Ultracold quantum matter

Abstract:

We suggest an atomic dipole trap which is produced by two counter-propagating Gaussian beams with different waists. This set-up creates an intensity dip in axial and radial directions near the node of the standing wave and can be used as an atom trap for blue detuning of the light. We simulated the behaviour of two level atoms in this trap using the dressed state Monte-Carlo method and we show that it gives a goad trapping when the residual intensity at the bottom of traps is small.

Abstract:

We have simulated the evaporative cooling of a dilute gas of Bose particles including quantum statistics using a Monte Carlo method. This approach can model situations which are far away from quasiequilibrium such as occur during forced evaporative cooling. We have also simulated the dynamical formation of Bose-Einstein condensate for homogeneous and inhomogeneous Bose gases under the random-phase approximation. It was found that the rate of accumulation of Bose particles into low-energy states through collisions is increased by forced evaporation; and a macroscopic population at ground state can be reached at a time scale characterized by classical collision time for an inhomogeneous gas in a harmonic potential. We present the results of simulations for the evaporative cooling and formation of a Bose-Einstein condensate in one-, two-, and three-dimensional position cuts and energy cuts. © 1997 The American Physical Society.

Abstract:

An investigation focusing on the nonequilibrium evolution of a trapped Bose gas revealed that the formation of a Bose gas can be classified into three stages, namely: kinetic evolution; the formation of short range order; and the off-diagonal long-range order. Using a homogeneous gas consisting of sodium atoms. The evolution of non-Gaussian velocity distribution was observed. This indicates the accumulation of Bose particles at low energy states, but not necessarily the signature of the appearance of a macroscopic Bose-Einstein condensate. The condensate fraction will be very small when the ensemble of sodium atoms magnetically trapped in a harmonic potential is considered.

Abstract:

We propose an efficient method of cooling atoms in a strong Gaussian standing wave. The steep gradients of the atomic potential energy give rise to large dipole forces, which can be much stronger than the maximum radiation pressure force and can therefore stop atoms in a much shorter distance. We have simulated the cooling process using a Semi-classical Monte-Carlo method, which includes the radial motion, in addition to the motion along the beams. Both motions are calculated directly without separation the dynamics into force and diffusion terms. To cool a large range of atomic velocities the frame in which the standing wave is at rest was swept by changing the frequencies of the counter-propagating beams, in a similar way to the well-known chirp cooling technique using the radiation pressure force. If the curvature of Gaussian beams far from beam waist is employed the radial motion and velocities can be reduced even for the blue detuning comparing to the near waist case. The simulations show that it ipossible to stop caesium atoms in a distance of several centimetres (the exact value depends on the laser power, beam waist radius and acceptable chirping force) starting from the most probable velocity at room temperature. Narrower radial and wider axial velocity distribution was obtained for red detuning comparing with the blue one.