Superradiant scattering of laser light from a Bose-Einstein condensate

Abstract

We describe the setup to create a large Bose-Einstein condensate (BEC) containing more than 120 million atoms. In the experiment a thermal beam is slowed by a Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). The sample is subsequently spin polarized in a high magnetic field, before the atoms are loaded in the magnetic trap. Spin polarizing in a high magnetic field results in an increase in the transfer efficiency by a factor of 2 compared to experiments without spin polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by evaporative cooling. This large atom number BEC yields the possibility to enter the hydrodynamic regime by evaporative cooling at a relative low density suppressing the effect of avalanches. The collisional opacity can be tuned from the collisionless regime to a collisional opacity of more than 3 by compressing the trap after condensation. In the collisional opaque regime a significant heating of the cloud at time scales shorter than half of the radial trap period is measured. This is direct proof that the BEC is hydrodynamic. Furthermore a detailed study is presented of superradiant scattering from a BEC. The relation between the depletion of the BEC and the onset of the different side modes as well as the transition from the weak to the strong pulse regime is studied yielding a good insight in the coupling processes between the BEC and the side modes. Finally, a simplified model to describe the time evolution of the process is presented. The model gives a good quantitative description of the fraction of atoms coupled to different side modes, showing that the approximations made in the modeling are valid. Also the atom pair production by superradiant backward-scattering from a Bose-Einstein condensate is investigated. In our experiment the energy in the process is conserved by using a pulse with two frequencies. This makes the process resonant, which gives full control over the number of atoms in the different recoil modes. It is shown that resonant backward-scattering produces strongly correlated side modes in which number squeezing and phase correlation are likely. This makes resonant superradiant backward-scattering a promising candidate for many-particle entanglement.