521 research outputs found
Long-lived non-thermal states realized by atom losses in one-dimensional quasi-condensates
We investigate the cooling produced by a loss process non selective in energy
on a one-dimensional (1D) Bose gas with repulsive contact interactions in the
quasi-condensate regime. By performing nonlinear classical field calculations
for a homogeneous system, we show that the gas reaches a non-thermal state
where different modes have acquired different temperatures. After losses have
been turned off, this state is robust with respect to the nonlinear dynamics,
described by the Gross-Pitaevskii equation. We argue that the integrability of
the Gross-Pitaevskii equation is linked to the existence of such long-lived
non-thermal states, and illustrate this by showing that such states are not
supported within a non-integrable model of two coupled 1D gases of different
masses. We go beyond a classical field analysis, taking into account the
quantum noise introduced by the discreteness of losses, and show that the
non-thermal state is still produced and its non-thermal character is even
enhanced. Finally, we extend the discussion to gases trapped in a harmonic
potential and present experimental observations of a long-lived non-thermal
state within a trapped 1D quasi-condensate following an atom loss process
Continuous measurement feedback control of a Bose-Einstein condensate using phase contrast imaging
We consider the theory of feedback control of a Bose-Einstein condensate
(BEC) confined in a harmonic trap under a continuous measurement constructed
via non-destructive imaging. A filtering theory approach is used to derive a
stochastic master equation (SME) for the system from a general Hamiltonian
based upon system-bath coupling. Numerical solutions for this SME in the limit
of a single atom show that the final steady state energy is dependent upon the
measurement strength, the ratio of photon kinetic energy to atomic kinetic
energy, and the feedback strength. Simulations indicate that for a weak
measurement strength, feedback can be used to overcome heating introduced by
the scattering of light, thereby allowing the atom to be driven towards the
ground state.Comment: 4 figures, 11 page
Ab initio Wannier-function-based correlated calculations of Born effective charges of crystalline LiO and LiCl
In this paper we have used our recently developed ab initio
Wannier-function-based methodology to perform extensive Hartree-Fock and
correlated calculations on LiO and LiCl to compute their Born effective
charges. Results thus obtained are in very good agreement with the experiments.
In particular, for the case of LiO, we resolve a controversy originating
in the experiment of Osaka and Shindo {[}Solid State Commun. 51 (1984) 421] who
had predicted the effective charge of Li ions to be in the range 0.58--0.61, a
value much smaller compared to its nominal value of unity, thereby, suggesting
that the bonding in the material could be partially covalent. We demonstrate
that effective charge computed by Osaka and Shindo is the Szigeti charge, and
once the Born charge is computed, it is in excellent agreement with our
computed value. Mulliken population analysis of LiO also confirms ionic
nature of the bonding in the substance.Comment: 11 pages, 1 figure. To appear in Phys. Rev. B (Feb 2008
Quantum tunneling dynamics of an interacting Bose-Einstein condensate through a Gaussian barrier
The transmission of an interacting Bose-Einstein condensate incident on a
repulsive Gaussian barrier is investigated through numerical simulation. The
dynamics associated with interatomic interactions are studied across a broad
parameter range not previously explored. Effective 1D Gross-Pitaevskii equation
(GPE) simulations are compared to classical Boltzmann-Vlasov equation (BVE)
simulations in order to isolate purely coherent matterwave effects. Quantum
tunneling is then defined as the portion of the GPE transmission not described
by the classical BVE. An exponential dependence of transmission on barrier
height is observed in the purely classical simulation, suggesting that
observing such exponential dependence is not a sufficient condition for quantum
tunneling. Furthermore, the transmission is found to be predominately described
by classical effects, although interatomic interactions are shown to modify the
magnitude of the quantum tunneling. Interactions are also seen to affect the
amount of classical transmission, producing transmission in regions where the
non-interacting equivalent has none. This theoretical investigation clarifies
the contribution quantum tunneling makes to overall transmission in
many-particle interacting systems, potentially informing future tunneling
experiments with ultracold atoms.Comment: Close to the published versio
PREVENTING DATA LEAKS FROM APPLICATION SCREEN-SHARING
According to a recent and comprehensive analysis of information security breaches, 23% of attacks are attributable to internal instances. Presented herein are techniques for protecting businesses against the sharing of confidential information within applications with unauthorized meeting participants. In particular, techniques presented herein restrict screen sharing of confidential information by preventing confidential content from being displayed on an unauthorized user’s endpoint device during a collaboration session
Integrating process design and control: An application of optimal control to chemical processes
In this paper, the optimal design of process systems generically used in chemical industries is studied. The closely coupled nature of optimal design specification of the equipment, the determination of the optimal process parameters in steady-state, moreover, some issues of the application of optimal control is shown. The solution of the overall optimization problem including (i) optimal design of the equipment and (ii) specification of its optimal control strategy can be found relying on two different design concepts, namely, on the conventionally used sequential or, on the newly emerged simultaneous design approaches. This paper gives the theoretical background of the ideas and presents a comparative summary of the approaches. The two approaches are contrasted to each other in which the effects of the interaction of optimal process design and optimal control is highlighted. A new simultaneous optimization procedure providing economic and operability benefits over the traditional stand-alone approach is proposed. The applicability of the idea is demonstrated by means of a design study carried out for optimal design of a coaxial heat exchanger and a reactive distillation column for the synthesis of ethyl tert butyl ether (ETBE), relying on the benefits of the utilization of optimal control
Intensity profiles of superdeformed bands in Pb isotopes in a two-level mixing model
A recently developed two-level mixing model of the decay out of superdeformed bands is applied to examine the loss of flux from the yrast superdeformed bands in Pb192, Pb194, and Pb196. Probability distributions for decay to states at normal deformations are calculated at each level. The sensitivity of the results to parameters describing the levels at normal deformation and their coupling to levels in the superdeformed well is explored. It is found that except for narrow ranges of the interaction strength coupling the states, the amount of intensity lost is primarily determined by the ratio of γ decay widths in the normal and superdeformed wells. It is also found that while the model can accommodate the observed fractional intensity loss profiles for decay from bands at relatively high excitation, it cannot accommodate the similarly abrupt decay from bands at lower energies if standard estimates of the properties of the states in the first minimum are employed
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