536 research outputs found
Unconditional two-mode squeezing of separated atomic ensembles
We propose schemes for the unconditional preparation of a two-mode squeezed
state of effective bosonic modes realized in a pair of atomic ensembles
interacting collectively with optical cavity and laser fields. The scheme uses
Raman transitions between stable atomic ground states and under ideal
conditions produces pure entangled states in the steady state. The scheme works
both for ensembles confined within a single cavity and for ensembles confined
in separate, cascaded cavities.Comment: 4 pages, 2 figure
Collective spin systems in dispersive optical cavity QED: Quantum phase transitions and entanglement
We propose a cavity QED setup which implements a dissipative
Lipkin-Meshkov-Glick model -- an interacting collective spin system. By varying
the external model parameters the system can be made to undergo both first-and
second-order quantum phase transitions, which are signified by dramatic changes
in cavity output field properties, such as the probe laser transmission
spectrum. The steady-state entanglement between pairs of atoms is shown to peak
at the critical points and can be experimentally determined by suitable
measurements on the cavity output field. The entanglement dynamics also
exhibits pronounced variations in the vicinities of the phase transitions.Comment: 19 pages, 18 figures, shortened versio
Coupling of effective one-dimensional two-level atoms to squeezed light
A cavity QED system is analyzed which duplicates the dynamics of a two-level
atom in free space interacting exclusively with broadband squeezed light. We
consider atoms in a three or four-level Lambda-configuration coupled to a
high-finesse optical cavity which is driven by a squeezed light field. Raman
transitions are induced between a pair of stable atomic ground states via the
squeezed cavity mode and coherent driving fields. An analysis of the reduced
master equation for the atomic ground states shows that a three-level atomic
system has insufficient parameter flexibility to act as an effective two-level
atom interacting exclusively with a squeezed reservoir. However, the inclusion
of a fourth atomic level, coupled dispersively to one of the two ground states
by an auxiliary laser field, introduces an extra degree of freedom and enables
the desired interaction to be realised. As a means of detecting the reduced
quadrature decay rate of the effective two-level system, we examine the
transmission spectrum of a weak coherent probe field incident upon the cavity
Counter-Intuitive Vacuum-Stimulated Raman Scattering
Vacuum-stimulated Raman scattering in strongly coupled atom-cavity systems
allows one to generate free-running single photon pulses on demand. Most
properties of the emitted photons are well defined, provided spontaneous
emission processes do not contribute. Therefore, electronic excitation of the
atom must not occur, which is assured for a system adiabatically following a
dark state during the photon-generation process. We experimentally investigate
the conditions that must be met for adiabatic following in a time-of-flight
driven system, with atoms passing through a cavity and a pump beam oriented
transverse to the cavity axis. From our results, we infer the optimal intensity
and relative pump-beam position with respect to the cavity axis.Comment: 4 pages, 4 figure
Propagation management for no-acknowledge HF communications links
The problems in the engineering of no-acknowledge HF links are
reviewed with particular application to naval applications.
Models for predicting the performance of the ionospheric path and
techniques for measurement of channel quality are considered. Manual
methods of propagation management are found to be limited by speed and
accuracy in conducting measurements. Automation of the channel quality
measurement process allowed observation of short term behaviour of the link
for comparison with predictions. Long term behaviour tends toward the
prediction but large deviations are found in the short term.
Techniques for improving the estimates of the remote ionosphere
were investigated. Updating predictions using a derived ionospheric index
and the use of a backscatter sounder were attempted with limited success.
Concepts for improvement in no-acknowledge links include
networking between transmitting stations, modulation diversity and coding
techniques, and development of 'intelligent receiving terminals with
facilities for rapid selection of operating channels and overcoming unwanted
signal interference. Recommendations for further work to investigate these
concepts is presented
Surgeons or Scribes? The Role of United States Court of Appeals Law Clerks in Appellate Triage
Using original survey data, we explore how federal courts of appeals judges select and use their law clerks—a question that we answered in an earlier article about federal district court clerks. As with that first article, we do not intend to tackle such normative issues as whether courts of appeals law clerks possess too much influence over the judicial process or whether the selection criteria used by these judges is appropriate. What we will present, however, is descriptive data on the criteria that courts of appeals judges use to pick their law clerks as well as the tasks assigned to those clerks. We believe that our findings, namely, that courts of appeals judges delegate substantial job duties to their clerks, should serve as the springboard for a future debate over the wisdom of such delegation
Generation of two-mode nonclassical states and a quantum phase gate operation in trapped ion cavity QED
We propose a scheme to generate nonclassical states of a quantum system,
which is composed of the one-dimensional trapped ion motion and a single cavity
field mode. We show that two-mode SU(2) Schr\"odinger-cat states, entangled
coherent states, two-mode squeezed vacuum states and their superposition can be
generated. If the vibration mode and the cavity mode are used to represent
separately a qubit, a quantum phase gate can be implemented.Comment: to appear in PR
Static and dynamic characteristics of an hydrodynamic journal bearing
Hydrodynamic oil film bearings exhibit lateral flexibility which
influences the dynamics of rotors they support. This lateral flexibility is specified by coefficients which relate forces generated by
the oil film to the instantaneous journal centre velocity and its
displacement from an equilibrium position. Previous investigators
adopted a linear treatment by taking uniform viscosity with small
displacement and velocity increments. Relatively large journal
centre velocities are possible in rotating machinery. Therefore,
this thesis investigates the non-linear behaviour of these oil film
coefficients.
Coefficient calculations allowed viscosity to vary with temperature
and pressure rendering the governing Reynolds Equation non-linear.
A range of positive and negative displacement and velocity increments
were examined. Novel experimental techniques have been developed
which allow determination of coefficient variation with respective
displacement and velocity. Coefficients were deduced from specially
chosen, imposed vibration orbits arising from two mutually perpendicular
external oscillating forces of variable relative magnitude and phase.
Journal centre displacement and velocity were measured using high
speed data logging equipment. A unique feature was the ability to
obtain, experimental displacement coefficients from the results of both
dynamic and incremental loading. It was found necessary to establish
the bearing centre separately for each warm-up/load combination.
Journal clearance in the hot rotating condition could not be measured
to the precision required by its sensitivity to calculated load.
Clearance and cavitation zone pressures were deduced from simultaneous
predictions of the measured vertical load and attitude angle.
Theoretical oil film tensile forces were necessary, a proposition
supported by recently published experimental findings. Theoretical
results for an equivalent uniform viscosity combined with experimental
data gave a simple static locus design procedure. A temperature
profile was assumed for theoretical work but choice thereof was found
to be not critical.
Coefficients are defined in terms of a "zero" value and linear gradient.
Using realistic criteria, measured coefficient variation was found to
be significant at eccentricity ratios greater than 0.78. Theory
adequately predicted most "zero" values but not gradients. It is
concluded that improvement in the coefficient prediction willPh
Surgeons or Scribes? The Role of United States Court of Appeals Law Clerks in Appellate Triage
non
Mimicking a Squeezed Bath Interaction: Quantum Reservoir Engineering with Atoms
The interaction of an atomic two-level system and a squeezed vacuum leads to
interesting novel effects in atomic dynamics, including line narrowing in
resonance fluorescence and absorption spectra, and a suppressed (enhanced)
decay of the in-phase and out-of phase component of the atomic polarization. On
the experimental side these predictions have so far eluded observation,
essentially due to the difficulty of embedding atoms in a 4 pi squeezed vacuum.
In this paper we show how to ``engineer'' a squeezed-bath-type interaction for
an effective two-level system. In the simplest example, our two-level atom is
represented by the two ground levels of an atom with angular momentum J=1/2 ->
J=1/2 transition (a four level system) which is driven by (weak) laser fields
and coupled to the vacuum reservoir of radiation modes. Interference between
the spontaneous emission channels in optical pumping leads to a squeezed bath
type coupling, and thus to symmetry breaking of decay on the Bloch sphere. With
this system it should be possible to observe the effects predicted in the
context of squeezed bath - atom interactions. The laser parameters allow one to
choose properties of the squeezed bath interaction, such as the (effective)
photon number expectation number N and the squeezing phase phi. We present
results of a detailed analytical and numerical study.Comment: 24 pages, 8 figure
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