76 research outputs found
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.
Optimal estimation of multiple phases
We study the issue of simultaneous estimation of several phase shifts induced
by commuting operators on a quantum state. We derive the optimal positive
operator-valued measure corresponding to the multiple-phase estimation. In
particular, we discuss the explicit case of the optimal detection of double
phase for a system of identical qutrits and generalise these results to optimal
multiple phase detection for d-dimensional quantum states.Comment: 6 page
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
Exploiting quantum parallelism of entanglement for a complete experimental quantum characterization of a single qubit device
We present the first full experimental quantum tomographic characterization
of a single-qubit device achieved with a single entangled input state. The
entangled input state plays the role of all possible input states in quantum
parallel on the tested device. The method can be trivially extended to any
n-qubits device by just replicating the whole experimental setup n times.Comment: 4 pages in revtex4 with 4 eps 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
Improving quantum interferometry by using entanglement (to take a decision you'd better use entanglement)
We address the use of entanglement to improve the precision of generalized
quantum interferometry, i.e. of binary measurements aimed to determine whether
or not a perturbation has been applied by a given device. For the most relevant
operations in quantum optics, we evaluate the optimal detection strategy and
the ultimate bounds to the minimum detectable perturbation. Our results
indicate that entanglement-assisted strategies improve the discrimination in
comparison with conventional schemes. A concrete setup to approach performances
of the optimal strategies is also suggested.Comment: 2 fig
Protocols for entanglement transformations of bipartite pure states
We present a general theoretical framework for both deterministic and
probabilistic entanglement transformations of bipartite pure states achieved
via local operations and classical communication. This framework unifies and
greatly simplifies previous works. A necessary condition for ``pure
contraction'' transformations is given. Finally, constructive protocols to
achieve both probabilistic and deterministic entanglement transformations are
presented.Comment: 7 pages, no figures. Version slightly modified on Physical Review A
reques
Unambiguous quantum state filtering
In this paper, we consider the generalized measurement where one particular
quantum signal is unambiguously extracted from a set of non-commutative quantum
signals and the other signals are filtered out. Simple expressions for the
maximum detection probability and its POVM are derived. We applyl such
unambiguous quantum state filtering to evaluation of the sensing of decoherence
channels. The bounds of the precision limit for a given quantum state of probes
and possible device implementations are discussed.Comment: 7 pages, 5 figure
Quantum State Reconstruction of Many Body System Based on Complete Set of Quantum Correlations Reduced by Symmetry
We propose and study a universal approach for the reconstruction of quantum
states of many body systems from symmetry analysis. The concept of minimal
complete set of quantum correlation functions (MCSQCF) is introduced to
describe the state reconstruction. As an experimentally feasible physical
object, the MCSQCF is mathematically defined through the minimal complete
subspace of observables determined by the symmetry of quantum states under
consideration. An example with broken symmetry is analyzed in detail to
illustrate the idea.Comment: 10 pages, n figures, Revte
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