3,690 research outputs found
Analysis of photon-atom entanglement generated by Faraday rotation in a cavity
Faraday rotation based on AC Stark shifts is a mechanism that can entangle
the polarization variables of photons and atoms. We analyze the structure of
such entanglement by using the Schmidt decomposition method. The
time-dependence of entanglement entropy and the effective Schmidt number are
derived for Gaussian amplitudes. In particular we show how the entanglement is
controlled by the initial fluctuations of atoms and photons.Comment: 6 pages, 3 figure
Quantum turbulence in condensate collisions: an application of the classical field method
We apply the classical field method to simulate the production of correlated
atoms during the collision of two Bose-Einstein condensates. Our
non-perturbative method includes the effect of quantum noise, and provides for
the first time a theoretical description of collisions of high density
condensates with very large out-scattered fractions. Quantum correlation
functions for the scattered atoms are calculated from a single simulation, and
show that the correlation between pairs of atoms of opposite momentum is rather
small. We also predict the existence of quantum turbulence in the field of the
scattered atoms--a property which should be straightforwardly measurable.Comment: 5 pages, 3 figures: Rewritten text, replaced figure
Tripartite entanglement and threshold properties of coupled intracavity downconversion and sum-frequency generation
The process of cascaded downconversion and sum-frequency generation inside an
optical cavity has been predicted to be a potential source of three-mode
continuous-variable entanglement. When the cavity is pumped by two fields, the
threshold properties have been analysed, showing that these are more
complicated than in well-known processes such as optical parametric
oscillation. When there is only a single pumping field, the entanglement
properties have been calculated using a linearised fluctuation analysis, but
without any consideration of the threshold properties or critical operating
points of the system. In this work we extend this analysis to demonstrate that
the singly pumped system demonstrates a rich range of threshold behaviour when
quantisation of the pump field is taken into account and that asymmetric
polychromatic entanglement is available over a wide range of operational
parameters.Comment: 24 pages, 15 figure
Adaptive homodyne phase discrimination and qubit measurement
Fast and accurate measurement is a highly desirable, if not vital, feature of
quantum computing architectures. In this work we investigate the usefulness of
adaptive measurements in improving the speed and accuracy of qubit measurement.
We examine a particular class of quantum computing architectures, ones based on
qubits coupled to well controlled harmonic oscillator modes (reminiscent of
cavity-QED), where adaptive schemes for measurement are particularly
appropriate. In such architectures, qubit measurement is equivalent to phase
discrimination for a mode of the electromagnetic field, and we examine adaptive
techniques for doing this. In the final section we present a concrete example
of applying adaptive measurement to the particularly well-developed circuit-QED
architecture.Comment: 9 pages, 8 figures. Published versio
Biased EPR entanglement and its application to teleportation
We consider pure continuous variable entanglement with non-equal correlations
between orthogonal quadratures. We introduce a simple protocol which equates
these correlations and in the process transforms the entanglement onto a state
with the minimum allowed number of photons. As an example we show that our
protocol transforms, through unitary local operations, a single squeezed beam
split on a beam splitter into the same entanglement that is produced when two
squeezed beams are mixed orthogonally. We demonstrate that this technique can
in principle facilitate perfect teleportation utilising only one squeezed beam.Comment: 8 pages, 5 figure
Loss-Induced Limits to Phase Measurement Precision with Maximally Entangled States
The presence of loss limits the precision of an approach to phase measurement
using maximally entangled states, also referred to as NOON states. A
calculation using a simple beam-splitter model of loss shows that, for all
nonzero values L of the loss, phase measurement precision degrades with
increasing number N of entangled photons for N sufficiently large. For L above
a critical value of approximately 0.785, phase measurement precision degrades
with increasing N for all values of N. For L near zero, phase measurement
precision improves with increasing N down to a limiting precision of
approximately 1.018 L radians, attained at N approximately equal to 2.218/L,
and degrades as N increases beyond this value. Phase measurement precision with
multiple measurements and a fixed total number of photons N_T is also examined.
For L above a critical value of approximately 0.586, the ratio of phase
measurement precision attainable with NOON states to that attainable by
conventional methods using unentangled coherent states degrades with increasing
N, the number of entangled photons employed in a single measurement, for all
values of N. For L near zero this ratio is optimized by using approximately
N=1.279/L entangled photons in each measurement, yielding a precision of
approximately 1.340 sqrt(L/N_T) radians.Comment: Additional references include
Quadripartite continuous-variable entanglement via quadruply concurrent downconversion
We investigate an intra-cavity coupled down-conversion scheme to generate
quadripartite entanglement using concurrently resonant nonlinearities. We
verify that quadripartite entanglement is present in this system by calculating
the output fluctuation spectra and then considering violations of optimized
inequalities of the van Loock-Furusawa type. The entanglement characteristics
both above and below the oscillation threshold are considered. We also present
analytic solutions for the quadrature operators and the van Loock-Furusawa
correlations in the undepleted pump approximation.Comment: 9 pages, 5 figure
A comparative study of dynamical simulation methods for the dissociation of molecular Bose-Einstein condensates
We describe a pairing mean-field theory related to the
Hartree-Fock-Bogoliubov approach, and apply it to the dynamics of dissociation
of a molecular Bose-Einstein condensate (BEC) into correlated bosonic atom
pairs. We also perform the same simulation using two stochastic phase-space
techniques for quantum dynamics -- the positive P-representation method and the
truncated Wigner method. By comparing the results of our calculations we are
able to assess the relative strength of these theoretical techniques in
describing molecular dissociation in one spatial dimension. An important aspect
of our analysis is the inclusion of atom-atom interactions which can be
problematic for the positive-P method. We find that the truncated Wigner method
mostly agrees with the positive-P simulations, but can be simulated for
significantly longer times. The pairing mean-field theory results diverge from
the quantum dynamical methods after relatively short times.Comment: 11 pages, 7 figures, corrected typos, minor content change
A Model for the Production of Regular Fluorescent Light from Coherently Driven Atoms
It has been shown in recent years that incoherent pumping through multiple
atomic levels provides a mechanism for the production of highly anti-bunched
light, and that as the number of incoherent steps is increased the light
becomes increasingly regular. We show that in a resonance fluorescence
situation, a multi-level atom may be multiply coherently driven so that the
fluorescent light is highly anti-bunched. We show that as the number of
coherently driven levels is increased, the spontaneous emissions may be made
increasingly more regular. We present a systematic method for designing the
level structure and driving required to produce highly anti-bunched light in
this manner for an arbitrary even number of levels.Comment: 6 pages multicol revtex, including figure
Decoherence induced by a phase-damping reservoir
A phase damping reservoir composed by -bosons coupled to a system of
interest through a cross-Kerr interaction is proposed and its effects on
quantum superpo sitions are investigated. By means of analytical calculations
we show that: i-) the reservoir induces a Gaussian decay of quantum coherences,
and ii-) the inher ent incommensurate character of the spectral distribution
yields irreversibility . A state-independent decoherence time and a master
equation are both derived an alytically. These results, which have been
extended for the thermodynamic limit, show that nondissipative decoherence can
be suitably contemplated within the EI D approach. Finally, it is shown that
the same mechanism yielding decoherence ar e also responsible for inducing
dynamical disentanglement.Comment: 8 pages, 3 figure
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