Harald Ellmann

Non-Gaussian velocity distributions in optical lattices

We present a detailed experimental study of the velocity distribution of atoms cooled in an optical lattice. Our results are supported by full-quantum numerical simulations. Even though the Sisyphus effect, the responsible cooling mechanism, has been used extensively in many cold atom experiments, no detailed study of the velocity distribution has been reported previously. For the experimental as well as for the numerical investigation, it turns out that a Gaussian function is not the one that best reproduces the data for all parameters. We also fit the data to alternative functions, such as Lorentzians, Tsallis functions, and double Gaussians. In particular, a double Gaussian provides a more precise fitting to our results.

Anisotropic velocity distributions in 3D dissipative optical lattices

Abstract:We present a direct measurement of velocity distributions in two dimensions by using an absorption imaging technique in a 3D near resonant optical lattice. The results show a clear difference in the velocity distributions for the different directions. The experimental results are compared with a numerical 3D semi-classical Monte-Carlo simulation. The numerical simulations are in good qualitative agreement with the experimental results.

Assessment of a time-of-flight detection technique for measuring small velocities of cold atoms

A low noise time-of-flight detection system for laser cooled atoms has been constructed and incrementally optimized. Here, a thorough description of the construction is presented along with an analysis of the capabilities of the system. The quality of the detection (the resolution, the reproducibility, the sensitivity, etc.) is crucial for, e.g., the ability to see details in the velocity distribution profile, which is of interest for fundamental studies of statistical physics and of the laser cooling processes, and for detection of small initial velocities of an atomic cloud, important, e.g., when studying small drifts induced by Brownian motors and ratchets. We estimate the signal-to-noise ratio of our signal to be better than 1000:1 for a typical single shot, and we discuss the effect of the initial atomic cloud size, the probe size, and the effects of the wave packet spread during the fall time on the measured quantities. We show that the shape of the velocity distribution is well conserved during the mapping done in the detection, i.e., in the convolution with the probe beam, and that velocities as small as a few percent of the single photon recoil velocity can be resolved.

Anisotropic velocity distributions in 3D optical lattices

Summary form only given. We have measured velocity distributions in two dimensions, as a function of intensity, in a 3D near resonant optical lattice (NROL) in Cs. The NROL is created by two orthogonally polarized pairs of laser beams that propagate in the yz and xz plane respectively. The angle between the beams of each pair is 90/spl deg/ and each beam forms an angle of /spl theta/=45/spl deg/ with the (vertical) quantization (z-) axis. This results in a tetragonal structure in which the lattice constants are a/sub z/=/spl lambda//(2/spl radic/2) and a/sub x,y/=/spl lambda//2, where /spl lambda/=852 nm. The difference in lattice constants means that lattice sites have anisotropy trapping potentials, where the oscillation frequencies differ with a factor of 2 along x and z, in a harmonic approximation.

Using Talbot lattices for quantum state manipulation

One basic feature of optical lattices is that they hardly interact at all with the environment. Therefore, they have recently been suggested as a platform for implementing quantum logic gates. An additional attraction comes from the fact that by modifying the light field, a large ensemble of atoms can be manipulated in parallel.