6,187 research outputs found
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 shared ebits: i) 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 -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
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