Entanglement at distance: qubits versus continuous variables

We consider the problem of obtaining maximally entangled photon states at distance in the presence of loss. We compare the efficiency of two different schemes in establishing $N$ shared ebits: i) $N$ single ebit states with the qubit encoded on polarization; ii) a single continuous variable entangled state (emode) assisted by optimal local operation and classical communication (LOCC) protocol in order to obtain a $2^N$-dimensional maximally entangled state, with qubits encoded on the photon number.Comment: 5 pages. 4 eps files. Use fortschritte.sty (included

Purification of noisy quantum measurements

We consider the problem of improving noisy quantum measurements by suitable preprocessing strategies making many noisy detectors equivalent to a single ideal detector. For observables pertaining to finite-dimensional systems (e.g. qubits or spins) we consider preprocessing strategies that are reminiscent of quantum error correction procedures and allows one to perfectly measure an observable on a single quantum system for increasing number of inefficient detectors. For measurements of observables with unbounded spectrum (e.g. photon number, homodyne and heterodyne detection), the purification of noisy quantum measurements can be achieved by preamplification as suggested by H. P. Yuen [1].Comment: 13 pages, 8 figures; minor correction

Joint estimation of real squeezing and displacement

We study the problem of joint estimation of real squeezing and amplitude of the radiation field, deriving the measurement that maximizes the probability density of detecting the true value of the unknown parameters. More generally, we provide a solution for the problem of estimating the unknown unitary action of a nonunimodular group in the maximum likelihood approach. Remarkably, in this case the optimal measurements do not coincide with the so called square-root measurements. In the case of squeezing and displacement we analyze in detail the sensitivity of estimation for coherent states and displaced squeezed states, deriving the asymptotic relation between the uncertainties in the joint estimation and the corresponding uncertainties in the optimal separate measurements of squeezing and displacement. A two-mode setup is also analyzed, showing how entanglement between optical modes can be used to approximate perfect estimation.Comment: 14 pages, 3 eps figures; a section has been added with new results in terms of Heisenberg uncertainty relations for the joint measuremen

Optimization of quantum universal detectors

The expectation value of an arbitrary operator O can be obtained via a universal measuring apparatus that is independent of O, by changing only the data-processing of the outcomes. Such a ``universal detector'' performs a joint measurement on the system and on a suitable ancilla prepared in a fixed state, and is equivalent to a positive operator valued measure (POVM) for the system that is ``informationally complete''. The data processing functions generally are not unique, and we pose the problem of their optimization, providing some examples for covariant POVM's, in particular for SU(d) covariance group.Comment: 8 pages, no figures. Proceedingsof the 8th International Conference on Squeezed States and Uncertainty Relations ICSSUR' 2003, Puebla, Mexico - June 9-13, 200