143 research outputs found
Locality of three-qubit Greenberger-Horne-Zeilinger-symmetric states
The hierarchy of nonlocality and entanglement in multipartite systems is one
of the fundamental problems in quantum physics. Existing studies on this topic
to date were limited to the entanglement classification according to the
numbers of particles enrolled. Equivalence under stochastic local operations
and classical communication provides a more detailed classification, e. g. the
genuine three-qubit entanglement being divided into W and GHZ classes. We
construct two families of local models for the three-qubit
Greenberger-Horne-Zeilinger (GHZ)-symmetric states, whose entanglement classes
have a complete description. The key technology of construction the local
models in this work is the GHZ symmetrization on tripartite extensions of the
optimal local-hidden-state models for Bell diagonal states. Our models show
that entanglement and nonlocality are inequivalent for all the entanglement
classes (biseparable, W, and GHZ) in three-qubit systems.Comment: 3 figures, 6 pages. Many spelling errors have been corrected.
Submitted versio
Device-independent tomography of multipartite quantum states
In the usual tomography of multipartite entangled quantum states one assumes
that the measurement devices used in the laboratory are under perfect control
of the experimenter. In this paper, using the so-called SWAP concept introduced
recently, we show how one can remove this assumption in realistic experimental
conditions and nevertheless be able to characterize the produced multipartite
state based only on observed statistics. Such a black box tomography of quantum
states is termed self-testing. As a function of the magnitude of the Bell
violation, we are able to self-test emblematic multipartite quantum states such
as the three-qubit W state, the three- and four-qubit
Greenberger-Horne-Zeilinger states, and the four-qubit linear cluster state.Comment: See also the related work of arXiv:1407.576
Exploring quantum properties of bipartite mixed states under coherent and incoherent basis
Quantum coherence and quantum entanglement are two different manifestations
of the superposition principle. In this article we show that the right choice
of basis to be used to estimate coherence is the separable basis. The quantum
coherence estimated using the Bell basis does not represent the coherence in
the system, since there is a coherence in the system due to the choice of the
basis states. We first compute the entanglement and quantum coherence in the
two qubit mixed states prepared using the Bell states and one of the states
from the computational basis. The quantum coherence is estimated using the
l1-norm of coherence, the entanglement is measured using the concurrence and
the mixedness is measured using the linear entropy. Then we estimate these
quantities in the Bell basis and establish that coherence should be measured
only in separable basis, whereas entanglement and mixedness can be measured in
any basis. We then calculate the teleportation fidelity of these mixed states
and find the regions where the states have a fidelity greater than the
classical teleportation fidelity. We also examine the violation of the
Bell-CHSH inequality to verify the quantum nonlocal correlations in the system.
The estimation of the above mentioned quantum correlations, teleportation
fidelity and the verification of Bell-CHSH inequality is also done for
bipartite states obtained from the tripartite systems by the tracing out of one
of their qubits. We find that for some of these states teleportation is
possible even when the Bell-CHSH inequality is not violated, signifying that
nonlocality is not a necessary condition for quantum teleportation.Comment: 18 pages, 3 figure
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