519 research outputs found
Dependence of the BEC transition temperature on interaction strength: a perturbative analysis
We compute the critical temperature T_c of a weakly interacting uniform Bose
gas in the canonical ensemble, extending the criterion of condensation provided
by the counting statistics for the uniform ideal gas. Using ordinary
perturbation theory, we find in first order , where T_c^0 is the transition temperature of the corresponding
ideal Bose gas, a is the scattering length, and is the particle number
density.Comment: 14 pages (RevTeX
Realistic continuous-variable quantum teleportation with non-Gaussian resources
We present a comprehensive investigation of nonideal continuous-variable
quantum teleportation implemented with entangled non-Gaussian resources. We
discuss in a unified framework the main decoherence mechanisms, including
imperfect Bell measurements and propagation of optical fields in lossy fibers,
applying the formalism of the characteristic function. By exploiting
appropriate displacement strategies, we compute analytically the success
probability of teleportation for input coherent states, and two classes of
non-Gaussian entangled resources: Two-mode squeezed Bell-like states (that
include as particular cases photon-added and photon-subtracted de-Gaussified
states), and two-mode squeezed cat-like states. We discuss the optimization
procedure on the free parameters of the non-Gaussian resources at fixed values
of the squeezing and of the experimental quantities determining the
inefficiencies of the non-ideal protocol. It is found that non-Gaussian
resources enhance significantly the efficiency of teleportation and are more
robust against decoherence than the corresponding Gaussian ones. Partial
information on the alphabet of input states allows further significant
improvement in the performance of the non-ideal teleportation protocol.Comment: 14 pages, 6 figure
Geometric measures of quantum correlations : characterization, quantification, and comparison by distances and operations
We investigate and compare three distinguished geometric measures of bipartite quantum correlations that have been recently introduced in the literature: the geometric discord, the measurement-induced geometric discord, and the discord of response, each one defined according to three contractive distances on the set of quantum states, namely the trace, Bures, and Hellinger distances. We establish a set of exact algebraic relations and inequalities between the different measures. In particular, we show that the geometric discord and the discord of response based on the Hellinger distance are easy to compute analytically for all quantum states whenever the reference subsystem is a qubit. These two measures thus provide the first instance of discords that are simultaneously fully computable, reliable (since they satisfy all the basic Axioms that must be obeyed by a proper measure of quantum correlations), and operationally viable (in terms of state distinguishability). We apply the general mathematical structure to determine the closest classical-quantum state of a given state and the maximally quantum-correlated states at fixed global state purity according to the different distances, as well as a necessary condition for a channel to be quantumness breaking
Quasideterministic generation of maximally entangled states of two mesoscopic atomic ensembles by adiabatic quantum feedback
We introduce an efficient, quasideterministic scheme to generate maximally
entangled states of two atomic ensembles. The scheme is based on quantum
nondemolition measurements of total atomic populations and on adiabatic quantum
feedback conditioned by the measurements outputs. The high efficiency of the
scheme is tested and confirmed numerically for ideal photodetection as well as
in the presence of losses.Comment: 7 pages, 6 figures, title changed, revised version published on Phys.
Rev
Tunable non-Gaussian resources for continuous-variable quantum technologies
We introduce and discuss a set of tunable two-mode states of
continuous-variable systems, as well as an efficient scheme for their
experimental generation. This novel class of tunable entangled resources is
defined by a general ansatz depending on two experimentally adjustable
parameters. It is very ample and flexible as it encompasses Gaussian as well as
non-Gaussian states. The latter include, among others, known states such as
squeezed number states and de-Gaussified photon-added and photon-subtracted
squeezed states, the latter being the most efficient non-Gaussian resources
currently available in the laboratory. Moreover, it contains the classes of
squeezed Bell states and even more general non-Gaussian resources that can be
optimized according to the specific quantum technological task that needs to be
realized. The proposed experimental scheme exploits linear optical operations
and photon detections performed on a pair of uncorrelated two--mode Gaussian
squeezed states. The desired non-Gaussian state is then realized via ancillary
squeezing and conditioning. Two independent, freely tunable experimental
parameters can be exploited to generate different states and to optimize the
performance in implementing a given quantum protocol. As a concrete instance,
we analyze in detail the performance of different states considered as
resources for the realization of quantum teleportation in realistic conditions.
For the fidelity of teleportation of an unknown coherent state, we show that
the resources associated to the optimized parameters outperform, in a
significant range of experimental values, both Gaussian twin beams and
photon-subtracted squeezed states.Comment: 13 pages, 7 figure
Geometric Effects and Computation in Spin Networks
When initially introduced, a Hamiltonian that realises perfect transfer of a
quantum state was found to be analogous to an x-rotation of a large spin. In
this paper we extend the analogy further to demonstrate geometric effects by
performing rotations on the spin. Such effects can be used to determine
properties of the chain, such as its length, in a robust manner. Alternatively,
they can form the basis of a spin network quantum computer. We demonstrate a
universal set of gates in such a system by both dynamical and geometrical
means
Exact Multiplicities in the Three-Anyon Spectrum
Using the symmetry properties of the three-anyon spectrum, we obtain exactly
the multiplicities of states with given energy and angular momentum. The
results are shown to be in agreement with the proper quantum mechanical and
semiclassical considerations, and the unexplained points are indicated.Comment: 16 pages plus 3 postscript figures, Kiev Institute for Theoretical
Physics preprint ITP-93-32
Quantifying nonclassicality: global impact of local unitary evolutions
We show that only those composite quantum systems possessing nonvanishing
quantum correlations have the property that any nontrivial local unitary
evolution changes their global state. We derive the exact relation between the
global state change induced by local unitary evolutions and the amount of
quantum correlations. We prove that the minimal change coincides with the
geometric measure of discord (defined via the Hilbert- Schmidt norm), thus
providing the latter with an operational interpretation in terms of the
capability of a local unitary dynamics to modify a global state. We establish
that two-qubit Werner states are maximally quantum correlated, and are thus the
ones that maximize this type of global quantum effect. Finally, we show that
similar results hold when replacing the Hilbert-Schmidt norm with the trace
norm.Comment: 5 pages, 1 figure. To appear in Physical Review
Engineering massive quantum memories by topologically time-modulated spin rings
We introduce a general scheme to realize perfect storage of quantum
information in systems of interacting qubits. This novel approach is based on
{\it global} external controls of the Hamiltonian, that yield time-periodic
inversions in the dynamical evolution, allowing a perfect periodic quantum
state recontruction. We illustrate the method in the particularly interesting
and simple case of spin systems affected by XY residual interactions with or
without static imperfections. The global control is achieved by step
time-inversions of an overall topological phase of the Aharonov-Bohm type. Such
a scheme holds both at finite size and in the thermodynamic limit, thus
enabling the massive storage of arbitrarily large numbers of local states, and
is stable against several realistic sources of noise and imperfections.Comment: 12 pages, 9 figure
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