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 $n$ 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