12,013 research outputs found
Correlations in a BEC collision: First-principles quantum dynamics with 150 000 atoms
The quantum dynamics of colliding Bose-Einstein condensates with 150 000
atoms are simulated directly from the Hamiltonian using the stochastic
positive-P method. Two-body correlations between the scattered atoms and their
velocity distribution are found for experimentally accessible parameters.
Hanbury Brown-Twiss or thermal-like correlations are seen for copropagating
atoms, while number correlations for counterpropagating atoms are even stronger
than thermal correlations at short times. The coherent phase grains grow in
size as the collision progresses with the onset of growth coinciding with the
beginning of stimulated scattering. The method is versatile and usable for a
range of cold atom systems.Comment: 4 pages, 4 figures. v2: Rewording and style changes, minor except for
rewrite of background on the positive-P representation. Original research
unchange
Many-body quantum dynamics of polarisation squeezing in optical fibre
We report new experiments that test quantum dynamical predictions of
polarization squeezing for ultrashort photonic pulses in a birefringent fibre,
including all relevant dissipative effects. This exponentially complex
many-body problem is solved by means of a stochastic phase-space method. The
squeezing is calculated and compared to experimental data, resulting in
excellent quantitative agreement. From the simulations, we identify the
physical limits to quantum noise reduction in optical fibres. The research
represents a significant experimental test of first-principles time-domain
quantum dynamics in a one-dimensional interacting Bose gas coupled to
dissipative reservoirs.Comment: 4 pages, 4 figure
Quantum theory of dispersive electromagnetic modes
A quantum theory of dispersion for an inhomogeneous solid is obtained, from a
starting point of multipolar coupled atoms interacting with an electromagnetic
field. The dispersion relations obtained are equivalent to the standard
classical Sellmeir equations obtained from the Drude-Lorentz model. In the
homogeneous (plane-wave) case, we obtain the detailed quantum mode structure of
the coupled polariton fields, and show that the mode expansion in all branches
of the dispersion relation is completely defined by the refractive index and
the group-velocity for the polaritons. We demonstrate a straightforward
procedure for exactly diagonalizing the Hamiltonian in one, two or
three-dimensional environments, even in the presence of longitudinal
phonon-exciton dispersion, and an arbitrary number of resonant transitions with
different frequencies. This is essential, since it is necessary to include at
least one phonon (I.R.) and one exciton (U.V.) mode, in order to accurately
represent dispersion in transparent solid media. Our method of diagonalization
does not require an explicit solution of the dispersion relation, but relies
instead on the analytic properties of Cauchy contour integrals over all
possible mode frequencies. When there is longitudinal phonon dispersion, the
relevant group-velocity term is modified so that it only includes the purely
electromagnetic part of the group velocity
Analyzing differences in the costs of treatment across centers within economic evaluations
Objectives: Assessments of health technologies increasingly include economic evaluations conducted alongside clinical trials. One particular concern with economic evaluations conducted alongside clinical trials is the generalizability of results from one setting to another. Much of the focus relating to this topic has been on the generalizability of results between countries, However, the characteristics of clinical trial design require further consideration of the generalizability of cost data between centers within a single country, which could be important in decisions about adoption of the new technology. Methods: We used data from a multicenter clinical trial conducted in the United Kingdom to assess the degree of variation in costs between patients and between treatment centers and the determinants of the degree of such variation. Results: The variation between patients was statistically significant for both the experimental and conventional treatments. However, the degree of variation between centers was only statistically significant for the experimental treatment. Such variation appeared to be a result of hospital practice, such as pay ment mechanisms for staff and provision of hostel accommodation, rather than variations in physical resource use or substantive differences in cost structure. Conclusions: Multicenter economic evaluations are necessary for determining the variations in hospital practice and characteristics that can in turn determine the generalizability of study results to other settings. Such analyses can identify issues that may be important in adopting a new health technology. Analysis is required of similar large multicenter trials to confirm these conclusions
Functional Wigner representation of BEC quantum dynamics
We develop a method of simulating the full quantum field dynamics of
multi-mode multi-component Bose-Einstein condensates in a trap. We use the
truncated Wigner representation to obtain a probabilistic theory that can be
sampled. This method produces c-number stochastic equations which may be solved
using conventional stochastic methods. The technique is valid for large mode
occupation numbers. We give a detailed derivation of methods of functional
Wigner representation appropriate for quantum fields. Our approach describes
spatial evolution of spinor components and properly accounts for nonlinear
losses. Such techniques are applicable to calculating the leading quantum
corrections, including effects like quantum squeezing, entanglement, EPR
correlations and interactions with engineered nonlinear reservoirs. By using a
consistent expansion in the inverse density, we are able to explain an
inconsistency in the nonlinear loss equations found by earlier authors
Gaussian quantum Monte Carlo methods for fermions
We introduce a new class of quantum Monte Carlo methods, based on a Gaussian
quantum operator representation of fermionic states. The methods enable
first-principles dynamical or equilibrium calculations in many-body Fermi
systems, and, combined with the existing Gaussian representation for bosons,
provide a unified method of simulating Bose-Fermi systems. As an application,
we calculate finite-temperature properties of the two dimensional Hubbard
model.Comment: 4 pages, 3 figures, Revised version has expanded discussion,
simplified mathematical presentation, and application to 2D Hubbard mode
JPL spacecraft sterilization technology program - A status report
Facility description and procedures for heat and ethylene oxide sterilization of spacecraft instrumentation, components, and material
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