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Q: Quantenoptik

Q 17: Poster: Cooling and Trapping

Q 17.5: Poster

Wednesday, April 4, 2001, 12:30–15:00, AT2

The puzzle of heating in the gravito-optical surface trap — •Markus Hammes, Jens Herbig, David Rychtarik, and Rudi Grimm — Institut für Experimentalphysik, Universität Innsbruck, A-6020 Innsbruck

In order to understand the limitations for evaporative cooling we have investigated heating of an ensemble of ultracold cesium atoms in the gravito-optical surface trap (GOST). To measure the temperature evolution with time a sample of cesium atoms at 3 µK was prepared and stored in a nearly conservative trapping potential. At a detuning of the evanescent wave (EW) of about 30 GHz we found a prominent heating of ∼ 700 nK/s, which is three times larger than calculated for photon scattering. Measuring the decay rate of the population of the lower hyperfine level nevertheless indicates that this heating is due to photon scattering. This apparent discrepancy could fully be explained by the existence of an incoherent background of resonant photons originating from amplified spontaneous emission into side modes of the diode laser which we use to form the EW. This emission has its maximum at a detuning of 36 GHz which is consistent with the cavity parameters of the laser diode. In a preliminary set of measurements we removed the resonant photons by shining the EW-laser through a heated cesium vapor cell and thus reduced heating to 200 nK/s. Before the introduction of this filtering cell temperatures as low as 300 nK could already be achieved. We therefore expect substantial improvements for the efficiency of evaporation towards a quantum degeneracy of a surface gas in the GOST.

[1] M. Hammes et al., J. Mod. Opt. 47, 2755 (2000)