1,270 research outputs found
Entanglement and nonlocality are inequivalent for any number of particles
Understanding the relation between nonlocality and entanglement is one of the
fundamental problems in quantum physics. In the bipartite case, it is known
that the correlations observed for some entangled quantum states can be
explained within the framework of local models, thus proving that these
resources are inequivalent in this scenario. However, except for a single
example of an entangled three-qubit state that has a local model, almost
nothing is known about such relation in multipartite systems. We provide a
general construction of genuinely multipartite entangled states that do not
display genuinely multipartite nonlocality, thus proving that entanglement and
nonlocality are inequivalent for any number of particles.Comment: submitted version, 7 pages (4.25 + appendix), 1 figur
Entangled symmetric states of N qubits with all positive partial transpositions
From both theoretical and experimental points of view symmetric states
constitute an important class of multipartite states. Still, entanglement
properties of these states, in particular those with positive partial
transposition (PPT), lack a systematic study. Aiming at filling in this gap, we
have recently affirmatively answered the open question of existence of
four-qubit entangled symmetric states with positive partial transposition and
thoroughly characterized entanglement properties of such states [J. Tura et
al., Phys. Rev. A 85, 060302(R) (2012)] With the present contribution we
continue on characterizing PPT entangled symmetric states. On the one hand, we
present all the results of our previous work in a detailed way. On the other
hand, we generalize them to systems consisting of arbitrary number of qubits.
In particular, we provide criteria for separability of such states formulated
in terms of their ranks. Interestingly, for most of the cases, the symmetric
states are either separable or typically separable. Then, edge states in these
systems are studied, showing in particular that to characterize generic PPT
entangled states with four and five qubits, it is enough to study only those
that assume few (respectively, two and three) specific configurations of ranks.
Finally, we numerically search for extremal PPT entangled states in such
systems consisting of up to 23 qubits. One can clearly notice regularity behind
the ranks of such extremal states, and, in particular, for systems composed of
odd number of qubits we find a single configuration of ranks for which there
are extremal states.Comment: 16 pages, typos corrected, some other improvements, extension of
arXiv:1203.371
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