7,243 research outputs found
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
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
A multidomain spectral collocation method for the Stokes problem
A multidomain spectral collocation scheme is proposed for the approximation of the two-dimensional Stokes problem. It is shown that the discrete velocity vector field is exactly divergence-free and we prove error estimates both for the velocity and the pressure
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
Efficient use of quantum resources for the transmission of a reference frame
We propose a covariant protocol for transmitting reference frames encoded on
spins, achieving sensitivity without the need of a pre-established
reference frame and without using entanglement between sender and receiver. The
protocol exploits the use of equivalent representations, which were overlooked
in the previous literature.Comment: 4 pages, no figures; added new references and improved introduction.
Accepted for publication on PR
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