344 research outputs found
Group Theoretical Quantum Tomography
The paper is devoted to the mathematical foundation of the quantum tomography
using the theory of square-integrable representations of unimodular Lie groups.Comment: 13 pages, no figure, Latex2e. Submitted to J.Math.Phy
Adaptive Quantum Homodyne Tomography
An adaptive optimization technique to improve precision of quantum homodyne
tomography is presented. The method is based on the existence of so-called null
functions, which have zero average for arbitrary state of radiation. Addition
of null functions to the tomographic kernels does not affect their mean values,
but changes statistical errors, which can then be reduced by an optimization
method that "adapts" kernels to homodyne data. Applications to tomography of
the density matrix and other relevant field-observables are studied in detail.Comment: Latex (RevTex class + psfig), 9 Figs, Submitted to PR
Quantum tomography of mesoscopic superpositions of radiation states
We show the feasibility of a tomographic reconstruction of Schr\"{o}dinger
cat states generated according to the scheme proposed by S. Song, C.M. Caves
and B. Yurke [Phys. Rev. A 41, 5261 (1990)]. We present a technique that
tolerates realistic values for quantum efficiency at photodetectors. The
measurement can be achieved by a standard experimental setup.Comment: Submitted to Phys. Rev. Lett.; 4 pages including 6 ps figure
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.
Efficient universal programmable quantum measurements
A universal programmable detector is a device that can be tuned to perform
any desired measurement on a given quantum system, by changing the state of an
ancilla. With a finite dimension d for the ancilla only approximate universal
programmability is possible, with "size" d=f(1/e) increasing function of the
"accuracy" 1/e. In this letter we show that, much better than the exponential
size known in the literature, one can achieve polynomial size. An explicit
example with linear size is also presented. Finally, we show that for covariant
measurements exact programmability is feasible.Comment: 4 pages, RevTex
Quantum state engineering assisted by entanglement
We suggest a general scheme for quantum state engineering based on
conditional measurements carried out on entangled twin-beam of radiation.
Realistic detection schemes such as {\sc on/off} photodetection, homodyne
detection and joint measurement of two-mode quadratures are analyzed in
details. Imperfections of the apparatuses, such as nonunit quantum efficiency
and finite resolution, are taken into account. We show that conditional {\sc
on/off} photodetection provides a reliable scheme to verify nonclassicality,
whereas conditional homodyning represents a tunable and robust source of
squeezed light. We also describe optical teleportation as a conditional
measurement, and evaluate the degrading effects of finite amount of
entanglement, decoherence due to losses, and nonunit quantum efficiency.Comment: Some pics with low resolution. Originals at http://www.qubit.i
No-signaling, entanglement-breaking, and localizability in bipartite channels
A bipartite quantum channel represents the interaction between systems,
generally allowing for exchange of information. A special class of bipartite
channels are the no-signaling ones, which do not allow communication. In Ref.
[1] it has been conjectured that all no-signaling channels are mixtures of
entanglement-breaking and localizable channels, which require only local
operations and entanglement. Here we provide the general realization scheme,
giving a counterexample to the conjecture.Comment: 4 pages, revtex
Optimal Non-Universally Covariant Cloning
We consider non-universal cloning maps, namely cloning transformations which
are covariant under a proper subgroup G of the universal unitary group U(d),
where d is the dimension of the Hilbert space H of the system to be cloned. We
give a general method for optimizing cloning for any cost-function. Examples of
applications are given for the phase-covariant cloning (cloning of equatorial
qubits) and for the Weyl-Heisenberg group (cloning of "continuous variables").Comment: 6 page
Characterising a universal cloning machine by maximum-likelihood estimation
We apply a general method for the estimation of completely positive maps to
the 1-to-2 universal covariant cloning machine. The method is based on the
maximum-likelihood principle, and makes use of random input states, along with
random projective measurements on the output clones. The downhill simplex
algorithm is applied for the maximisation of the likelihood functional.Comment: 5 pages, 2 figure
Local observables for entanglement witnesses
We present an explicit construction of entanglement witnesses for depolarized
states in arbitrary finite dimension. For infinite dimension we generalize the
construction to twin-beams perturbed by Gaussian noises in the phase and in the
amplitude of the field. We show that entanglement detection for all these
families of states requires only three local measurements. The explicit form of
the corresponding set of local observables (quorom) needed for entanglement
witness is derived.Comment: minor corrections, title change
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