Optimal dimensionality for quantum cryptography

We perform a comparison of two protocols for generating a cryptographic key composed from d-valued symbols: one exploiting a string of independent qubits and another one utilizing d-level systems prepared in states belonging to d+1 mutually unbiased bases. We show that the protocol based on qubits is optimal for quantum cryptography, since it provides higher security and higher key generation rate.Comment: Revtex, 4 pages, 1 eps figur

The effectiveness of quantum operations for eavesdropping on sealed messages

A quantum protocol is described which enables a user to send sealed messages and that allows for the detection of active eavesdroppers. We examine a class of eavesdropping strategies, those that make use of quantum operations, and we determine the information gain versus disturbance caused by these strategies. We demonstrate this tradeoff with an example and we compare this protocol to quantum key distribution, quantum direct communication, and quantum seal protocols.Comment: 10 pages, 2 figures. Third Feynman Festival, 25 -- 29 August 2006, University of Maryland, College Park, Maryland, U.S.

Aspects of mutually unbiased bases in odd prime power dimensions

We rephrase the Wootters-Fields construction [Ann. Phys., {\bf 191}, 363 (1989)] of a full set of mutually unbiased bases in a complex vector space of dimensions $N=p^r$, where $p$ is an odd prime, in terms of the character vectors of the cyclic group $G$ of order $p$. This form may be useful in explicitly writing down mutually unbiased bases for $N=p^r$.Comment: 3 pages, latex, no figure

Quantum Key Distribution between N partners: optimal eavesdropping and Bell's inequalities

Quantum secret-sharing protocols involving N partners (NQSS) are key distribution protocols in which Alice encodes her key into $N-1$ qubits, in such a way that all the other partners must cooperate in order to retrieve the key. On these protocols, several eavesdropping scenarios are possible: some partners may want to reconstruct the key without the help of the other ones, and consequently collaborate with an Eve that eavesdrops on the other partners' channels. For each of these scenarios, we give the optimal individual attack that the Eve can perform. In case of such an optimal attack, the authorized partners have a higher information on the key than the unauthorized ones if and only if they can violate a Bell's inequality.Comment: 14 pages, 1 figur

Orthogonality of Biphoton Polarization States

Orthogonality of two-photon polarization states belonging to a single frequency and spatial mode is demonstrated experimentally, in a generalization of the well-known anti-correlation 'dip' experiment.Comment: Submitted to Phys.Rev.Let

Optimal eavesdropping in cryptography with three-dimensional quantum states

We study optimal eavesdropping in quantum cryptography with three-dimensional systems, and show that this scheme is more secure than protocols using two-dimensional states. We generalize the according eavesdropping transformation to arbitrary dimensions, and discuss the connection with optimal quantum cloning.Comment: 4 pages, 2 figure

On Preparing Entangled Pairs of Polarization Qubits in the Frequency Non-Degenerate Regime

The problems associated with practical implementation of the scheme proposed for preparation of arbitrary states of polarization ququarts based on biphotons are discussed. The influence of frequency dispersion effects are considered, and the necessity of group velocities dispersion compensation in the frequency non-degenerate case even for continuous pumping is demonstrated. A method for this compensation is proposed and implemented experimentally. Physical restrictions on the quality of prepared two-photon states are revealed.Comment: 9 pages, 6 figure

Investigaciones actuales en ligandos de receptores opiáceos

Relevant developments have been achieved in the last twenty years in the search for opioid agonists and antagonists with selectivity for each receptor subpopulation. Recently, new benzomorphan derivatives have been synthesized and compounds with substituted cyclopropylmethyl functionalities at N-l position showed high affinity and selectivity for K opioid receptor subpopulations. MPCB and CCB were selected as specific K agonists. The affinity ofCCB was two-fold the USO,488H one. Mixed peptide-heterocyclic compounds have been derived from these compounds and important informations on binding processes of K ligands have been obtained.En los últimos veinte años se ha llevado a cabo una intensa investigación sobre la búsqueda de agonistas y antagonistas selectivos de cada subtipo de receptores opiáceos. Recientemente, se han sintetizado nuevos derivados benzomorfánicos y compuestos con restos ciclopropilmetílicos sobre N-l que muestran alta afinidad y selectividad por los receptores K . Los compuestos MPCB y CCB se han seleccionado como agonistas específicos K. La afinidad de CCB es dos veces mayor que la del compuesto USO,488H. De estos compuestos se han derivado interesantes compuestos con estructura de peptidoheterocido y se han obtenido importantes informaciones sobre los procesos de binding a los receptores K

Measurement of qutrits

We proposed the procedure of measuring the unknown state of the three-level system - the qutrit, which was realized as the arbitrary polarization state of the single-mode biphoton field. This procedure is accomplished for the set of the pure states of qutrits; this set is defined by the properties of SU(2) transformations, that are done by the polarization transformers.Comment: 9 pages, 9 figure