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

ensemble walks into this region, its velocity is compared to the velocity trappable by
the optical trap. If the atom is cold enough to be trapped, the single photon recoil
velocity due to the demon beam is added. Following the probabilities given by the
branching ratios atoms are then sorted into one of three categories: trapped in state
|F = 1,mF = 0〉, trapped in state |F = 1,mF = 1〉, or untrapped. If its velocity is too
high, the atom is removed from the simulation, mimicking trap loss.
These simulations confirm that only a subset of the total number of atoms has
the chance to be transferred into the optical trap via single-photon cooling. A large
number of the trajectories does not overlap with the optical trap volume or the atoms
have too high a remaining velocity along the x and y directions to be trapped inside the
optical trap.
These findings indicate that either an effective mixing of the atomic trajectories
or a true three-dimensional cooling scheme would improve the performance of single-
photon cooling.
5.7 Concluding Remarks
The largest amount of phase-space compression achieved in this implementation
of single-photon cooling was a factor of 350 over the phase-space density of the magnetic
trap. The final phase-space density was 4.9(3)×10 −4 , with a magnetic trap temperature
of T = 53 µK and a corresponding radius of 515 µm. The transfer efficiency was 0.3%,
leading to 3×10 5 atoms trapped in the optical trough at a temperature of T ′ = 4.3 µK.
This is a clear indication of the power of the single-photon cooling process, how-
ever, future improvements to the process should result in even larger increases in phase-
space density. Even though it will probably remain challenging to generate a degenerate
gas of atoms directly via single-photon cooling due to the single-photon recoil limit, it
seems feasible to increase phase-space density enough to be able to initialize evaporative
cooling.
This implementation was fundamentally limited by dynamics inside the magnetic
trap. Only a subset of the total phase-space had the potential to be cooled and trans-
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