Metastable helium Bose-Einstein condensate with a large number of atoms

We have produced a Bose-Einstein condensate of metastable helium 4 He * containing over 1.510 7 atoms, which is a factor of 25 higher than previously achieved. The improved starting conditions for evaporative cooling are obtained by applying one-dimensional Doppler cooling inside a magnetic trap. The same technique is successfully used to cool the spin-polarized fermionic isotope 3 He * , for which thermalizing collisions are highly suppressed. Our detection techniques include absorption imaging, time-of-flight measurements on a microchannel plate detector, and ion counting to monitor the formation and decay of the condensate. Ten years after the first experimental realization of BoseEinstein condensation BEC in dilute, weakly interacting atomic systems 1, the field of degenerate quantum gases has developed into a major area of research. For most elements it has not yet been possible to produce Bose-Einstein condensates containing large numbers of atoms. Only for hydrogen, sodium, and rubidium have condensates with more than 10 6 atoms been realized 2. Large condensates provide a better signal-to-noise ratio, allow a study of both the collisionless and the hydrodynamic regime, and are especially useful for sympathetic cooling and atom optics applications. In this realm metastable atoms are of particular interest, offering alternative and more sensitive detection methods due to their high internal energy. The first He * BEC was detected using a microchannel plate MCP detector 3. In the second experiment in which BEC was realized with He * 4, optical detection was used and up to 610 5 atoms could be condensed. The value of the scattering length, recently reported by this group 5, is favorable for experiments with the fermionic isotope 3 He * . It ensures a stable ultracold 4 He * - 3 He * boson-fermion mixture, as the interisotope scattering length will be large and positive 6. Large numbers of 4 He * atoms down to the critical temperature provide an efficient reservoir for sympathetic cooling and will facilitate the production of degenerate 3 He * clouds with large numbers of atoms. In this Rapid Communication we present an experiment that combines the various detection methods used previously 3,4 and describes the realization of a BEC of 4 He * containing more than 1.510 7 atoms. This large improvement is primarily due to the application of one-dimensional Doppler cooling inside the magnetic trap rather than threedimensional Doppler cooling prior to magnetic trapping. Doppler cooling of polarized atoms was originally proposed for atomic hydrogen 7, and has recently been demonstrated for optically dense samples of magnetically trapped chromium atoms 8. Compared to the laser cooling methods we investigated previously 9,10, this configuration is more efficient and simple. The experimental setup is an extended and improved version of our previous setup 9. In short, we start with a beam of metastable atoms produced by a liquid-nitrogen cooled dc-discharge source. The atomic beam is collimated, deflected, and slowed by applying laser beams resonant with the 2 3 S 1 →2 3 P 2 transition at 1083 nm. Typically 210 9 atoms are loaded into a magneto-optical trap MOT at a temperature of 1 mK. Since our previous experiments 9 we have installed a new ultrahigh vacuum UHV chamber and magnetic trap. The coils for our cloverleaf magnetic trap are placed in water-cooled plastic boxes and positioned in reentrant windows. Inside the UHV chamber two MCP detectors and an rf coil are mounted. The first MCP detector is positioned 10 cm from the trap center and attracts positively charged ions produced in Penning ionizing collisions: