3,532 research outputs found
Characterization and quantification of symmetric Gaussian state entanglement through a local classicality criterion
A necessary and sufficient condition for characterization and quantification
of entanglement of any bipartite Gaussian state belonging to a special symmetry
class is given in terms of classicality measures of one-party states. For
Gaussian states whose local covariance matrices have equal determinants it is
shown that separability of a two-party state and classicality of one party
state are completely equivalent to each other under a nonlocal operation,
allowing entanglement features to be understood in terms of any available
classicality measure.Comment: 5 pages, 1 figure. Replaced with final published versio
Nonzero Classical Discord
Quantum discord is the quantitative difference between two alternative
expressions for bipartite mutual information, given respectively in terms of
two distinct definitions for the conditional entropy. By constructing a
stochastic model of shared states, classical discord can be similarly defined,
quantifying the presence of some stochasticity in the measurement process.
Therefore, discord can generally be understood as a quantification of the
system's state disturbance due to local measurements, be it quantum or
classical. We establish an operational meaning of classical discord in the
context of state merging with noisy measurement and thereby show the
quantum-classical separation in terms of a negative conditional entropy.Comment: Replaced by the published versio
On the P-representable subset of all bipartite Gaussian separable states
P-representability is a necessary and sufficient condition for separability
of bipartite Gaussian states only for the special subset of states whose
covariance matrix are locally invariant. Although this
special class of states can be reached by a convenient
transformation over an arbitrary covariance matrix, it represents a loss of
generality, avoiding inference of many general aspects of separability of
bipartite Gaussian states.Comment: Final version with new results added. Slightly more detailed than the
accepted manuscript (to appear in Phys. Rev. A
Multipartite Entanglement Signature of Quantum Phase Transitions
We derive a general relation between the non-analyticities of the ground
state energy and those of a subclass of the multipartite generalized global
entanglement (GGE) measure defined by T. R. de Oliveira et al. [Phys. Rev. A
73, 010305(R) (2006)] for many-particle systems. We show that GGE signals both
a critical point location and the order of a quantum phase transition (QPT). We
also show that GGE allows us to study the relation between multipartite
entanglement and QPTs, suggesting that multipartite but not bipartite
entanglement is favored at the critical point. Finally, using GGE we were able,
at a second order QPT, to define a diverging entanglement length (EL) in terms
of the usual correlation length. We exemplify this with the XY spin-1/2 chain
and show that the EL is half the correlation length.Comment: Published version. Incorporates correction made in erratu
Solovay-Kitaev Decomposition Strategy for Single-Qubit Channels
Inspired by the Solovay-Kitaev decomposition for approximating unitary
operations as a sequence of operations selected from a universal quantum
computing gate set, we introduce a method for approximating any single-qubit
channel using single-qubit gates and the controlled-NOT (CNOT). Our approach
uses the decomposition of the single-qubit channel into a convex combination of
"quasiextreme" channels. Previous techniques for simulating general
single-qubit channels would require as many as 20 CNOT gates, whereas ours only
needs one, bringing it within the range of current experiments
Genuine Multipartite Entanglement in Quantum Phase Transitions
We demonstrate that the Global Entanglement (GE) measure defined by Meyer and
Wallach, J. Math. Phys. 43, 4273 (2002), is maximal at the critical point for
the Ising chain in a transverse magnetic field. Our analysis is based on the
equivalence of GE to the averaged linear entropy, allowing the understanding of
multipartite entanglement (ME) features through a generalization of GE for
bipartite blocks of qubits. Moreover, in contrast to GE, the proposed ME
measure can distinguish three paradigmatic entangled states: ,
, and . As such the generalized measure can detect
genuine ME and is maximal at the critical point.Comment: 4 pages, 3 figures. Replaced with final published versio
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