Bose-Einstein Condensation of Metastable Helium
Condensation and Coherence in Condensed Matter
We have observed a Bose-Einstein condensate in a dilute gas of 4 He in the 2 3 S 1 metastable state. We find a critical temperature of ͑4.7 6 0.5͒ mK and a typical number of atoms at the threshold of 8 3 10 6 . The maximum number of atoms in our condensate is about 5 3 10 5 . An approximate value for the scattering length a ͑16 6 8͒ nm is measured. The mean elastic collision rate at threshold is then estimated to be about 2 3 10 4 s 21 , indicating that we are deeply in the hydrodynamic regime.
... ydrodynamic regime. The typical decay time of the condensate is 2 s, which places an upper bound on the rate constants for twobody and three-body inelastic collisions. Bose-Einstein condensation (BEC) of dilute atomic gases was first observed in alkali atoms in 1995 and then, a few years later, in atomic hydrogen. Since then, the field has developed in a spectacular way both experimentally and theoretically  . So far only condensates with atoms in their electronic ground state have been produced. Several laboratories are currently involved in the search for BEC of atoms in an excited state, namely, noble gases in an excited metastable state. Helium in its triplet metastable 2 3 S 1 state ( 4 He ء ) is of particular interest. The first advantage of 4 He ء is its large internal energy (19.8 eV). It allows for very efficient detection of the atom by ionization after collision with another atom or a surface, which can be of interest for atomic lithography [2, 3] . Second, helium is a relatively simple atom which allows for quasiexact calculations that are useful in metrological applications. Third, mixtures of 3 He and 4 He can be used to study quantum degenerate mixtures of bosons and fermions. Finally, Penning collisions are expected to be inhibited for spin polarized atoms due to spin selection rules. This effect, first pointed out in , was confirmed by subsequent calculations  . The present Letter describes the observation of BEC of 4 He ء atoms. Similar results have also been obtained at IOTA, Orsay . The two experiments differ by their detection methods. The IOTA group detects the atoms falling on a microchannel plate, whereas we use an optical absorption imaging of the atomic cloud on a charge-coupled device (CCD) camera. The two experiments therefore give different and complementary information on the physics of BEC in 4 He ء . The first step of our experiment is the efficient loading of a magneto-optical trap (MOT). The experimental setup is described in detail in  . A discharge atomic source ensures a high flux of triplet metastable atoms of 10 14 atoms͞s.sr, with a mean velocity of about 1000 m͞s. The atomic beam is collimated  and Zeeman slowed by laser light at 1083 nm (2 3 S 1 -2 3 P 2 transition). A narrow frequency band master oscillator (distributed Bragg reflector diode laser) injects a Yb-doped fiber amplifier with an output power of 500 mW. Using this setup, it is possible to trap ϳ8 3 10 8 atoms in the MOT at a temperature of 1 mK. 4 He ء atoms are confined at the center of a small (4 3 4 3 5 cm) quartz cell. All coils are external to the cell (see Fig. 1 ). Coils Q 1 and Q 2 (144 turns and 7 cm diameter) combined with coil C 3 (108 turns and 4 cm diameter) produce an anisotropic magnetic Ioffe-Pritchard trap. Two additional Helmholtz coils reduce the bias field, in order to increase the radial confinement of the trap. A current of 45 A in all the coils produces a 4.2 G bias field, radial gradients of 280 G͞cm, and an axial curvature of 200 G͞cm 2 . 0031-9007͞01͞86(16)͞3459(4)$15.00