The architecture of DDMl: a recursively structured data driven machine

Journal ArticleAn architecture for a highly modular, recursively structured class of machines is presented. DDMl is an instance of such a machine structure, and is capable of executing machine language programs which are data driven (data flow) nets. These nets may represent arbitrary amounts of concurrency as well as arbitrary amounts of pipelining. DDMl is a fully distributed multi-processing system composed of completely asynchronous modules. The architecture allows for limitless physical extensibility without necessitating special programming or special hardware to support individual machines of widely varying sizes. DDMl is capable of automatically and dynamically allocating concurrent tasks to the available physical resources. The essential characteristics of the highly parallel, pipelined machine language are also described along with its method for execution on DDMl

Dataflow computers: a tutorial and survey

Journal ArticleThe demand for very high performance computer has encouraged some researchers in the computer science field to consider alternatives to the conventional notions of program and computer organization. The dataflow computer is one attempt to form a new collection of consistent systems ideas to improve both computer performance and to alleviate the software design problems induced by the construction of highly concurrent programs

Small-amplitude normal modes of a vortex in a trapped Bose-Einstein condensate

We consider a cylindrically symmetric trap containing a small Bose-Einstein condensate with a singly quantized vortex on the axis of symmetry. A time-dependent variational Lagrangian analysis yields the small-amplitude dynamics of the vortex and the condensate, directly determining the equations of motion of the coupled normal modes. As found previously from the Bogoliubov equations, there are two rigid dipole modes and one anomalous mode with a negative frequency when seen in the laboratory frame.Comment: 4 pages, no figures, Revte

Conserving and Gapless Approximations for an Inhomogeneous Bose Gas at Finite Temperatures

We derive and discuss the equations of motion for the condensate and its fluctuations for a dilute, weakly interacting Bose gas in an external potential within the self--consistent Hartree--Fock--Bogoliubov (HFB) approximation. Account is taken of the depletion of the condensate and the anomalous Bose correlations, which are important at finite temperatures. We give a critical analysis of the self-consistent HFB approximation in terms of the Hohenberg--Martin classification of approximations (conserving vs gapless) and point out that the Popov approximation to the full HFB gives a gapless single-particle spectrum at all temperatures. The Beliaev second-order approximation is discussed as the spectrum generated by functional differentiation of the HFB single--particle Green's function. We emphasize that the problem of determining the excitation spectrum of a Bose-condensed gas (homogeneous or inhomogeneous) is difficult because of the need to satisfy several different constraints.Comment: plain tex, 19 page

Stability of Bose condensed atomic Li-7

We study the stability of a Bose condensate of atomic $^7$Li in a (harmonic oscillator) magnetic trap at non-zero temperatures. In analogy to the stability criterion for a neutron star, we conjecture that the gas becomes unstable if the free energy as a function of the central density of the cloud has a local extremum which conserves the number of particles. Moreover, we show that the number of condensate particles at the point of instability decreases with increasing temperature, and that for the temperature interval considered, the normal part of the gas is stable against density fluctuations at this point.Comment: Submitted for publication in Physical Review

Condensate Heating by Atomic Losses

Atomic Bose-Einstein condensate is heated by atomic losses. Predicted depletion ranges from 1% for a uniform 3D condensate to around 10% for a quasi-1D condensate in a harmonic trap.Comment: 4 pages in RevTex, 1 eps figur

Theory of coherent Bragg spectroscopy of a trapped Bose-Einstein condensate

We present a detailed theoretical analysis of Bragg spectroscopy from a Bose-Einstein condensate at T=0K. We demonstrate that within the linear response regime, both a quantum field theory treatment and a meanfield Gross-Pitaevskii treatment lead to the same value for the mean evolution of the quasiparticle operators. The observable for Bragg spectroscopy experiments, which is the spectral response function of the momentum transferred to the condensate, can therefore be calculated in a meanfield formalism. We analyse the behaviour of this observable by carrying out numerical simulations in axially symmetric three-dimensional cases and in two dimensions. An approximate analytic expression for the observable is obtained and provides a means for identifying the relative importance of three broadening and shift mechanisms (meanfield, Doppler, and finite pulse duration) in different regimes. We show that the suppression of scattering at small values of q observed by Stamper-Kurn et al. [Phys. Rev. Lett. 83, 2876 (1999)] is accounted for by the meanfield treatment, and can be interpreted in terms of the interference of the u and v quasiparticle amplitudes. We also show that, contrary to the assumptions of previous analyses, there is no regime for trapped condensates for which the spectral response function and the dynamic structure factor are equivalent. Our numerical calculations can also be performed outside the linear response regime, and show that at large laser intensities a significant decrease in the shift of the spectral response function can occur due to depletion of the initial condensate.Comment: RevTeX4 format, 16 pages plus 7 eps figures; Update to published version: minors changes and an additional figure. (To appear in Phys. Rev. A

Free expansion of Bose-Einstein condensates with quantized vortices

The expansion of Bose-Einstein condensates with quantized vortices is studied by solving numerically the time-dependent Gross-Pitaevskii equation at zero temperature. For a condensate initially trapped in a spherical harmonic potential, we confirm previous results obtained by means of variational methods showing that, after releasing the trap, the vortex core expands faster than the radius of the atomic cloud. This could make the detection of vortices feasible, by observing the depletion of the density along the axis of rotation. We find that this effect is significantly enhanced in the case of anisotropic disc-shaped traps. The results obtained as a function of the anisotropy of the initial configuration are compared with the analytic solution for a noninteracting gas in 3D as well as with the scaling law predicted for an interacting gas in 2D.Comment: 5 pages, 6 postscript figure

Superradiation from Crystals of High-Spin Molecular Nanomagnets

Phenomenological theory of superradiation from crystals of high-spin molecules is suggested. We show that radiation friction can cause a superradiation pulse and investigate the role of magnetic anisotropy, external magnetic field and dipole-dipole interactions. Depending on the contribution of all these factors at low temperature, several regimes of magnetization of crystal sample are described. Very fast switch of magnetization's direction for some sets of parameters is predicted.Comment: 10 pages, 3 figure