280 research outputs found
Optical state engineering, quantum communication, and robustness of entanglement promiscuity in three-mode Gaussian states
We present a novel, detailed study on the usefulness of three-mode Gaussian
states states for realistic processing of continuous-variable quantum
information, with a particular emphasis on the possibilities opened up by their
genuine tripartite entanglement. We describe practical schemes to engineer
several classes of pure and mixed three-mode states that stand out for their
informational and/or entanglement properties. In particular, we introduce a
simple procedure -- based on passive optical elements -- to produce pure
three-mode Gaussian states with {\em arbitrary} entanglement structure (upon
availability of an initial two-mode squeezed state). We analyze in depth the
properties of distributed entanglement and the origin of its sharing structure,
showing that the promiscuity of entanglement sharing is a feature peculiar to
symmetric Gaussian states that survives even in the presence of significant
degrees of mixedness and decoherence. Next, we discuss the suitability of the
considered tripartite entangled states to the implementation of quantum
information and communication protocols with continuous variables. This will
lead to a feasible experimental proposal to test the promiscuous sharing of
continuous-variable tripartite entanglement, in terms of the optimal fidelity
of teleportation networks with Gaussian resources. We finally focus on the
application of three-mode states to symmetric and asymmetric telecloning, and
single out the structural properties of the optimal Gaussian resources for the
latter protocol in different settings. Our analysis aims to lay the basis for a
practical quantum communication with continuous variables beyond the bipartite
scenario.Comment: 33 pages, 10 figures (some low-res due to size constraints), IOP
style; (v2) improved and reorganized, accepted for publication in New Journal
of Physic
Theory of ground state factorization in quantum cooperative systems
We introduce a general analytic approach to the study of factorization points
and factorized ground states in quantum cooperative systems. The method allows
to determine rigorously existence, location, and exact form of separable ground
states in a large variety of, generally non-exactly solvable, spin models
belonging to different universality classes. The theory applies to
translationally invariant systems, irrespective of spatial dimensionality, and
for spin-spin interactions of arbitrary range.Comment: 4 pages, 1 figur
Harmonics of the AC susceptibility as probes to differentiate the various creep models
We measured the temperature dependence of the 1st and the 3rd harmonics of
the AC magnetic susceptibility on some type II superconducting samples at
different AC field amplitudes, hAC. In order to interpret the measurements, we
computed the harmonics of the AC susceptibility as function of the temperature
T, by integrating the non-linear diffusion equation for the magnetic field with
different creep models, namely the vortex glass-collective creep
(single-vortex, small bundle and large bundle) and Kim-Anderson model. We also
computed them by using a non-linear phenomenological I-V characteristics,
including a power law dependence of the pinning potential on hAC. Our
experimental results were compared with the numerically computed ones, by the
analysis of the Cole-Cole plots. This method results more sensitive than the
separate component analysis, giving the possibility to obtain detailed
information about the contribution of the flux dynamic regimes in the magnetic
response of the analysed samples.Comment: 9 pages, 6 figures, submitted to Physica
Controllable Gaussian-qubit interface for extremal quantum state engineering
We study state engineering through bilinear interactions between two remote
qubits and two-mode Gaussian light fields. The attainable two-qubit states span
the entire physically allowed region in the entanglement-versus-global-purity
plane. Two-mode Gaussian states with maximal entanglement at fixed global and
marginal entropies produce maximally entangled two-qubit states in the
corresponding entropic diagram. We show that a small set of parameters
characterizing extremally entangled two-mode Gaussian states is sufficient to
control the engineering of extremally entangled two-qubit states, which can be
realized in realistic matter-light scenarios.Comment: 4+3 pages, 6 figures, RevTeX4. Close to published version with
appendi
Continuous variable tangle, monogamy inequality, and entanglement sharing in Gaussian states of continuous variable systems
For continuous-variable systems, we introduce a measure of entanglement, the
continuous variable tangle ({\em contangle}), with the purpose of quantifying
the distributed (shared) entanglement in multimode, multipartite Gaussian
states. This is achieved by a proper convex roof extension of the squared
logarithmic negativity. We prove that the contangle satisfies the
Coffman-Kundu-Wootters monogamy inequality in all three--mode Gaussian states,
and in all fully symmetric --mode Gaussian states, for arbitrary . For
three--mode pure states we prove that the residual entanglement is a genuine
tripartite entanglement monotone under Gaussian local operations and classical
communication. We show that pure, symmetric three--mode Gaussian states allow a
promiscuous entanglement sharing, having both maximum tripartite residual
entanglement and maximum couplewise entanglement between any pair of modes.
These states are thus simultaneous continuous-variable analogs of both the GHZ
and the states of three qubits: in continuous-variable systems monogamy
does not prevent promiscuity, and the inequivalence between different classes
of maximally entangled states, holding for systems of three or more qubits, is
removed.Comment: 13 pages, 1 figure. Replaced with published versio
Entanglement quantification by local unitaries
Invariance under local unitary operations is a fundamental property that must
be obeyed by every proper measure of quantum entanglement. However, this is not
the only aspect of entanglement theory where local unitaries play a relevant
role. In the present work we show that the application of suitable local
unitary operations defines a family of bipartite entanglement monotones,
collectively referred to as "mirror entanglement". They are constructed by
first considering the (squared) Hilbert-Schmidt distance of the state from the
set of states obtained by applying to it a given local unitary. To the action
of each different local unitary there corresponds a different distance. We then
minimize these distances over the sets of local unitaries with different
spectra, obtaining an entire family of different entanglement monotones. We
show that these mirror entanglement monotones are organized in a hierarchical
structure, and we establish the conditions that need to be imposed on the
spectrum of a local unitary for the associated mirror entanglement to be
faithful, i.e. to vanish on and only on separable pure states. We analyze in
detail the properties of one particularly relevant member of the family, the
"stellar mirror entanglement" associated to traceless local unitaries with
nondegenerate spectrum and equispaced eigenvalues in the complex plane. This
particular measure generalizes the original analysis of [Giampaolo and
Illuminati, Phys. Rev. A 76, 042301 (2007)], valid for qubits and qutrits. We
prove that the stellar entanglement is a faithful bipartite entanglement
monotone in any dimension, and that it is bounded from below by a function
proportional to the linear entropy and from above by the linear entropy itself,
coinciding with it in two- and three-dimensional spaces.Comment: 13 pages, 3 figures. Improved and generalized proof of monotonicity
of the mirror and stellar entanglemen
Asymptotically optimal quantum channel reversal for qudit ensembles and multimode Gaussian states
We investigate the problem of optimally reversing the action of an arbitrary
quantum channel C which acts independently on each component of an ensemble of
n identically prepared d-dimensional quantum systems. In the limit of large
ensembles, we construct the optimal reversing channel R* which has to be
applied at the output ensemble state, to retrieve a smaller ensemble of m
systems prepared in the input state, with the highest possible rate m/n. The
solution is found by mapping the problem into the optimal reversal of Gaussian
channels on quantum-classical continuous variable systems, which is here solved
as well. Our general results can be readily applied to improve the
implementation of robust long-distance quantum communication. As an example, we
investigate the optimal reversal rate of phase flip channels acting on a
multi-qubit register.Comment: 17 pages, 3 figure
Multipartite entanglement in three-mode Gaussian states of continuous variable systems: Quantification, sharing structure and decoherence
We present a complete analysis of multipartite entanglement of three-mode
Gaussian states of continuous variable systems. We derive standard forms which
characterize the covariance matrix of pure and mixed three-mode Gaussian states
up to local unitary operations, showing that the local entropies of pure
Gaussian states are bound to fulfill a relationship which is stricter than the
general Araki-Lieb inequality. Quantum correlations will be quantified by a
proper convex roof extension of the squared logarithmic negativity (the
contangle), satisfying a monogamy relation for multimode Gaussian states, whose
proof will be reviewed and elucidated. The residual contangle, emerging from
the monogamy inequality, is an entanglement monotone under Gaussian local
operations and classical communication and defines a measure of genuine
tripartite entanglement. We analytically determine the residual contangle for
arbitrary pure three-mode Gaussian states and study the distribution of quantum
correlations for such states. This will lead us to show that pure, symmetric
states allow for a promiscuous entanglement sharing, having both maximum
tripartite residual entanglement and maximum couplewise entanglement between
any pair of modes. We thus name these states GHZ/ states of continuous
variable systems because they are simultaneous continuous-variable counterparts
of both the GHZ and the states of three qubits. We finally consider the
action of decoherence on tripartite entangled Gaussian states, studying the
decay of the residual contangle. The GHZ/ states are shown to be maximally
robust under both losses and thermal noise.Comment: 20 pages, 5 figures. (v2) References updated, published versio
Composite Cluster States and Alternative Architectures for One- Way Quantum Computation
We propose a new architecture for the measurement-based quantum computation
model. The new design relies on small composite light-atom primary clusters.
These are then assembled into cluster arrays using ancillary light modes and
the actual computation is run on such a cellular cluster. We show how to create
the primary clusters, which are Gaussian cluster states composed of both light
and atomic modes. These are entangled via QND interactions and beamsplitters
and the scheme is well described within the continuous-variable covariance
matrix formalism.Comment: arXiv admin note: text overlap with arXiv:1007.040
A new method to detect the vortex glass phase and its evidence in YBCO
The Vortex Glass phase has been unequivocally identified by analyzing the non
linear magnetic response of type II superconductors. The method here
introduced, more effective than the study of direct transport measurements, is
based on a combined frequency dependence analysis of the real and imaginary
part of the 1st and 3rd harmonics of the AC magnetic susceptibility. The
analysis has been performed by taking into account both the components and the
Cole-Cole plots (i.e. the imaginary part as a function of the real part).
Numerical simulations have been used to individuate the fingerprints of the
magnetic behaviour in the Vortex Glass phase. These characteristics allowed to
distinguish the Vortex Glass phase from the other disordered phases, even those
showing similar electrical properties. Finally, this method has been
successfully applied to detect the Vortex Glass Phase in an YBCO bulk
melt-textured sample.Comment: 15 pages, 12 figure
- …