69 research outputs found

### Entanglement of multiparty stabilizer, symmetric, and antisymmetric states

We study various distance-like entanglement measures of multipartite states
under certain symmetries. Using group averaging techniques we provide
conditions under which the relative entropy of entanglement, the geometric
measure of entanglement and the logarithmic robustness are equivalent. We
consider important classes of multiparty states, and in particular show that
these measures are equivalent for all stabilizer states, symmetric basis and
antisymmetric basis states. We rigorously prove a conjecture that the closest
product state of permutation symmetric states can always be chosen to be
permutation symmetric. This allows us to calculate the explicit values of
various entanglement measures for symmetric and antisymmetric basis states,
observing that antisymmetric states are generally more entangled. We use these
results to obtain a variety of interesting ensembles of quantum states for
which the optimal LOCC discrimination probability may be explicitly determined
and achieved. We also discuss applications to the construction of optimal
entanglement witnesses

### A complete criterion for separability detection

Using new results on the separability properties of bosonic systems, we
provide a new complete criterion for separability. This criterion aims at
characterizing the set of separable states from the inside by means of a
sequence of efficiently solvable semidefinite programs. We apply this method to
derive arbitrarily good approximations to the optimal measure-and-prepare
strategy in generic state estimation problems. Finally, we report its
performance in combination with the criterion developed by Doherty et al. [1]
for the calculation of the entanglement robustness of a relevant family of
quantum states whose separability properties were unknown

### Bounds on Multipartite Entangled Orthogonal State Discrimination Using Local Operations and Classical Communication

We show that entanglement guarantees difficulty in the discrimination of
orthogonal multipartite states locally. The number of pure states that can be
discriminated by local operations and classical communication is bounded by the
total dimension over the average entanglement. A similar, general condition is
also shown for pure and mixed states. These results offer a rare operational
interpretation for three abstractly defined distance like measures of
multipartite entanglement.Comment: 4 pages, 1 figure. Title changed in accordance with jounral request.
Major changes to the paper. Intro rewritten to make motivation clear, and
proofs rewritten to be clearer. Picture added for clarit

### Local distinguishability of quantum states in infinite dimensional systems

We investigate local distinguishability of quantum states by use of the
convex analysis about joint numerical range of operators on a Hilbert space. We
show that any two orthogonal pure states are distinguishable by local
operations and classical communications, even for infinite dimensional systems.
An estimate of the local discrimination probability is also given for some
family of more than two pure states

### 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

### Linear amplification and quantum cloning for non-Gaussian continuous variables

We investigate phase-insensitive linear amplification at the quantum limit
for single- and two-mode states and show that there exists a broad class of
non-Gaussian states whose nonclassicality survives even at an arbitrarily large
gain. We identify the corresponding observable nonclassical effects and find
that they include, remarkably, two-mode entanglement. The implications of our
results for quantum cloning outside the Gaussian regime are also addressed.Comment: published version with reference updat

### Group theoretical study of LOCC-detection of maximally entangled state using hypothesis testing

In the asymptotic setting, the optimal test for hypotheses testing of the
maximally entangled state is derived under several locality conditions for
measurements. The optimal test is obtained in several cases with the asymptotic
framework as well as the finite-sample framework. In addition, the experimental
scheme for the optimal test is presented

### The power of symmetric extensions for entanglement detection

In this paper, we present new progress on the study of the symmetric
extension criterion for separability. First, we show that a perturbation of
order O(1/N) is sufficient and, in general, necessary to destroy the
entanglement of any state admitting an N Bose symmetric extension. On the other
hand, the minimum amount of local noise necessary to induce separability on
states arising from N Bose symmetric extensions with Positive Partial Transpose
(PPT) decreases at least as fast as O(1/N^2). From these results, we derive
upper bounds on the time and space complexity of the weak membership problem of
separability when attacked via algorithms that search for PPT symmetric
extensions. Finally, we show how to estimate the error we incur when we
approximate the set of separable states by the set of (PPT) N -extendable
quantum states in order to compute the maximum average fidelity in pure state
estimation problems, the maximal output purity of quantum channels, and the
geometric measure of entanglement.Comment: see Video Abstract at
http://www.quantiki.org/video_abstracts/0906273

### Quantum memory for entangled two-mode squeezed states

A quantum memory for light is a key element for the realization of future
quantum information networks. Requirements for a good quantum memory are (i)
versatility (allowing a wide range of inputs) and (ii) true quantum coherence
(preserving quantum information). Here we demonstrate such a quantum memory for
states possessing Einstein-Podolsky-Rosen (EPR) entanglement. These
multi-photon states are two-mode squeezed by 6.0 dB with a variable orientation
of squeezing and displaced by a few vacuum units. This range encompasses
typical input alphabets for a continuous variable quantum information protocol.
The memory consists of two cells, one for each mode, filled with cesium atoms
at room temperature with a memory time of about 1msec. The preservation of
quantum coherence is rigorously proven by showing that the experimental memory
fidelity 0.52(2) significantly exceeds the benchmark of 0.45 for the best
possible classical memory for a range of displacements.Comment: main text 5 pages, supplementary information 3 page

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