Deterministic Quantum Distribution of a d-ary key

We present an extension to a d-ary alphabet of a recently proposed deterministic quantum key distribution protocol. It relies on the use of mutually unbiased bases in prime power dimension d, for which we provide an explicit expression. Then, by considering a powerful individual attack, we show that the security of the protocol is maximal for d=3.Comment: 12 pages, 2 figure

Temporal imperfections building up correcting codes

We address the timing problem in realizing correcting codes for quantum information processing. To deal with temporal uncertainties we employ a consistent quantum mechanical approach. The conditions for optimizing the effect of error correction in such a case are determined.Comment: 5 pages, 2 eps figures, to appear in J. Mod. Op

Characterizing the Depolarizing Quantum Channel in Terms of Riemannian Geometry

We explore the conceptual usefulness of Riemannian geometric tools induced by the statistical concept of distinguishability in quantifying the effect of a depolarizing channel on quantum states. Specifically, we compare the geometries of the interior of undeformed and deformed Bloch spheres related to density operators on a two-dimensional Hilbert space. We show that randomization emerges geometrically through a smaller infinitesimal quantum line element on the deformed Bloch sphere while the uniform contraction manifests itself via a deformed set of geodesics where the spacial components of the deformed four-Bloch vector are simply the contracted versions of the undeformed Bloch vector components.Comment: 7 pages, 0 figures; Accepted contribution to "Folding and Unfolding: Interactions from Geometry", Workshop in honour of Giuseppe Marmo's 65th birthday; 8-12 June 2011, Ischia (ITALY

Estimation of two-qubit interactions through channels with environment assistance

We consider the estimation of two-qubit interactions when initial states of both qubit can be locally controlled, while the final state of only one qubit can be measured. This amounts to realize a model of quantum channel communication with environment assistance. In such a framework the unitaries' parameters space results a tetrahedron in $\mathbb{R}^3$. On its edges the problem, becoming of single parameter estimation, can be exactly solved and we derive optimal probe states and estimators. Our results show that the possibility of environment assistance is always beneficial, while the usage of entanglement at channels' input is not

Quantum Zeno-like effect due to competing decoherence mechanisms

We propose a selfconsistent quantum mechanical approach to study the dynamics of a two-level system subject to random time evolution. This randomness gives rise to competing effects between dissipative and non-dissipative decoherence with a consequent slow down of the atomic decay rate.Comment: 4 pages, ReVTeX file, 2 eps figures, to appear in Phys. Rev.

Information Dissipation in Random Quantum Networks

We study the information dynamics in a network of spin-$1/2$ particles when edges representing $XY$ interactions are randomly added to a disconnected graph accordingly to a probability distribution characterized by a "weighting" parameter. In this way we model dissipation of information initially localized in single or two qubits all over the network. We then show the dependence of this phenomenon from weighting parameter and size of the network.Comment: 9 pages, 5 figure

Noise enhancing the classical information capacity of a quantum channel

We present a simple model of quantum communication where a noisy quantum channel may benefit from the addition of further noise at the decoding stage. We demonstrate enhancement of the classical information capacity of an amplitude damping channel, with a predetermined detection threshold, by the addition of noise in the decoding measurement.Comment: 4 pages, 2 figure

Macroscopic Coherence for a Trapped Electron

We investigate the possibility of generating quantum macroscopic coherence phenomena by means of relativistic effects on a trapped electron.Comment: 6 pages, RevTex, accepted by Phys. Rev.