229 research outputs found
Direct measurement of the Wigner function by photon counting
We report a direct measurement of the Wigner function characterizing the
quantum state of a light mode. The experimental scheme is based on the
representation of the Wigner function as an expectation value of a displaced
photon number parity operator. This allowed us to scan the phase space
point-by-point, and obtain the complete Wigner function without using any
numerical reconstruction algorithms.Comment: 4 pages, REVTe
A measure of the non-Gaussian character of a quantum state
We address the issue of quantifying the non-Gaussian character of a bosonic
quantum state and introduce a non-Gaussianity measure based on the
Hilbert-Schmidt distance between the state under examination and a reference
Gaussian state. We analyze in details the properties of the proposed measure
and exploit it to evaluate the non-Gaussianity of some relevant single- and
multi-mode quantum states. The evolution of non-Gaussianity is also analyzed
for quantum states undergoing the processes of Gaussification by loss and
de-Gaussification by photon-subtraction. The suggested measure is easily
computable for any state of a bosonic system and allows to define a
corresponding measure for the non-Gaussian character of a quantum operation.Comment: revised and enlarged version, 7 pages, 4 figure
Iterative maximum-likelihood reconstruction in quantum homodyne tomography
I propose an iterative expectation maximization algorithm for reconstructing
a quantum optical ensemble from a set of balanced homodyne measurements
performed on an optical state. The algorithm applies directly to the acquired
data, bypassing the intermediate step of calculating marginal distributions.
The advantages of the new method are made manifest by comparing it with the
traditional inverse Radon transformation technique
Optimization of Bell's Inequality Violation For Continuous Variable Systems
Two mode squeezed vacuum states allow Bell's inequality violation (BIQV) for
all non-vanishing squeezing parameter . Maximal violation occurs at
when the parity of either component averages to zero. For a
given entangled {\it two spin} system BIQV is optimized via orientations of the
operators entering the Bell operator (cf. S. L. Braunstein, A. Mann and M.
Revzen: Phys. Rev. Lett. {\bf68}, 3259 (1992)). We show that for finite
in continuous variable systems (and in general whenever the dimensionality of
the subsystems is greater than 2) additional parameters are present for
optimizing BIQV. Thus the expectation value of the Bell operator depends, in
addition to the orientation parameters, on configuration parameters.
Optimization of these configurational parameters leads to a unique maximal BIQV
that depends only on The configurational parameter variation is used
to show that BIQV relation to entanglement is, even for pure state, not
monotonic.Comment: An example added; shows that the amount of Bell's inequality
violation as a measure of entanglement is doubtfu
Maximum likelihood estimation of photon number distribution from homodyne statistics
We present a method for reconstructing the photon number distribution from
the homodyne statistics based on maximization of the likelihood function
derived from the exact statistical description of a homodyne experiment. This
method incorporates in a natural way the physical constraints on the
reconstructed quantities, and the compensation for the nonunit detection
efficiency.Comment: 3 pages REVTeX. Final version, to appear in Phys. Rev. A as a Brief
Repor
Effect of noise and enhancement of nonlocality in on/off photodetection
Nonlocality of two-mode states of light is addressed by means of CHSH
inequality based on displaced on/off photodetection. Effects due to non-unit
quantum efficiency and nonzero dark counts are taken into account. Nonlocality
of both balanced and unbalanced superpositions of few photon-number states, as
well as that of multiphoton twin beams, is investigated. We find that
unbalanced superpositions show larger nonlocality than balanced one when noise
affects the photodetection process. De-Gaussification by means of
(inconclusive) photon subtraction is shown to enhance nonlocality of twin beams
in the low energy regime. We also show that when the measurement is described
by a POVM, rather than a set of projectors, the maximum achievable value of the
Bell parameter in the CHSH inequality is decreased, and is no longer given by
the Cirel'son bound.Comment: 21 Figure
Nonlocality of Two-Mode Squeezing with Internal Noise
We examine the quantum states produced through parametric amplification with
internal quantum noise. The internal diffusion arises by coupling both modes of
light to a reservoir for the duration of the interaction time. The Wigner
function for the diffused two-mode squeezed state is calculated. The
nonlocality, separability, and purity of these quantum states of light are
discussed. In addition, we conclude by studying the nonlocality of two other
continuous variable states: the Werner state and the phase-diffused state for
two light modes.Comment: 7 pages, 5 figures, submitted to PR
Parameters estimation in quantum optics
We address several estimation problems in quantum optics by means of the
maximum-likelihood principle. We consider Gaussian state estimation and the
determination of the coupling parameters of quadratic Hamiltonians. Moreover,
we analyze different schemes of phase-shift estimation. Finally, the absolute
estimation of the quantum efficiency of both linear and avalanche
photodetectors is studied. In all the considered applications, the Gaussian
bound on statistical errors is attained with a few thousand data.Comment: 11 pages. 6 figures. Accepted on Phys. Rev.
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