Ultracold quantum matter

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.

Abstract:

We have simulated the evaporative cooling of trapped atoms using a very efficient method originally introduced for the study of molecular gas dynamics. This straightforward and intuitive method allows the dynamics of the evaporative cooling process to be studied and requires fewer simplifications and assumptions than other methods. In particular, the method is not restricted to distributions close to equilibrium and therefore it can model accurately rapid forced evaporative cooling, which is an important technique for cooling trapped atoms. We present the results of simulations for forced evaporative cooling in one, two and three dimensions.

Abstract:

Evaporative cooling is a simple and very effective way of cooling atoms in a magnetic trap. A modelling method for this technique was developed by considering the physics of gas flow. Using this method, cross-dimensional mixing in homogeneous and inhomogeneous gases and continuous cuts in two and three dimensions are studied. The two-dimensional cut model is similar to the evaporative process in a TOP trap because atoms in this trap are removed in the basis of their radial positions. Initially, a two dimension cut retains atoms in the trap but atom loss becomes greater than with a three dimension cut because the velocity component along z is relatively hot and gives up more energetic atoms.