C. J. Foot

Vortex Nucleation in Bose-Einstein Condensates in an Oblate, Purely Magnetic Potential

We have investigated the formation of vortices by rotating the purely magnetic potential confining a Bose-Einstein condensate. We modified the bias field of an axially symmetric TOP trap to create an elliptical potential that rotates in the radial plane. This enabled us to study the conditions for vortex nucleation over a wide range of eccentricities and rotation rates.

Observation of the scissors mode and evidence for superfluidity of a trapped bose-einstein condensed Gas

We report the observation of the scissors mode of a Bose-Einstein condensed gas of 87Rb atoms in a magnetic trap, which gives direct evidence of superfluidity in this system. The scissors mode of oscillation is excited by a sudden rotation of the anisotropic trapping potential. For a gas above T(c) (normal fluid) we detect the occurrence of oscillations at two frequencies, with the lower frequency corresponding to the rigid body value of the moment of inertia. Well below T(c) the condensate oscillates at a single frequency, without damping, as expected for a superfluid.

Dynamic manipulation of Bose-Einstein condensates with a spatial light modulator

We manipulate a Bose-Einstein condensate using the optical trap created by the diffraction of a laser beam on a fast ferroelectric liquid crystal spatial light modulator. The modulator acts as a phase grating which can generate arbitrary diffraction patterns and be rapidly reconfigured at rates up to 1 kHz to create smooth, time-varying optical potentials. The flexibility of the device is demonstrated with our experimental results for splitting a Bose-Einstein condensate and independently transporting the separate parts of the atomic cloud.

A pyramidal magneto-optical trap as a source of slow atoms

Abstract We have constructed and characterised a novel source of slow atoms based on a pyramidal magneto optical trap with a small hole at its vertex. Atoms are first captured in the trap and then pushed through the hole by a laser beam. The size and velocity of the resulting pulses of atoms were measured. The flux of cold atoms was 1.1×10 9 atoms/s and the apparatus is readily scaleable to obtain higher fluxes.

Quasi-2D confinement of a BEC in a combined optical and magnetic potential

We have added an optical potential to a conventional time-averaged orbiting potential (TOP) trap to create a highly anisotropic hybrid trap for ultracold atoms. Axial confinement is provided by the optical potential; the maximum frequency currently obtainable in this direction is 2.2 kHz for rubidium. The radial confinement is independently controlled by the magnetic trap and can be a factor of 700 times smaller than in the axial direction. This large anisotropy is more than sufficient to confine condensates with ~105 atoms in a quasi-2D (Q2D) regime, and we have verified this by measuring a change in the free expansion of the condensate; our results agree with a variational model.

A ring trap for ultracold atoms in an RF-dressed state

We combine an RF-dressed magnetic trap with an optical potential to produce a toroidal trapping potential for ultracold 87Rb atoms. We load atoms into this ring trap from a conventional magnetic trap and compare the measured oscillation frequencies with theoretical predictions. This method of making a toroidal trap gives a high degree of flexibility such as a tuneable radius and variable transverse oscillation frequency. The ring trap is ideal for the creation of a multiply connected Bose–Einstein condensate (BEC) and the study of persistent flow and we propose a scheme for introducing a flow of the atoms around the ring.

Temperature Dependence of Damping and Frequency Shifts of the Scissors Mode of a Trapped Bose-Einstein Condensate

We have studied the properties of the scissors mode of a trapped Bose-Einstein condensate of 87Rb atoms at finite temperature. We measured a significant shift in the frequency of the mode below the hydrodynamic limit and a strong dependence of the damping rate as the temperature increased. We compared our damping rate results to recent theoretical calculations for other observed collective modes, finding a fair agreement. From the frequency measurements we deduce the moment of inertia of the gas and show that it is quenched below the transition point, because of the superfluid nature of the condensed gas.

Direct simulation of evaporative cooling

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.

Laser spectroscopy of calcium isotopes

Improved measurements of isotope shifts in the 4s2 1S0-4s5s 1S0 transition of calcium are reported for the stable isotopes. A comparison with isotope shift measurements in other transitions by means of a King plot shows satisfactory agreement. Values of the changes in mean-square nuclear charge radius delta (r2) from a combined analysis of muonic isotope shifts and electron scattering data are used to separate the mass and field shifts in the optical lines. This procedure leads to values of delta (r2) for the calcium isotopes from 40Ca to 48Ca using all available high-precision data. The results for delta (r2)A,40 are 3.2(2.5), 215.3 (4.9), 125.4 (3.2), 283.2 (6.4), 118.8 (5.9), 124.2 (5.0), 5 (13) and -4.4(6.0)*10-3 fm2 for A=41 to 48 respectively. Values of the electronic factors relating the observed shifts of delta (r2) are deduced, and discussed in terms of configuration mixing in calcium.

Influence of background pressure on the stability region of a Paul trap

We report experimental measurements of the stability region of a Paul trap with very strong damping where the trapping parameter q was up to 25 times greater than the maximum value with no damping. We measured the region of stable trapping of charged particles for background pressures from atmospheric down to 5×10-2 mbar. The large increase in the size of the stability region that we observed at the highest pressure is explained by numerical solution of the Mathieu differential equation including a damping term. We show that stability regions that are separate under conditions of low damping merge together at higher pressures.