29 research outputs found
Approximation and limit theorems for quantum stochastic models with unbounded coefficients
We prove a limit theorem for quantum stochastic differential equations with
unbounded coefficients which extends the Trotter-Kato theorem for contraction
semigroups. From this theorem, general results on the convergence of
approximations and singular perturbations are obtained. The results are
illustrated in several examples of physical interest.Comment: 23 page
Quantum state reconstruction via continuous measurement
We present a new procedure for quantum state reconstruction based on weak
continuous measurement of an ensemble average. By applying controlled evolution
to the initial state new information is continually mapped onto the measured
observable. A Bayesian filter is then used to update the state-estimate in
accordance with the measurement record. This generalizes the standard paradigm
for quantum tomography based on strong, destructive measurements on separate
ensembles. This approach to state estimation can be non-destructive and
real-time, giving information about observables whose evolution cannot be
described classically, opening the door to new types of quantum feedback
control.Comment: 4 pages, 2 figure
A quantum stochastic calculus approach to modeling double-pass atom-field coupling
We examine a proposal by Sherson and Moelmer to generate
polarization-squeezed light in terms of the quantum stochastic calculus (QSC).
We investigate the statistics of the output field and confirm their results
using the QSC formalism. In addition, we study the atomic dynamics of the
system and find that this setup can produce up to 3 dB of atomic spin
squeezing.Comment: Minor corrections to Section II
Physical model of continuous two-qubit parity measurement in a cavity-QED network
We propose and analyze a physical implementation of two-qubit parity
measurements as required for continuous error correction, assuming a setup in
which the individual qubits are strongly coupled to separate optical cavities.
A single optical probe beam scatters sequentially from the two cavities and the
continuous parity measurement is realized via fixed quadrature homodyne
photo-detection. We present models based on quantum stochastic differential
equations (QSDE's) for both an ideal continuous parity measurement and our
proposed cavity quantum electrodynamics (cavity QED) implementation; a recent
adiabatic elimination theorem for QSDE's is used to assert strong convergence
of the latter to the former in an appropriate limit of physical parameters.
Performance of the cavity QED scheme is studied via numerical simulation with
experimentally realistic parameters.Comment: 4 pages, 3 figure
Quantum Control of the Hyperfine Spin of a Cs Atom Ensemble
We demonstrate quantum control of a large spin-angular momentum associated
with the F=3 hyperfine ground state of 133Cs. A combination of time dependent
magnetic fields and a static tensor light shift is used to implement
near-optimal controls and map a fiducial state to a broad range of target
states, with yields in the range 0.8-0.9. Squeezed states are produced also by
an adiabatic scheme that is more robust against errors. Universal control
facilitates the encoding and manipulation of qubits and qudits in atomic ground
states, and may lead to improvement of some precision measurements.Comment: 4 pages, 4 figures (color
Generalized Pseudopotentials for Higher Partial Wave Scattering
We derive a generalized zero-range pseudopotential applicable to all partial
wave solutions to the Schroedinger equation based on a delta-shell potential in
the limit that the shell radius approaches zero. This properly models all
higher order multipole moments not accounted for with a monopolar delta
function at the origin, as used in the familiar Fermi pseudopotential for
s-wave scattering. By making the strength of the potential energy dependent, we
derive self-consistent solutions for the entire energy spectrum of the
realistic potential. We apply this to study two particles in an isotropic
harmonic trap, interacting through a central potential, and derive analytic
expressions for the energy eigenstates and eigenvalues.Comment: RevTeX 4 pages, 1 figure, final published versio