242 research outputs found
Entanglement assisted random access codes
An (n,m,p) Random Access Code (RAC) allows to encode n bits in an m bit
message, in such a way that a receiver of the message can guess any of the
original bits with probability p, greater than 1/2. In Quantum RAC's
(QRACs) one transmits n qubits. The full set of primitive Entanglement Assisted
Random Access Codes (EARACs) is introduced, in which parties are allowed to
share a two-qubit singlet. It is shown that via a concatenation of these, one
can build for any n an (n,1,p) EARAC. QRAC's for n>3 exist only if parties
additionally share classical randomness (SR). We show that EARACs outperform
the best of known QRACs not only in the success probabilities but also in the
amount of communication needed in the preparatory stage of the protocol. Upper
bounds on the performance of EARACs are given, and shown to limit also QRACs.Comment: 4 pages, 1 figure, published versio
Experimental observation of four-photon entanglement from down-conversion
We observe polarization-entanglement between four photons produced from a
single down-conversion source. The non-classical correlations between the
measurement results violate a generalized Bell inequality for four qubits. The
characteristic properties and its easy generation with high interferometric
contrast make the observed four-photon state well-suited for implementing
advanced quantum communication schemes such as multi-party quantum key
distribution, secret sharing and telecloning.Comment: 4 pages, 3 figure
Bell Theorem for Nonclassical Part of Quantum Teleportation Process
The quantum teleportation process is composed of a joint measurement
performed upon two subsystems A and B (uncorrelated), followed by a unitary
transformation (parameters of which depend on the outcome of the measurement)
performed upon a third subsystem C (EPR correlated with system B). The
information about the outcome of the measurement is transferred by classical
means. The measurement performed upon the systems A and B collapses their joint
wavefunction into one of the four {\it entangled} Bell states. It is shown here
that this measurement process plus a possible measurement on the third
subsystem (with classical channel switched off - no additional unitary
transformation performed) cannot be described by a local realistic theory.Comment: 4 pages, RevTeX, no figure
Greenberger-Horne-Zeilinger paradoxes for N quNits
In this paper we show the series of Greenberger-Horne-Zeilinger paradoxes for
N maximally entangled N-dimensional quantum systems.Comment: 6 page
Information theoretic approach to single-particle and two-particle interference in multi-path interferometers
We propose entropic measures for the strength of single-particle and
two-particle interference in interferometric experiments where each particle of
a pair traverses a multi-path interferometer. Optimal single-particle
interference excludes any two-particle interference, and vice versa. We report
an inequality that states the compromises allowed by quantum mechanics in
intermediate situations, and identify a class of two-particle states for which
the upper bound is reached. Our approach is applicable to symmetric two-partite
systems of any finite dimension.Comment: RevTex 4, 4 pages, 2 figure
Detection of N-particle entanglement with generalized Bell inequalities
We show that the generalized Bell-type inequality, explicitly involving
rotational symmetry of physical laws, is very efficient in distinguishing
between true N-particle quantum correlations and correlations involving less
particles. This applies to various types of generalized partial separabilities.
We also give a rigorous proof that the new Bell inequalities are maximally
violated by the GHZ states, and find a very handy description of the N-qubit
correlation function.Comment: 5 pages, minor typos corrected, journal versio
Single Qubit Quantum Secret Sharing
We present a simple and practical protocol for the solution of a secure
multiparty communication task, the secret sharing, and its experimental
realization. In this protocol, a secret message is split among several parties
in a way that its reconstruction require the collaboration of the participating
parties. In the proposed scheme the parties solve the problem by a sequential
communication of a single qubit. Moreover we show that our scheme is equivalent
to the use of a multiparty entangled GHZ state but easier to realize and better
scalable in practical applications.Comment: 5 pages, 2 figures, submitted December 29, 200
Bell's theorem for general N-qubit states
We derive a single general Bell inequality which is a necessary and
sufficient condition for the correlation function for N particles to be
describable in a local and realistic picture, for the case in which
measurements on each particle can be chosen between two arbitrary dichotomic
observables. We also derive a necessary and sufficient condition for an
arbitrary N-qubit mixed state to violate this inequality. This condition is a
generalization and reformulation of the Horodeccy family condition for two
qubits.Comment: 4 pages, journal versio
Bipartite all-versus-nothing proofs of Bell's theorem with single-qubit measurements
If we distribute n qubits between two parties, which quantum pure states and
distributions of qubits would allow all-versus-nothing (or
Greenberger-Horne-Zeilinger-like) proofs of Bell's theorem using only
single-qubit measurements? We show a necessary and sufficient condition for the
existence of these proofs for any number of qubits, and provide all distinct
proofs up to n=7 qubits. Remarkably, there is only one distribution of a state
of n=4 qubits, and six distributions, each for a different state of n=6 qubits,
which allow these proofs.Comment: REVTeX4, 4 pages, 2 figure
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