38 research outputs found
Sampling the canonical phase from phase-space functions
We discuss the possibility of sampling exponential moments of the canonical
phase from the s-parametrized phase space functions. We show that the sampling
kernels exist and are well-behaved for any s>-1, whereas for s=-1 the kernels
diverge in the origin. In spite of that we show that the phase space moments
can be sampled with any predefined accuracy from the Q-function measured in the
double-homodyne scheme with perfect detectors. We discuss the effect of
imperfect detection and address sampling schemes using other measurable
phase-space functions. Finally, we discuss the problem of sampling the
canonical phase distribution itself.Comment: 10 pages, 7 figures, REVTe
Quantum inference of states and processes
The maximum-likelihood principle unifies inference of quantum states and
processes from experimental noisy data. Particularly, a generic quantum process
may be estimated simultaneously with unknown quantum probe states provided that
measurements on probe and transformed probe states are available. Drawbacks of
various approximate treatments are considered.Comment: 7 pages, 4 figure
Maximum likelihood estimation of photon number distribution from homodyne statistics
We present a method for reconstructing the photon number distribution from
the homodyne statistics based on maximization of the likelihood function
derived from the exact statistical description of a homodyne experiment. This
method incorporates in a natural way the physical constraints on the
reconstructed quantities, and the compensation for the nonunit detection
efficiency.Comment: 3 pages REVTeX. Final version, to appear in Phys. Rev. A as a Brief
Repor
Least-squares inversion for density-matrix reconstruction
We propose a method for reconstruction of the density matrix from measurable
time-dependent (probability) distributions of physical quantities. The
applicability of the method based on least-squares inversion is - compared with
other methods - very universal. It can be used to reconstruct quantum states of
various systems, such as harmonic and and anharmonic oscillators including
molecular vibrations in vibronic transitions and damped motion. It also enables
one to take into account various specific features of experiments, such as
limited sets of data and data smearing owing to limited resolution. To
illustrate the method, we consider a Morse oscillator and give a comparison
with other state-reconstruction methods suggested recently.Comment: 16 pages, REVTeX, 6 PS figures include
Matter-wave entanglement and teleportation by molecular dissociation and collisions
We propose dissociation of cold diatomic molecules as a source of atom pairs
with highly correlated (entangled) positions and momenta, approximating the
original quantum state introduced by Einstein, Podolsky and Rosen (EPR) [Phys.
Rev. 47, 777 (1935)]. Wavepacket teleportation is shown to be achievable by its
collision with one of the EPR correlated atoms and manipulation of the other
atom in the pair.Comment: REVTeX, 4 pages, 3 figures. Text reformulated, modified figs. 1 and
2. Accepted by Phys. Rev. Let
Direct sampling of exponential phase moments of smoothed Wigner functions
We investigate exponential phase moments of the s-parametrized
quasidistributions (smoothed Wigner functions). We show that the knowledge of
these moments as functions of s provides, together with photon-number
statistics, a complete description of the quantum state. We demonstrate that
the exponential phase moments can be directly sampled from the data recorded in
balanced homodyne detection and we present simple expressions for the sampling
kernels. The phase moments are Fourier coefficients of phase distributions
obtained from the quasidistributions via integration over the radial variable
in polar coordinates. We performed Monte Carlo simulations of the homodyne
detection and we demonstrate the feasibility of direct sampling of the moments
and subsequent reconstruction of the phase distribution.Comment: RevTeX, 8 pages, 6 figures, accepted Phys. Rev.
Braggoriton--Excitation in Photonic Crystal Infiltrated with Polarizable Medium
Light propagation in a photonic crystal infiltrated with polarizable
molecules is considered. We demonstrate that the interplay between the spatial
dispersion caused by Bragg diffraction and polaritonic frequency dispersion
gives rise to novel propagating excitations, or braggoritons, with intragap
frequencies. We derive the braggoriton dispersion relation and show that it is
governed by two parameters, namely, the strength of light-matter interaction
and detuning between the Bragg frequency and that of the infiltrated molecules.
We also study defect-induced states when the photonic band gap is divided into
two subgaps by the braggoritonic branches and find that each defect creates two
intragap localized states inside each subgap.Comment: LaTeX, 8 pages, 5 figure
Continuous-variable optical quantum state tomography
This review covers latest developments in continuous-variable quantum-state
tomography of optical fields and photons, placing a special accent on its
practical aspects and applications in quantum information technology. Optical
homodyne tomography is reviewed as a method of reconstructing the state of
light in a given optical mode. A range of relevant practical topics are
discussed, such as state-reconstruction algorithms (with emphasis on the
maximum-likelihood technique), the technology of time-domain homodyne
detection, mode matching issues, and engineering of complex quantum states of
light. The paper also surveys quantum-state tomography for the transverse
spatial state (spatial mode) of the field in the special case of fields
containing precisely one photon.Comment: Finally, a revision! Comments to lvov(at)ucalgary.ca and
raymer(at)uoregon.edu are welcom
The characterization of Gaussian operations and Distillation of Gaussian States
We characterize the class of all physical operations that transform Gaussian
states to Gaussian states. We show that this class coincides with that of all
operations which can be performed on Gaussian states using linear optical
elements and homodyne measurements. For bipartite systems we characterize the
processes which can be implemented by local operations and classical
communication, as well as those that can be implemented using positive partial
transpose preserving maps. As an application, we show that Gaussian states
cannot be distilled by local Gaussian operations and classical communication.
We also define and characterize positive (but not completely positive) Gaussian
maps.Comment: 8 pages, revtex4; v4: published version; v3: more details on
V(gamma), some typos corrected, formulations clarifie
Non-classical correlations from dissociation time entanglement
We discuss a strongly entangled two-particle state of motion that emerges
naturally from the double-pulse dissociation of a diatomic molecule. This
state, which may be called dissociation-time entangled, permits the unambiguous
demonstration of non-classical correlations by violating a Bell inequality
based on switched single particle interferometry and only position
measurements. We apply time-dependent scattering theory to determine the
detrimental effect of dispersion. The proposed setup brings into reach the
possibility of establishing non-classical correlations with respect to system
properties that are truly macroscopically distinct.Comment: 8 pages, 2 figures; corresponds to published versio