269 research outputs found
Discrete Wigner functions and the phase space representation of quantum teleportation
We present a phase space description of the process of quantum teleportation
for a system with an dimensional space of states. For this purpose we
define a discrete Wigner function which is a minor variation of previously
existing ones. This function is useful to represent composite quantum system in
phase space and to analyze situations where entanglement between subsystems is
relevant (dimensionality of the space of states of each subsystem is
arbitrary). We also describe how a direct tomographic measurement of this
Wigner function can be performed.Comment: 8 pages, 1 figure, to appear in Phys Rev
Quantum computers in phase space
We represent both the states and the evolution of a quantum computer in phase
space using the discrete Wigner function. We study properties of the phase
space representation of quantum algorithms: apart from analyzing important
examples, such as the Fourier Transform and Grover's search, we examine the
conditions for the existence of a direct correspondence between quantum and
classical evolutions in phase space. Finally, we describe how to directly
measure the Wigner function in a given phase space point by means of a
tomographic method that, itself, can be interpreted as a simple quantum
algorithm.Comment: 16 pages, 7 figures, to appear in Phys Rev
Quantum properties of the codirectional three-mode Kerr nonlinear coupler
We investigate the quantum properties for the codirectional three-mode Kerr
nonlinear coupler. We investigate single-, two- and three-mode quadrature
squeezing, Wigner function and purity. We prove that this device can provide
richer nonclassical effects than those produced by the conventional coupler,
i.e. the two-mode Kerr coupler. We show that it can provide squeezing and the
quadrature squeezing exhibiting leaf-revival-collapse phenomenon in dependence
on the values of the interaction parameters. In contrast to the conventional
Kerr coupler two different forms of cat states can be simultaneously generated
in the waveguides. We deduce conditions required for the complete
disentanglement between the components of the system.Comment: 23 pages, 6 figure
Test of quantum nonlocality for cavity fields
There have been studies on formation of quantum-nonlocal states in spatially
separate two cavities. We suggest a nonlocal test for the field prepared in the
two cavities. We couple classical driving fields with the cavities where a
nonlocal state is prepared. Two independent two-level atoms are then sent
through respective cavities to interact off-resonantly with the cavity fields.
The atomic states are measured after the interaction. Bell's inequality can be
tested by the joint probabilities of two-level atoms being in their excited or
ground states. We find that quantum nonlocality can also be tested using a
single atom sequentially interacting with the two cavities. Potential
experimental errors are also considered. We show that with the present
experimental condition of 5% error in the atomic velocity distribution, the
violation of Bell's inequality can be measured.Comment: 14pages, 2figures. accepted to Phys. Rev.
Squeezing arbitrary cavity-field states through their interaction with a single driven atom
We propose an implementation of the parametric amplification of an arbitrary
radiation-field state previously prepared in a high-Q cavity. This nonlinear
process is accomplished through the dispersive interactions of a single
three-level atom (fundamental |g>, intermediate |i>, and excited |e> levels)
simultaneously with i) a classical driving field and ii) a previously prepared
cavity mode whose state we wish to squeeze. We show that, in the adiabatic
approximantion, the preparation of the initial atomic state in the intermediate
level |i> becomes crucial for obtaing the degenerated parametric amplification
process.Comment: Final published versio
Recovering coherence from decoherence: a method of quantum state reconstruction
We present a feasible scheme for reconstructing the quantum state of a field
prepared inside a lossy cavity. Quantum coherences are normally destroyed by
dissipation, but we show that at zero temperature we are able to retrieve
enough information about the initial state, making possible to recover its
Wigner function as well as other quasiprobabilities. We provide a numerical
simulation of a Schroedinger cat state reconstruction.Comment: 8 pages, in RevTeX, 4 figures, accepted for publication in Phys. Rev.
A (november 1999
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
Sampling functions for multimode homodyne tomography with a single local oscillator
We derive various sampling functions for multimode homodyne tomography with a
single local oscillator. These functions allow us to sample multimode
s-parametrized quasidistributions, density matrix elements in Fock basis, and
s-ordered moments of arbitrary order directly from the measured quadrature
statistics. The inevitable experimental losses can be compensated by proper
modification of the sampling functions. Results of Monte Carlo simulations for
squeezed three-mode state are reported and the feasibility of reconstruction of
the three-mode Q-function and s-ordered moments from 10^7 sampled data is
demonstrated.Comment: 12 pages, 8 figures, REVTeX, submitted Phys. Rev.
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
Quantum phase gate with a selective interaction
We present a proposal for implementing quantum phase gates using selective
interactions. We analize selectivity and the possibility to implement these
gates in two particular systems, namely, trapped ions and Cavity QED.Comment: Four pages of TEX file and two EPS figures. Submitted for publicatio
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