7,940 research outputs found
First-principles quantum dynamics in interacting Bose gases I: The positive P representation
The performance of the positive P phase-space representation for exact
many-body quantum dynamics is investigated. Gases of interacting bosons are
considered, where the full quantum equations to simulate are of a
Gross-Pitaevskii form with added Gaussian noise. This method gives tractable
simulations of many-body systems because the number of variables scales
linearly with the spatial lattice size. An expression for the useful simulation
time is obtained, and checked in numerical simulations. The dynamics of first-,
second- and third-order spatial correlations are calculated for a uniform
interacting 1D Bose gas subjected to a change in scattering length. Propagation
of correlations is seen. A comparison is made to other recent methods. The
positive P method is particularly well suited to open systems as no
conservation laws are hard-wired into the calculation. It also differs from
most other recent approaches in that there is no truncation of any kind.Comment: 21 pages, 7 figures, 2 tables, IOP styl
Naturally-phasematched second harmonic generation in a whispering gallery mode resonator
We demonstrate for the first time natural phase matching for optical
frequency doubling in a high-Q whispering gallery mode resonator made of
Lithium Niobate. A conversion efficiency of 9% is achieved at 30 micro Watt
in-coupled continuous wave pump power. The observed saturation pump power of
3.2 mW is almost two orders of magnitude lower than the state-of-the-art. This
suggests an application of our frequency doubler as a source of non-classical
light requiring only a low-power pump, which easily can be quantum noise
limited. Our theoretical analysis of the three-wave mixing in a whispering
gallery mode resonator provides the relative conversion efficiencies for
frequency doubling in various modes
Feshbach Resonance and Growth of a Bose-Einstein Condensate
Gross-Pitaevskii equation with gain is used to model Bose Einstein
condensation (BEC) fed by the surrounding thermal cloud. It is shown that the
number of atoms continuously injected into BEC from the reservoir can be
controlled by applying the external magnetic field via Feshbach resonance.Comment: 4 page
Some Aspects of the Biology of a Predaceous Anthomyiid Fly, \u3ci\u3eCoenosia Tigrina\u3c/i\u3e
The results of a two-year study in Michigan on the incidence of Coenosia tigrina adults under different onion production practices is presented. In Michigan, C. tigrina has three generations and is more abundant in organic agroecosystems than chemically-intensive onion production systems
Gaussian phase-space representations for fermions
We introduce a positive phase-space representation for fermions, using the
most general possible multi-mode Gaussian operator basis. The representation
generalizes previous bosonic quantum phase-space methods to Fermi systems. We
derive equivalences between quantum and stochastic moments, as well as operator
correspondences that map quantum operator evolution onto stochastic processes
in phase space. The representation thus enables first-principles quantum
dynamical or equilibrium calculations in many-body Fermi systems. Potential
applications are to strongly interacting and correlated Fermi gases, including
coherent behaviour in open systems and nanostructures described by master
equations. Examples of an ideal gas and the Hubbard model are given, as well as
a generic open system, in order to illustrate these ideas.Comment: More references and examples. Much less mathematical materia
Emergent classicality in continuous quantum measurements
We develop a classical theoretical description for nonlinear many-body
dynamics that incorporates the back-action of a continuous measurement process.
The classical approach is compared with the exact quantum solution in an
example with an atomic Bose-Einstein condensate in a double-well potential
where the atom numbers in both potential wells are monitored by light
scattering. In the classical description the back-action of the measurements
appears as diffusion of the relative phase of the condensates on each side of
the trap. When the measurements are frequent enough to resolve the system
dynamics, the system behaves classically. This happens even deep in the quantum
regime, and demonstrates how classical physics emerges from quantum mechanics
as a result of measurement back-action
The time-reversal test for stochastic quantum dynamics
The calculation of quantum dynamics is currently a central issue in
theoretical physics, with diverse applications ranging from ultra-cold atomic
Bose-Einstein condensates (BEC) to condensed matter, biology, and even
astrophysics. Here we demonstrate a conceptually simple method of determining
the regime of validity of stochastic simulations of unitary quantum dynamics by
employing a time-reversal test. We apply this test to a simulation of the
evolution of a quantum anharmonic oscillator with up to
(Avogadro's number) of particles. This system is realisable as a Bose-Einstein
condensate in an optical lattice, for which the time-reversal procedure could
be implemented experimentally.Comment: revtex4, two figures, four page
Quantum squeezing of optical dissipative structures
We show that any optical dissipative structure supported by degenerate
optical parametric oscillators contains a special transverse mode that is free
from quantum fluctuations when measured in a balanced homodyne detection
experiment. The phenomenon is not critical as it is independent of the system
parameters and, in particular, of the existence of bifurcations. This result is
a consequence of the spatial symmetry breaking introduced by the dissipative
structure. Effects that could degrade the squeezing level are considered.Comment: 4 pages and a half, 1 fugure. Version to appear in Europhysics
Letter
Differential equations for multi-loop integrals and two-dimensional kinematics
In this paper we consider multi-loop integrals appearing in MHV scattering
amplitudes of planar N=4 SYM. Through particular differential operators which
reduce the loop order by one, we present explicit equations for the two-loop
eight-point finite diagrams which relate them to massive hexagons. After the
reduction to two-dimensional kinematics, we solve them using symbol technology.
The terms invisible to the symbols are found through boundary conditions coming
from double soft limits. These equations are valid at all-loop order for double
pentaladders and allow to solve iteratively loop integrals given lower-loop
information. Comments are made about multi-leg and multi-loop integrals which
can appear in this special kinematics. The main motivation of this
investigation is to get a deeper understanding of these tools in this
configuration, as well as for their application in general four-dimensional
kinematics and to less supersymmetric theories.Comment: 25 pages, 7 figure
EPR entanglement strategies in two-well BEC
Criteria suitable for measuring entanglement between two different potential
wells in a Bose- Einstein condensation (BEC) are evaluated. We show how to
generate the required entanglement, utilizing either an adiabatic two-mode or
dynamic four-mode interaction strategy, with techniques that take advantage of
s-wave scattering interactions to provide the nonlinear coupling. The dynamic
entanglement method results in an entanglement signature with spatially
separated detectors, as in the Einstein-Podolsky-Rosen (EPR) paradox.Comment: 4 pages, 4 figure
- ā¦