Inelastic Collisions in Optically Trapped Ultracold Metastable Ytterbium

Abstract

We report measurement of inelastic loss in dense and cold metastable ytterbium (Yb[ 3 P2]). Use of an optical far-off-resonance trap enables us to trap atoms in all magnetic sublevels, removing multichannel collisional trap loss from the system. Trapped samples of Yb[ 3 P2] are produced at a density of 2×10 13 cm −3 and temperature of 2 µK. We observe rapid two-body trap loss of Yb[ 3 P2] and measure the inelastic collision rate constant 1.0(3)×10 −11 cm 3 s −1 . The existence of the finestructure changing collisions between atoms in the 3 P2 state is strongly suggested.PACS numbers: 37.10. De, There is increasing interest in ultracold two-electron atoms [1,2], such as the alkaline earth metals (e.g. Ca and Sr) and Yb. In particular, novel characteristics of the metastable 3 P 2 atoms have recently attracted attention, both for applications and for the study of their collisional properties [3]. These atoms are set apart from the more commonly studied alkali metal atoms because collisions between 3 P 2 atoms are intrinsically anisotropic. Recent theory has investigated the effects of this anisotropy, including its interplay with magnetic field effects, which enable novel control of the scattering length [4], and multichannel collisions due to a strong coupling among the partial waves of relative motion [5,6]. Also, the magnetic dipole-dipole interaction between 3 P 2 atoms is 9 times larger than that between alkali metal atoms. This has led to theoretical predictions such as novel quantum phases and use in quantum information systems [7,8].In order to move toward study of these new possible features of 3 P 2 atoms, several laboratories have realized laser cooling and trapping of metastable two-electron atoms. Ca and Sr atoms decaying to the 3 P 2 state from the 1 P 1 state, which is the upper state in the 1 S 0 ↔ 1 P 1 magneto-optical trap (MOT) transition, have been successfully trapped in a magnetic trap [9]. Also, a MOT operating on the 3 P 2 ↔ 3 D 3 transition has been used to load a magnetic trap [10]. In spite of successes of these approaches, evaporative cooling of 3 P 2 atoms in a magnetic trap to reach Bose-Einstein condensation (BEC) turned out to be unsuccessful due to trap loss caused by strong multichannel collision processes [5]. More recently, a similar large inelastic collision rate in Ca[ 3 P 2 , m J =2] was observed [11].The loss induced by multichannel collisions in a magnetic trap can be overcome by employing, instead of a magnetic trap, an optical far-off-resonance trap (FORT). According to Ref. [12], the FORT wavelength can be chosen so that atoms in every magnetic sublevel of the 3 P 2 state can be trapped with the same strength. As a result, although multichannel collisions can still occur, which distributes the atoms over the different magnetic sublevels, they will not lead to trap loss. Thus, any trap loss observed in such a trap must be due to a different physical mechanism. Study of these collisional properties is crucial to understanding the physics of these important class of atom...

Magnetic relaxation in dysprosium-dysprosium collisions

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