysis of a conditional measure of
separability under the condition of fixed purity shows a clear dualism between
purity and separability: entanglement is typical for pure states, while
separability is connected with quantum mixtures. In particular, states of
sufficiently low purity are necessarily separable.Comment: 10 pages in LaTex - RevTex + 4 figures in eps. submitted to Phys.
Rev.
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