575 research outputs found
Multiparty Quantum Secret Sharing Based on Entanglement Swapping
A multiparty quantum secret sharing (QSS) protocol is proposed by using
swapping quantum entanglement of Bell states. The secret messages are imposed
on Bell states by local unitary operations. The secret messages are split into
several parts and each part is distributed to a party so that no action of a
subset of all the parties but their entire cooperation is able to read out the
secret messages. In addition, the dense coding is used in this protocol to
achieve a high efficiency. The security of the present multiparty QSS against
eavesdropping has been analyzed and confirmed even in a noisy quantum channel.Comment: 5 page
Realization of the Optimal Universal Quantum Entangler
We present the first experimental demonstration of the ''optimal'' and
''universal'' quantum entangling process involving qubits encoded in the
polarization of single photons. The structure of the ''quantum entangling
machine'' consists of the quantum injected optical parametric amplifier by
which the contextual realization of the 1->2 universal quantum cloning and of
the universal NOT (U-NOT) gate has also been achieved.Comment: 10 pages, 3 figures, to appear in Physical Review
Two-dimensional quantum repeaters
The endeavor to develop quantum networks gave rise to a rapidly developing field with far-reaching applications such as secure communication and the realization of distributed computing tasks. This ultimately calls for the creation of flexible multiuser structures that allow for quantum communication between arbitrary pairs of parties in the network and facilitate also multiuser applications. To address this challenge, we propose a two-dimensional quantum repeater architecture to establish long-distance entanglement shared between multiple communication partners in the presence of channel noise and imperfect local control operations. The scheme is based on the creation of self-similar multiqubit entanglement structures at growing scale, where variants of entanglement swapping and multiparty entanglement purification are combined to create high-fidelity entangled states. We show how such networks can be implemented using trapped ions in cavities
Remote information concentration by GHZ state and by bound entangled state
We compare remote information concentration by a maximally entangled GHZ
state with by an unlockable bound entangled state. We find that the bound
entangled state is as useful as the GHZ state, even do better than the GHZ
state in the context of communication security.Comment: 4 pages,1 figur
Preserving entanglement under decoherence and sandwiching all separable states
Every entangled state can be perturbed, for instance by decoherence, and stay
entangled. For a large class of pure entangled states, we show how large the
perturbation can be. Our class includes all pure bipartite and all maximally
entangled states. For an entangled state, E, the constucted neighborhood of
entangled states is the region outside two parallel hyperplanes, which sandwich
the set of all separable states. The states for which these neighborhoods are
largest are the maximally entangled ones. As the number of particles, or the
dimensions of the Hilbert spaces for two of the particles increases, the
distance between two of the hyperplanes which sandwich the separable states
goes to zero. It is easy to decide if a state Q is in the neighborhood of
entangled states we construct for an entangled state E. One merely has to check
if the trace of EQ is greater than a constant which depends upon E and which we
determine.Comment: Corrected first author's e-mail address. All the rest remains
unchange
A note on bound entanglement and local realism
We show using a numerical approach that gives necessary and sufficient
conditions for the existence of local realism, that the bound entangled state
presented in Bennett et. al. Phys. Rev. Lett. 82, 5385 (1999) admits a local
and realistic description. We also find the lowest possible amount of some
appropriate entangled state that must be ad-mixed to the bound entangled state
so that the resulting density operator has no local and realistic description
and as such can be useful in quantum communication and quantum computation.Comment: 5 page
New classes of n-copy undistillable quantum states with negative partial transposition
The discovery of entangled quantum states from which one cannot distill pure
entanglement constitutes a fundamental recent advance in the field of quantum
information. Such bipartite bound-entangled (BE) quantum states \emph{could}
fall into two distinct categories: (1) Inseparable states with positive partial
transposition (PPT), and (2) States with negative partial transposition (NPT).
While the existence of PPT BE states has been confirmed, \emph{only one} class
of \emph{conjectured} NPT BE states has been discovered so far. We provide
explicit constructions of a variety of multi-copy undistillable NPT states, and
conjecture that they constitute families of NPT BE states. For example, we show
that for every pure state of Schmidt rank greater than or equal to three, one
can construct n-copy undistillable NPT states, for any . The abundance
of such conjectured NPT BE states, we believe, considerably strengthens the
notion that being NPT is only a necessary condition for a state to be
distillable.Comment: Latex, 10 page
Better detection of Multipartite Bound Entanglement with Three-Setting Bell Inequalities
It was shown in Phys. Rev. Lett., 87, 230402 (2001) that N (N >= 4) qubits
described by a certain one parameter family F of bound entangled states violate
Mermin-Klyshko inequality for N >= 8. In this paper we prove that the states
from the family F violate Bell inequalities derived in Phys. Rev. A, 56, R1682
(1997), in which each observer measures three non-commuting sets of orthogonal
projectors, for N >=7. We also derive a simple one parameter family of
entanglement witnesses that detect entanglement for all the states belonging to
F. It is possible that these new entanglement witnesses could be generated by
some Bell inequalities.Comment: Revtex4, 1 figur
Quantum central limit theorem for continuous-time quantum walks on odd graphs in quantum probability theory
The method of the quantum probability theory only requires simple structural
data of graph and allows us to avoid a heavy combinational argument often
necessary to obtain full description of spectrum of the adjacency matrix. In
the present paper, by using the idea of calculation of the probability
amplitudes for continuous-time quantum walk in terms of the quantum probability
theory, we investigate quantum central limit theorem for continuous-time
quantum walks on odd graphs.Comment: 19 page, 1 figure
Entanglement, Mixedness, and Spin-Flip Symmetry in Multiple-Qubit Systems
A relationship between a recently introduced multipartite entanglement
measure, state mixedness, and spin-flip symmetry is established for any finite
number of qubits. It is also shown that, within those classes of states
invariant under the spin-flip transformation, there is a complementarity
relation between multipartite entanglement and mixedness. A number of example
classes of multiple-qubit systems are studied in light of this relationship.Comment: To appear in Physical Review A; submitted 14 May 200
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