5,545 research outputs found
Maximum-likelihood method in quantum estimation
The maximum-likelihood method for quantum estimation is reviewed and applied
to the reconstruction of density matrix of spin and radiation as well as to the
determination of several parameters of interest in quantum optics.Comment: 12 pages, 4 figure
Non-divisibility vs backflow of information in understanding revivals of quantum correlations for continuous-variable systems interacting with fluctuating environments
We address the dynamics of quantum correlations for a bipartite
continuous-variable quantum system interacting with its fluctuating
environment. In particular, we consider two independent quantum oscillators
initially prepared in a Gaussian state, e.g. a squeezed thermal state, and
compare the dynamics resulting from local noise, i.e. oscillators coupled to
two independent external fields, to that originating from common noise, i.e.
oscillators interacting with a single common field. We prove non-Markovianity
(non-divisibility) of the dynamics in both regimes and analyze the connections
between non-divisibility, backflow of information and revivals of quantum
correlations. Our main results may be summarized as follows: (i) revivals of
quantumness are present in both scenarios, however, the interaction with a
common environment better preserves the quantum features of the system; (ii)
the dynamics is always non-divisible but revivals of quantum correlations are
present only when backflow of information is present as well. We conclude that
non-divisibility in its own is not a resource to preserve quantum correlations
in our system, i.e. it is not sufficient to observe recoherence phenomena.
Rather, it represents a necessary prerequisite to obtain backflow of
information, which is the true ingredient to obtain revivals of quantumness
Entanglement as a resource for discrimination of classical environments
We address extended systems interacting with classical fluctuating
environments and analyze the use of quantum probes to discriminate local noise,
described by independent fluctuating fields, from common noise, corresponding
to the interaction with a common one. In particular, we consider a bipartite
system made of two non interacting harmonic oscillators and assess
discrimination strategies based on homodyne detection, comparing their
performances with the ultimate bounds on the error probabilities of
quantum-limited measurements. We analyze in details the use of Gaussian probes,
with emphasis on experimentally friendly signals. Our results show that a joint
measurement of the position-quadrature on the two oscillators outperforms any
other homodyne-based scheme for any input Gaussian state
Experimental reconstruction of photon statistics without photon counting
Experimental reconstructions of photon number distributions of both
continuous-wave and pulsed light beams are reported. Our scheme is based on
on/off avalanche photodetection assisted by maximum-likelihood estimation and
does not involve photon counting. Reconstructions of the distribution for both
semiclassical and quantum states of light are reported for single-mode as well
as for multimode beams.Comment: Revised version: in press on PRL. 4 pages, 4 fig
About the use of entanglement in the optical implementation of quantum information processing
We review some applications of entanglement to improve quantum measurements
and communication, with the main focus on the optical implementation of quantum
information processing. The evolution of continuos variable entangled states in
active optical fibers is also analyzed.Comment: 8pages, invited contribution to Quant. Interf. IV (ICTP Trieste 2002
Input-output relations for multiport ring cavities
Quantum input-output relations for a generic -port ring cavity are
obtained by modeling the ring as a cascade of interlinked beam splitters.
Cavity response to a beam impinging on one port is studied as a function of the
beam-splitter reflectivities and the internal phase-shifts. Interferometric
sensitivity and stability are analyzed as a function of the number of ports.Comment: 6 pages, 5 figures (low-res
Conditional measurements on multimode pairwise entangled states from spontaneous parametric downconversion
We address the intrinsic multimode nature of the quantum state of light
obtained by pulsed spontaneous parametric downconversion and develop a
theoretical model based only on experimentally accessible quantities. We
exploit the pairwise entanglement as a resource for conditional multimode
measurements and derive closed formulas for the detection probability and the
density matrix of the conditional states. We present a set of experiments
performed to validate our model in different conditions that are in excellent
agreement with experimental data. Finally, we evaluate nonGaussianity of the
conditional states obtained from our source with the aim of discussing the
effects of the different experimental parameters on the efficacy of this type
of conditional state preparation
Experimental pre-assessing entanglement in Gaussian states mixing
We suggest and demonstrate a method to assess entanglement generation schemes
based on mixing of Gaussian states at a beam splitter (BS). Our method is based
on the fidelity criterion and represents a tool to analyze the effect of losses
and noise before the BS in both symmetric and asymmetric channels with and
without thermal effects. More generally, our scheme allows one to pre-assess
entanglement resources and to optimize the design of BS-based schemes for the
generation of continuous variable entanglement.Comment: 10 pages, 15 figure
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