Experiments to Control Atom Number and Phase-Space Density in ...

Chapter 5
Single-Photon Cooling of Rubidium
The first experiment showing single-photon cooling was a proof-of-principle demon-
stration using a magnetically trapped sample of 87 Rb. About 1.5×10 5 rubidium atoms
were irreversibly transferred into a small optical dipole trap from a magnetic trap reser-
voir. The phase-space density of this trapped sample was a factor of 23 higher than the
phase-space density that could be achieved by directly loading atoms into the optical
dipole trap without single-photon cooling [61].
This chapter discusses the second implementation of single-photon cooling, where
the amount of cooling was ultimately limited by trap dynamics [76]. Modifying the trap
geometry from an all-optical trap to a gravito-optical trap increased the trap depth,
allowing the capture of more atoms. In addition, a more detailed study of the cooling
process was possible due to the simpler geometry.
5.1 Implementation of Single-Photon Cooling in Rubidium
The starting point of the single-photon cooling scheme is a magnetically trapped
ensemble of atoms. For this proof-of-principle experiment this is achieved using standard
laser cooling techniques. Atoms are captured in the upper MOT from the background
rubidium vapor. The push beam transfers atoms into the glass cell, where the atoms
are recaptured in the lower MOT. Typically about 10 8 atoms are loaded during a time
of 1-2 seconds. The magnetic field gradient created by the lower MOT coils is about
8 G/cm. The MOT beam detuning is typically 15 MHz red detuned, and temperatures
of around 150 µK are usual.
The temperature of the atoms is further reduced to between 15 and 20 µK using
molasses cooling. For this stage the MOT beam detuning is usually increased to50 MHz.
67