3,500 research outputs found
Creation of macroscopic superposition states from arrays of Bose-Einstein condensates
We consider how macroscopic quantum superpositions may be created from arrays
of Bose-Einstein condensates. We study a system of three condensates in Fock
states, all with the same number of atoms and show that this has the form of a
highly entangled superposition of different quasi-momenta. We then show how, by
partially releasing these condensates and detecting an interference pattern
where they overlap, it is possible to create a macroscopic superposition of
different relative phases for the remaining portions of the condensates. We
discuss methods for confirming these superpositions.Comment: 7 pages, 5 figure
Classical field techniques for condensates in one-dimensional rings at finite temperatures
For a condensate in a one-dimensional ring geometry, we compare the
thermodynamic properties of three conceptually different classical field
techniques: stochastic dynamics, microcanonical molecular dynamics, and the
classical field method. Starting from non-equilibrium initial conditions, all
three methods approach steady states whose distribution and correlation
functions are in excellent agreement with an exact evaluation of the partition
function in the high-temperature limit. Our study helps to establish these
various classical field techniques as powerful non-perturbative tools for
systems at finite temperatures.Comment: 7 pages, 7 figures; minor changes, one reference adde
Greenhouse gas balance over thaw-freeze cycles in discontinuous zone permafrost
Peat in the discontinuous permafrost zone contains a globally significant reservoir of carbon that has undergone multiple permafrost-thaw cycles since the end of the mid-Holocene (~3700 years before present). Periods of thaw increase C decomposition rates which leads to the release of CO2 and CH4 to the atmosphere creating potential climate feedback. To determine the magnitude and direction of such feedback, we measured CO2 and CH4 emissions and modeled C accumulation rates and radiative fluxes from measurements of two radioactive tracers with differing lifetimes to describe the C balance of the peatland over multiple permafrost-thaw cycles since the initiation of permafrost at the site. At thaw features, the balance between increased primary production and higher CH4 emission stimulated by warmer temperatures and wetter conditions favors C sequestration and enhanced peat accumulation. Flux measurements suggest that frozen plateaus may intermittently (order of years to decades) act as CO2 sources depending on temperature and net ecosystem respiration rates, but modeling results suggest that—despite brief periods of net C loss to the atmosphere at the initiation of thaw—integrated over millennia, these sites have acted as net C sinks via peat accumulation. In greenhouse gas terms, the transition from frozen permafrost to thawed wetland is accompanied by increasing CO2 uptake that is partially offset by increasing CH4 emissions. In the short-term (decadal time scale) the net effect of this transition is likely enhanced warming via increased radiative C emissions, while in the long-term (centuries) net C deposition provides a negative feedback to climate warming
Tailoring discrete quantum walk dynamics via extended initial conditions: Towards homogeneous probability distributions
We study the evolution of initially extended distributions in the coined
quantum walk on the line by analyzing the dispersion relation of the process
and its associated wave equations. This allows us, in particular, to devise an
initially extended condition leading to a uniform probability distribution
whose width increases linearly with time, with increasing homogeneity.Comment: 4 pages, 2 figure
Temporal coherence, anomalous moments, and pairing correlations in the classical-field description of a degenerate Bose gas
The coherence properties of degenerate Bose gases have usually been expressed
in terms of spatial correlation functions, neglecting the rich information
encoded in their temporal behavior. In this paper we show, using a Hamiltonian
classical-field formalism, that temporal correlations can be used to
characterize familiar properties of a finite-temperature degenerate Bose gas.
The temporal coherence of a Bose-Einstein condensate is limited only by the
slow diffusion of its phase, and thus the presence of a condensate is indicated
by a sharp feature in the temporal power spectrum of the field. We show that
the condensate mode can be obtained by averaging the field for a short time in
an appropriate phase-rotating frame, and that for a wide range of temperatures,
the condensate obtained in this approach agrees well with that defined by the
Penrose-Onsager criterion based on one-body (spatial) correlations. For time
periods long compared to the phase diffusion time, the field will average to
zero, as we would expect from the overall U(1) symmetry of the Hamiltonian. We
identify the emergence of the first moment on short time scales with the
concept of U(1) symmetry breaking that is central to traditional mean-field
theories of Bose condensation. We demonstrate that the short-time averaging
procedure constitutes a general analog of the 'anomalous' averaging operation
of symmetry-broken theories by calculating the anomalous thermal density of the
field, which we find to have form and temperature dependence consistent with
the results of mean-field theories.Comment: 11 pages, 6 figures. v3: Final version. Typos fixed, and other minor
change
A Continuum Description of Rarefied Gas Dynamics (I)--- Derivation From Kinetic Theory
We describe an asymptotic procedure for deriving continuum equations from the
kinetic theory of a simple gas. As in the works of Hilbert, of Chapman and of
Enskog, we expand in the mean flight time of the constituent particles of the
gas, but we do not adopt the Chapman-Enskog device of simplifying the formulae
at each order by using results from previous orders. In this way, we are able
to derive a new set of fluid dynamical equations from kinetic theory, as we
illustrate here for the relaxation model for monatomic gases. We obtain a
stress tensor that contains a dynamical pressure term (or bulk viscosity) that
is process-dependent and our heat current depends on the gradients of both
temperature and density. On account of these features, the equations apply to a
greater range of Knudsen number (the ratio of mean free path to macroscopic
scale) than do the Navier-Stokes equations, as we see in the accompanying
paper. In the limit of vanishing Knudsen number, our equations reduce to the
usual Navier-Stokes equations with no bulk viscosity.Comment: 16 page
Gapless finite- theory of collective modes of a trapped gas
We present predictions for the frequencies of collective modes of trapped
Bose-condensed Rb atoms at finite temperature. Our treatment includes a
self-consistent treatment of the mean-field from finite- excitations and the
anomolous average. This is the first gapless calculation of this type for a
trapped Bose-Einstein condensed gas. The corrections quantitatively account for
the downward shift in the excitation frequencies observed in recent
experiments as the critical temperature is approached.Comment: 4 pages Latex and 2 postscript figure
Novel application of three-dimensional technologies in a case of dismemberment
This case study reports the novel application of three-dimensional technologies such as micro-CT and 3D printing to the forensic investigation of a complex case of dismemberment. Micro-CT was successfully employed to virtually align severed skeletal elements found in different locations, analyse tool marks created during the dismemberment process, and virtually dissect a charred piece of evidence. High resolution 3D prints of the burnt human bone contained within were created for physical visualisation to assist the investigation team. Micro-CT as a forensic radiological method provided vital information and the basis for visualisation both during the investigation and in the subsequent trial making it one of the first examples of such technology in a UK court
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