Observation of Bose-Einstein condensation in metastable helium

During the evening of 12 February 2001, one of the Cold Quantum Gases network partners, the atom optics group of the Laboratoire Charles Fabry de l’Institut d’Optique in Orsay, observed a BEC of He* (in the 23S1 state). The apparatus uses a cloverleaf-type magnetic trap with coils placed in re-entrant vacuum flanges. This design allows us to use a microchannel plate (MCP) 5 cm below the trap to detect the atoms after releasing them from the trap. Figure 1 shows a diagram of the apparatus.

Figure 1: Schematic diagram of the apparatus. The magnetic field coils are in red (the arrows show the direction of the currents), the MCP is the gray disc at the bottom (not to scale), and the cold atoms are in blue (also not to scale).

Figure 2: shows an example of a single-shot time-of-flight spectrum detected by the MCP is shown in the 2nd figure. The horizontal axis is the arrival time of the atoms, but the distribution closely corresponds to the spatial profile of the atoms along one of the strong axes of the trap.The red curve shows a fit to the wings of the distribution giving a temperature of 0.7 mK. 

Some experimental data from the MCP are shown in the second figure.The condensate peak shown contains about 4000 atoms, but we have determined that in this experiment we only detected about 10% of the atoms in the trap. The largest condensate we have observed contained about 105 atoms.

Some people, including some of us, had been skeptical about whether BEC would be attainable with He* fearing that Penning ionization would limit the density to below that which was necessary. Our condensate lifetime however, is about 1s and this gives an upper limit of 10-13 cm3s-1 for the Penning ionization rate constant. From our data we also estimate the s-wave scattering length to be 20±10 nm.

For more details, see A. Robert et al. Sciencexpress/ www.sciencexpress.org/ 22 March 2001/ page 1/ 10.1126/science.1060622, and A. Browaeys et al. http://arXiv/abs/physics/0102068, as well as the publication by the ENS group, F. Pereira dos Santos et al., Phys. Rev. Lett., 86, 3459 (2001).

The current team members are :

Alice Robert, Olivier Sirjean, Denis Boiron, Chris Westbrook, Alain Aspect

and some of the recent ‘graduates’, who made essential contributions are :

Antoine Browaeys, Julie Poupard, Stephan Nowak.

Orsay, 9 April 2001