170 research outputs found
All-optical versus electro-optical quantum-limited feedback
All-optical feedback can be effected by putting the output of a source cavity
through a Faraday isolator and into a second cavity which is coupled to the
source cavity by a nonlinear crystal. If the driven cavity is heavily damped,
then it can be adiabatically eliminated and a master equation or quantum
Langevin equation derived for the first cavity alone. This is done for an input
bath in an arbitrary state, and for an arbitrary nonlinear coupling. If the
intercavity coupling involves only the intensity (or one quadrature) of the
driven cavity, then the effect on the source cavity is identical to that which
can be obtained from electro-optical feedback using direct (or homodyne)
detection. If the coupling involves both quadratures, this equivalence no
longer holds, and a coupling linear in the source amplitude can produce a
nonclassical state in the source cavity. The analogous electro-optic scheme
using heterodyne detection introduces extra noise which prevents the production
of nonclassical light. Unlike the electro-optic case, the all-optical feedback
loop has an output beam (reflected from the second cavity). We show that this
may be squeezed, even if the source cavity remains in a classical state.Comment: 21 pages. This is an old (1994) paper, but one which I thought was
worth posting because in addition to what is described in abstract it has:
(1) the first formulation (to my knowledge) of quantum trajectories for an
arbitrary (i.e. squeezed, thermal etc.) broadband bath; (2) the prediction of
a periodic modification to the detuning and damping of an oscillator for the
simplest sort of all-optical feedback (i.e. a mirror) as seen in the recent
experiment "Forces between a Single Atom and Its Distant Mirror Image", P.
Bushev et al, Phys. Rev. Lett. 92, 223602 (2004
Removal of a single photon by adaptive absorption
We present a method to remove, using only linear optics, exactly one photon
from a field-mode. This is achieved by putting the system in contact with an
absorbing environment which is under continuous monitoring. A feedback
mechanism then decouples the system from the environment as soon as the first
photon is absorbed. We propose a possible scheme to implement this process and
provide the theoretical tools to describe it
Squeezing via feedback
We present the quantum theory of optical cavity feedback mediated by homodyne detection, with an arbitrary time delay. We apply this theory to a system with nonclassical dynamics, a sub-Poissonian pumped laser. By using the feedback to phase lock the laser it is possible to produce output light which exhibits perfect quadrature squeezing on resonance, rather than just sub-Poissonian intensity statistics. However, we also show that feedback mediated by homodyne detection (or any other extracavity measurement) cannot produce nonclassical light unless the cavity dynamics can do so without feedback. Furthermore, in systems which already exhibit squeezing, such feedback can only degrade the squeezing in the output. With feedback mediated by an intracavity measurement, these theorems do not apply. We show that an (admittedly unrealistic) intracavity quantum nondemolition quadrature measurement allows arbitrary squeezing to be produced by controlling the amplitude of a coherent driving field
Non Uniform Black Strings and Critical Dimensions in
We study the equations of black strings in spacetimes of arbitrary dimensions
with a negative cosmological constant and construct numerically non uniform
black strings solutions. Our results suggest the existence of a localised black
hole in asymptotically locally spacetime. We also present evidences for a
dependence of the critical dimension on the horizon radius.The critical
dimension represents the dimension where the order of the phase transition
between uniform and non uniform black string changes. Finally, we argue that
both, the regular asymptotically locally solution and black string
solutions with a very small horizon radius, present a negative tension. This
turns out to be an unexpected feature of the solutions.Comment: 13 pages, 5 figure
Three-Charge Black Holes on a Circle
We study phases of five-dimensional three-charge black holes with a circle in
their transverse space. In particular, when the black hole is localized on the
circle we compute the corrections to the metric and corresponding
thermodynamics in the limit of small mass. When taking the near-extremal limit,
this gives the corrections to the constant entropy of the extremal three-charge
black hole as a function of the energy above extremality. For the partial
extremal limit with two charges sent to infinity and one finite we show that
the first correction to the entropy is in agreement with the microscopic
entropy by taking into account that the number of branes shift as a consequence
of the interactions across the transverse circle. Beyond these analytical
results, we also numerically obtain the entire phase of non- and near-extremal
three- and two-charge black holes localized on a circle. More generally, we
find in this paper a rich phase structure, including a new phase of
three-charge black holes that are non-uniformly distributed on the circle. All
these three-charge black hole phases are found via a map that relates them to
the phases of five-dimensional neutral Kaluza-Klein black holes.Comment: 58 pages, 10 figures; v2: Corrected typos, version appearing in JHE
Control of an atom laser using feedback
A generalised method of using feedback to control Bose-Einstein condensates
is introduced. The condensates are modelled by the Gross-Pitaevskii equation,
so only semiclassical fluctations can be suppressed, and back-action from the
measurement is ignored. We show that for any available control, a feedback
scheme can be found to reduce the energy while the appropriate moment is still
dynamic. We demonstrate these schemes by considering a condensate trapped in a
harmonic potential that can be modulated in strength and position. The
formalism of our feedback scheme also allows the inclusion of certain types of
non-linear controls. If the non-linear interaction between the atoms can be
controlled via a Feshbach resonance, we show that the feedback process can
operate with a much higher efficiency.Comment: 6 pages, 7 figure
New Phases of Near-Extremal Branes on a Circle
We study the phases of near-extremal branes on a circle, by which we mean
near-extremal branes of string theory and M-theory with a circle in their
transverse space. We find a map that takes any static and neutral Kaluza-Klein
black hole, i.e. any static and neutral black hole on Minkowski-space times a
circle M^d x S^1, and map it to a corresponding solution for a near-extremal
brane on a circle. The map is derived using first a combined boost and
U-duality transformation on the Kaluza-Klein black hole, transforming it to a
solution for a non-extremal brane on a circle. The resulting solution for a
near-extremal brane on a circle is then obtained by taking a certain
near-extremal limit. As a consequence of the map, we can transform the neutral
non-uniform black string branch into a new non-uniform phase of near-extremal
branes on a circle. Furthermore, we use recently obtained analytical results on
small black holes in Minkowski-space times a circle to get new information
about the localized phase of near-extremal branes on a circle. This gives in
turn predictions for the thermal behavior of the non-gravitational theories
dual to these near-extremal branes. In particular, we give predictions for the
thermodynamics of supersymmetric Yang-Mills theories on a circle, and we find a
new stable phase of (2,0) Little String Theory in the canonical ensemble for
temperatures above its Hagedorn temperature.Comment: 72 pages, 5 figures. v2: Typos fixed, refs. added. v3: Sec. 3.2 fixe
The anisotropic Ashkin-Teller model: a renormalization group study
The two-dimensional ferromagnetic anisotropic Ashkin-Teller model is
investigated through a real-space renormalization-group approach. The critical
frontier, separating five distinct phases, recover all the known exacts results
for the square lattice. The correlation length and crossover
critical exponents are also calculated. With the only exception of the
four-state Potts critical point, the entire phase diagram belongs to the Ising
universality class.Comment: 3 ps figures, accepted for publication in Physica
New stable phase of non uniform black strings in
We consider the non uniform black string equations in arbitrary number
of dimension in a perturbative approach up to order 2 and in a non
perturbative. We restrict the study in the perturbative approach to the
backreacting modes, since they provide the first relevant corrections on the
thermodynamical quantities of the solutions. We also present some preliminary
results in the construction of non-perturbative solutions, in particular, we
present a first part of the non uniform - uniform black string phase diagram.
Our results suggests the existence of a new stable phase for non uniform
black strings, namely long non uniform black string, with the extra direction
length of the order of the curvature.Comment: Results extended. 14 pages, 5 figure
An entangled two photon source using biexciton emission of an asymmetric quantum dot in a cavity
A semiconductor based scheme has been proposed for generating entangled
photon pairs from the radiative decay of an electrically-pumped biexciton in a
quantum dot. Symmetric dots produce polarisation entanglement, but
experimentally-realised asymmetric dots produce photons entangled in both
polarisation and frequency. In this work, we investigate the possibility of
erasing the `which-path' information contained in the frequencies of the
photons produced by asymmetric quantum dots to recover polarisation-entangled
photons. We consider a biexciton with non-degenerate intermediate excitonic
states in a leaky optical cavity with pairs of degenerate cavity modes close to
the non-degenerate exciton transition frequencies. An open quantum system
approach is used to compute the polarisation entanglement of the two-photon
state after it escapes from the cavity, measured by the visibility of
two-photon interference fringes. We explicitly relate the two-photon visibility
to the degree of Bell-inequality violation, deriving a threshold at which
Bell-inequality violations will be observed. Our results show that an ideal
cavity will produce maximally polarisation-entangled photon pairs, and even a
non-ideal cavity will produce partially entangled photon pairs capable of
violating a Bell-inequality.Comment: 16 pages, 10 figures, submitted to PR
- …