371 research outputs found
Entanglement of mixed macroscopic superpositions: an entangling-power study
We investigate entanglement properties of a recently introduced class of
macroscopic quantum superpositions in two-mode mixed states. One of the tools
we use in order to infer the entanglement in this non-Gaussian class of states
is the power to entangle a qubit system. Our study reveals features which are
hidden in a standard approach to entanglement investigation based on the
uncertainty principle of the quadrature variables. We briefly describe the
experimental setup corresponding to our theoretical scenario and a suitable
modification of the protocol which makes our proposal realizable within the
current experimental capabilities.Comment: 9 pages, 7 figures, RevTeX
Entanglement generation in harmonic chains: tagging by squeezing
We address the problem of spring-like coupling between bosons in an open
chain configuration where the counter-rotating terms are explicitly included.
We show that fruitful insight can be gained by decomposing the time-evolution
operator of this problem into a pattern of linear-optics elements. This allows
us to provide a clear picture of the effects of the counter-rotating terms in
the important problem of long-haul entanglement distribution. The analytic
control over the variance matrix of the state of the bosonic register allows us
to track the dynamics of the entanglement. This helps in designing a global
addressing scheme, complemented by a proper initialization of the register,
which quantitatively improves the entanglement between the extremal oscillators
in the chain, thus providing a strategy for feasible long distance entanglement
distribution.Comment: 8 pages, 8 figures, RevTeX
Teleporting bipartite entanglement using maximally entangled mixed channels
The ability to teleport entanglement through maximally entangled mixed states
as defined by concurrence and linear entropy is studied. We show how the
teleported entanglement depends on the quality of the quantum channel used, as
defined through its entanglement and mixedness, as well as the form of the
target state to be teleported. We present new results based on the fidelity of
the teleported state as well as an experimental set-up that is immediately
implementable with currently available technology.Comment: 8 pages, 7 figures, RevTeX4, Accepted for publication in the IJQI
special issue on Distributed Quantum Information Processin
Robust quantum state engineering through coherent localization in biased-coin quantum walks
We address the performance of a coin-biased quantum walk as a generator for
non-classical position states of the walker. We exploit a phenomenon of
coherent localisation in the position space --- resulting from the choice of
small values of the coin parameter and assisted by post-selection --- to
engineer large-size coherent superpositions of distinguishable position states
of the walker. The protocol that we design appears to be remarkably robust
against both the actual value taken by the coin parameter and strong
dephasing-like noise acting on the spatial degree of freedom. We finally
illustrate a possible linear-optics implementation of our proposal, suitable
for both bulk and integrated-optics platforms.Comment: 7 pages, 7 figure
Work statistics, irreversible heat and correlations build-up in joining two spin chains
We investigate the influences of quantum many-body effects, such as
criticality and the existence of factorisation fields, in the thermodynamic
cost of establishing a bonding link between two independent quantum spin
chains. We provide a physical interpretation of the behavior of irreversible
work spent in such process by linking the phenomenology of such quantities to
the properties of the spectrum of the systemComment: 9 pages, 8 figures. Contribution to the FQMT13 special volum
Global Quantum Correlation in the Ising model
We study quantum correlations in an isotropic Ising ring under the effects of
a transverse magnetic field. After characterizing the behavior of two-spin
quantum correlations, we extend our analysis to global properties of the ring,
using a figure of merit for quantum correlations that shows enough sensitivity
to reveal the drastic changes in the properties of the system at criticality.
This opens up the possibility to relate statistical properties of quantum
many-body systems to suitably tailored measures of quantum correlations that
capture features going far beyond standard quantum entanglement.Comment: Published in the International Journal of Quantum Information as part
of the special issue devoted to "Quantum Correlations: entanglement and
beyond
Characterization of Bose-Hubbard Models with Quantum Non-demolition Measurements
We propose a scheme for the detection of quantum phase transitions in the 1D
Bose-Hubbard (BH) and 1D Extended Bose-Hubbard (EBH) models, using the
non-demolition measurement technique of quantum polarization spectroscopy. We
use collective measurements of the effective total angular momentum of a
particular spatial mode to characterise the Mott insulator to superfluid phase
transition in the BH model, and the transition to a density wave state in the
EBH model. We extend the application of collective measurements to the ground
states at various deformations of a super-lattice potential.Comment: 8 pages, 9 figures; published version in PRA, Editors' Suggestio
Orthogonality catastrophe as a consequence of qubit embedding in an ultra-cold Fermi gas
We investigate the behaviour of a single qubit coupled to a low-dimensional,
ultra-cold Fermi gas. The scattering between the system and the fermions leads
to the loss of any coherence in the initial state of the qubit and we show that
the exact dynamics of this process is strongly influenced by the effect of the
orthogonality catastrophe within the gas. We highlight the relationship between
the Loschmidt echo and the retarded Green's function - typically used to
formulate the dynamical theory of the catastrophe - and demonstrate that the
effect can be triggered and characterized via local operations on the qubit. We
demonstrate how the expected broadening of the spectral function can be
observed using Ramsey interferometry on the qubit.Comment: 4 and a bit pages, 3 figures. Updated versio
Quantum discord bounds the amount of distributed entanglement
The ability to distribute quantum entanglement is a prerequisite for many
fundamental tests of quantum theory and numerous quantum information protocols.
Two distant parties can increase the amount of entanglement between them by
means of quantum communication encoded in a carrier that is sent from one party
to the other. Intriguingly, entanglement can be increased even when the
exchanged carrier is not entangled with the parties. However, in light of the
defining property of entanglement stating that it cannot increase under
classical communication, the carrier must be quantum. Here we show that, in
general, the increase of relative entropy of entanglement between two remote
parties is bounded by the amount of non-classical correlations of the carrier
with the parties as quantified by the relative entropy of discord. We study
implications of this bound, provide new examples of entanglement distribution
via unentangled states and put further limits on this phenomenon.Comment: 8 pages, 1 figure, RevTeX4; Accepted for publication in Phys. Rev.
Let
Faithful test of non-local realism with entangled coherent states
We investigate the violation of Leggett's inequality for non-local realism
using entangled coherent states and various types of local measurements. We
prove mathematically the relation between the violation of the
Clauser-Horne-Shimony-Holt form of Bell's inequality and Leggett's one when
tested by the same resources. For Leggett inequalities, we generalize the
non-local realistic bound to systems in Hilbert spaces larger than
bidimensional ones and introduce an optimization technique that allows to
achieve larger degrees of violation by adjusting the local measurement
settings. Our work describes the steps that should be performed to produce a
self-consistent generalization of Leggett's original arguments to
continuous-variable states.Comment: 8 pages, 6 figures, to be published in Phys. Rev.
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