20 research outputs found
Quorum of observables for universal quantum estimation
Any method for estimating the ensemble average of arbitrary operator
(observables or not, including the density matrix) relates the quantity of
interest to a complete set of observables, i.e. a quorum}. This corresponds to
an expansion on an irreducible set of operators in the Liouville space. We give
two general characterizations of these sets. All the known unbiased
reconstruction techniques, i.e. ``quantum tomographies'', can be described in
this framework. New operatorial resolutions are given that can be used to
implement novel reconstruction schemes.Comment: Latex, no figure
Quantum Tomography
This is the draft version of a review paper which is going to appear in
"Advances in Imaging and Electron Physics"Comment: To appear in "Advances in Imaging and Electron Physics". Some figs
with low resolutio
Quantum Tomography of a system of three-level atoms
We analyze the possibility of tomographic reconstruction of a system of
three-level atoms in both non-degenerate and degenerate cases. In the
non-degenerate case (when both transitions can be accessed independently) a
complete reconstruction is possible. In the degenerate case (when both
transitions are excited simultaneously) the complete reconstruction is
achievable only for a single atom in the Sigma configuration. For multiple
Sigma atoms, or even a single atom in the Lambda configuration, only partial
reconstruction is possible. Examples of one and two-atom cases are explicitly
considered.Comment: accepted in J.Phys.A: Math.& Theo
To take a (binary) decision you'd better use entanglement
We address two-mode quantum interferometry as binary measurements aimed at determining whether or not a phase perturbation has occurred. We show that optimized measurements achieve the best sensitivity when the input state is entangled. A concrete set-up based on parametric sources of entanglement and photodetection is also suggested and shown to approach ideal sensitivity
Comment on ``Creating Metastable Schroedinger Cat States''
After a careful analysis of the feedback model recently proposed by Slosser
and Milburn [Phys. Rev. Lett. 75, 418 (1995)], we are led to the conclusion
that---under realistic conditions---their scheme is not significantly more
effective in the production of linear superpositions of macroscopically
distinguishable quantum states than the usual quantum-optical Kerr effect.Comment: 1 page, RevTeX, 1 eps figure (fig_1.eps), accepted for publication in
Physical Review Letters [Phys. Rev. Lett. 77 (9) (1996)
Using entanglement improves precision of quantum measurements
We show how entanglement can be used to improve the estimation of an unknown
transformation. Using entanglement is always of benefit, in improving either
the precision or the stability of the measurement. Examples relevant for
applications are illustrated, for either qubits and continuous variable
Optimal phase estimation in quantum networks
We address the problem of estimating the phase phi given N copies of the
phase rotation u(phi) within an array of quantum operations in finite
dimensions. We first consider the special case where the array consists of an
arbitrary input state followed by any arrangement of the N phase rotations, and
ending with a POVM. We optimise the POVM for a given input state and fixed
arrangement. Then we also optimise the input state for some specific cost
functions. In all cases, the optimal POVM is equivalent to a quantum Fourier
transform in an appropriate basis. Examples and applications are given.Comment: 9 pages, 2 figures; this is an extended version of
arXiv:quant-ph/0609160. v2: minor corrections in reference