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

On Series of Multiqubit Bell's Inequalities

We overview series of multiqubit Bell's inequalities which apply to correlation functions. We present conditions that quantum states must satisfy to violate such inequalities.Comment: 10 page

Comment on: Nonlocal Realistic Leggett Models Can be Considered Refuted by the Before-Before Experiment

It is shown here that Suarez [Found. Phys. 38, 583 (2008)] wrongly presents the assumptions behind the Leggett's inequalities, and their modified form used by Groeblacher et al. [Nature 446, 871 (2007)] for an experimental falsification of a certain class of non-local hidden variable models.Comment: comment submitted to Found. Phy

Quantum Clock Synchronization with a Single Qudit

Clock synchronization for nonfaulty processes in multiprocess networks is indispensable for a variety of technologies. A reliable system must be able to resynchronize the nonfaulty processes upon some components failing causing the distribution of incorrect or conflicting information in the network. The task of synchronizing such networks is related to detectable Byzantine agreement (DBA), which can classically be solved using recursive algorithms if and only if less than one-third of the processes are faulty. Here we introduce a nonrecursive quantum algorithm that solves the DBA and achieves clock synchronization in the presence of arbitrary many faulty processes by using only a single quantum system