153 research outputs found
MaxEnt assisted MaxLik tomography
Maximum likelihood estimation is a valuable tool often applied to inverse
problems in quantum theory. Estimation from small data sets can, however, have
non unique solutions. We discuss this problem and propose to use Jaynes maximum
entropy principle to single out the most unbiased maximum-likelihood guess.Comment: 10 pages, 5 figures, presented at MaxEnt conference in Jackson, WY,
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Quantum Zeno tomography
We show that the resolution "per absorbed particle" of standard absorption
tomography can be outperformed by a simple interferometric setup, provided that
the different levels of "gray" in the sample are not uniformly distributed. The
technique hinges upon the quantum Zeno effect and has been tested in numerical
simulations. The scheme we propose could be implemented in experiments with
UV-light, neutrons or X-rays.Comment: 8 pages, 5 figure
On the local unitary equivalence of states of multi-partite systems
Two pure states of a multi-partite system are alway are related by a unitary
transformation acting on the Hilbert space of the whole system. This
transformation involves multi-partite transformations. On the other hand some
quantum information protocols such as the quantum teleportation and quantum
dense coding are based on equivalence of some classes of states of bi-partite
systems under the action of local (one-particle) unitary operations. In this
paper we address the question: ``Under what conditions are the two states
states, and , of a multi-partite system locally unitary
equivalent?'' We present a set of conditions which have to be satisfied in
order that the two states are locally unitary equivalent. In addition, we study
whether it is possible to prepare a state of a multi-qudit system. which is
divided into two parts A and B, by unitary operations acting only on the
systems A and B, separately.Comment: 6 revtex pages, 1 figur
Informational completeness of continuous-variable measurements
We justify that homodyne tomography turns out to be informationally complete
when the number of independent quadrature measurements is equal to the
dimension of the density matrix in the Fock representation. Using this as our
thread, we examine the completeness of other schemes, when continuous-variable
observations are truncated to discrete finite-dimensional subspaces.Comment: To appear in Phys. Rev.
Efficient tomography with unknown detectors
We compare the two main techniques used for estimating the state of a
physical system from unknown measurements: standard detector tomography and
data-pattern tomography. Adopting linear inversion as a fair benchmark, we show
that the difference between these two protocols can be traced back to the
nonexistence of the reverse-order law for pseudoinverses. We capitalize on this
fact to identify regimes where the data-pattern approach outperforms the
standard one and vice versa. We corroborate these conclusions with numerical
simulations of relevant examples of quantum state tomography.Comment: 13 pages, 6 figures. Submitted for publication. Comments most
welcome
Optimal measurements for quantum spatial superresolution
We construct optimal measurements, achieving the ultimate precision predicted
by quantum theory, for the simultaneous estimation of centroid, separation, and
relative intensities of two incoherent point sources using a linear optical
system. We discuss the physical feasibility of the scheme, which could pave the
way for future practical implementations of quantum inspired imaging.Comment: 7 pages. 3 color figures. Title change
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