2,245 research outputs found
Decoherence of quantum wavepackets due to interaction with conformal spacetime fluctuations
One of the biggest problems faced by those attempting to combine quantum
theory and general relativity is the experimental inaccessibility of the
unification scale. In this paper we show how incoherent conformal waves in the
gravitational field, which may be produced by quantum mechanical zero-point
fluctuations, interact with the wavepackets of massive particles. The result of
this interaction is to produce decoherence within the wavepackets which could
be accessible in experiments at the atomic scale.
Using a simple model for the coherence properties of the gravitational field
we derive an equation for the evolution of the density matrix of such a
wavepacket. Following the primary state diffusion programme, the most promising
source of spacetime fluctuations for detection are the above zero-point energy
fluctuations. According to our model, the absence of intrinsic irremoveable
decoherence in matter interferometry experiments puts bounds on some of the
parameters of quantum gravity theories. Current experiments give \lambda > 18.
, where \lambda t_{Planck} is an effective cut-off for the validity of
low-energy quantum gravity theories.Comment: REVTeX forma
Quantum state diffusion with a moving basis: computing quantum-optical spectra
Quantum state diffusion (QSD) as a tool to solve quantum-optical master
equations by stochastic simulation can be made several orders of magnitude more
efficient if states in Hilbert space are represented in a moving basis of
excited coherent states. The large savings in computer memory and time are due
to the localization property of the QSD equation. We show how the method can be
used to compute spectra and give an application to second harmonic generation.Comment: 8 pages in RevTeX, 1 uuencoded postscript figure, submitted to Phys.
Rev.
Quantum state diffusion, localization and computation
Numerical simulation of individual open quantum systems has proven advantages
over density operator computations. Quantum state diffusion with a moving basis
(MQSD) provides a practical numerical simulation method which takes full
advantage of the localization of quantum states into wave packets occupying
small regions of classical phase space. Following and extending the original
proposal of Percival, Alber and Steimle, we show that MQSD can provide a
further gain over ordinary QSD and other quantum trajectory methods of many
orders of magnitude in computational space and time. Because of these gains, it
is even possible to calculate an open quantum system trajectory when the
corresponding isolated system is intractable. MQSD is particularly advantageous
where classical or semiclassical dynamics provides an adequate qualitative
picture but is numerically inaccurate because of significant quantum effects.
The principles are illustrated by computations for the quantum Duffing
oscillator and for second harmonic generation in quantum optics. Potential
applications in atomic and molecular dynamics, quantum circuits and quantum
computation are suggested.Comment: 16 pages in LaTeX, 2 uuencoded postscript figures, submitted to J.
Phys.
Sub-Shot-Noise Magnetometry with a Correlated Spin-Relaxation Dominated Alkali-Metal Vapor
Spin noise sets fundamental limits to the precision of measurements using
spin-polarized atomic vapors, such as performed with sensitive atomic
magnetometers. Spin squeezing offers the possibility to extend the measurement
precision beyond the standard quantum limit of uncorrelated atoms. Contrary to
the current understanding, we show that even in the presence of spin
relaxation, spin squeezing can lead to a significant reduction of spin noise,
and hence an increase in magnetometric sensitivity, for a long measurement
time. This is the case when correlated spin relaxation due to binary
alkali-atom collisions dominates independently acting decoherence processes.Comment: 4 pages, submitted to Phys. Rev. Let
Selected topics in Planck-scale physics
We review a few topics in Planck-scale physics, with emphasis on possible
manifestations in relatively low energy. The selected topics include quantum
fluctuations of spacetime, their cumulative effects, uncertainties in
energy-momentum measurements, and low energy quantum-gravity phenomenology. The
focus is on quantum-gravity-induced uncertainties in some observable
quantities. We consider four possible ways to probe Planck-scale physics
experimentally: 1. looking for energy-dependent spreads in the arrival time of
photons of the same energy from GRBs; 2. examining spacetime
fluctuation-induced phase incoherence of light from extragalactic sources; 3.
detecting spacetime foam with laser-based interferometry techniques; 4.
understanding the threshold anomalies in high energy cosmic ray and gamma ray
events. Some other experiments are briefly discussed. We show how some physics
behind black holes, simple clocks, simple computers, and the holographic
principle is related to Planck-scale physics. We also discuss a formulation of
the Dirac equation as a difference equation on a discrete Planck-scale
spacetime lattice, and a possible interplay between Planck-scale and
Hubble-scale physics encoded in the cosmological constant (dark energy).Comment: 31 pages, 1 figure; minor changes; to appear in Mod. Phys. Lett. A as
a Brief Revie
Non-monotonicity in the quantum-classical transition: Chaos induced by quantum effects
The transition from classical to quantum behavior for chaotic systems is
understood to be accompanied by the suppression of chaotic effects as the
relative size of is increased. We show evidence to the contrary in the
behavior of the quantum trajectory dynamics of a dissipative quantum chaotic
system, the double-well Duffing oscillator. The classical limit in the case
considered has regular behavior, but as the effective is increased we
see chaotic behavior. This chaos then disappears deeper into the quantum
regime, which means that the quantum-classical transition in this case is
non-monotonic in .Comment: 4 pages; presentation modified significantly to demonstrate that
quantum effects are indeed responsible for the `anomalous' chaos. 2 figures
adde
Tunable Whispering Gallery Mode Resonators for Cavity Quantum Electrodynamics
We theoretically study the properties of highly prolate shaped dielectric
microresonators. Such resonators sustain whispering gallery modes that exhibit
two spatially well separated regions with enhanced field strength. The field
per photon on the resonator surface is significantly higher than e.g. for
equatorial whispering gallery modes in microsphere resonators with a comparable
mode volume. At the same time, the frequency spacing of these modes is much
more favorable, so that a tuning range of several free spectral ranges should
be attainable. We discuss the possible application of such resonators for
cavity quantum electrodynamics experiments with neutral atoms and reveal
distinct advantages with respect to existing concepts.Comment: 4 pages, 3 figure
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