Efficient production of an 87Rb F = 2, mF = 2 Bose-Einstein condensate in a hybrid trap

Eur. Phys. J. D Efficient production of an 87Rb F = 2, mF = 2 Bose-Einstein condensate in a hybrid trap Hari Prasad Mishra 0 Adonis Silva Flores 0 Wim Vassen 0 Steven Knoop 0 0 LaserLaB, Department of Physics and Astronomy, VU University , De Boelelaan 1081, 1081 HV Amsterdam , The Netherlands We have realized Bose-Einstein condensation (BEC) of 87Rb in the F = 2, mF = 2 hyperfine substate in a hybrid trap, consisting of a quadrupole magnetic field and a single optical dipole beam. The symmetry axis of the quadrupole magnetic trap coincides with the optical beam axis, which gives stronger axial confinement than previous hybrid traps. After loading 2 106 atoms at 14 K from a quadrupole magnetic trap into the hybrid trap, we perform efficient forced evaporation and reach the onset of BEC at a temperature of 0.5 K and with 4 105 atoms. We also obtain thermal clouds of 1 106 atoms below 1 K in a pure single beam optical dipole trap, by ramping down the magnetic field gradient after evaporative cooling in the hybrid trap. 1 Introduction The experimental realization of Bose-Einstein condensates (BEC) of dilute atomic gases [1,2] is most often based on laser cooling and subsequent evaporative cooling in magnetic or optical dipole traps, or both, in either a sequential or combined way. A simple approach to achieve BEC is a hybrid trap that consists of a single beam optical dipole trap (ODT) and a quadrupole magnetic trap (QMT) [3]. This hybrid trap combines the most simple magnetic trap and optical dipole trap in a way that one benefits from their individual strengths, i.e. a large trap volume to capture the laser-cooled cloud of atoms, tight confinement and efficient evaporation, while minimizing their weaknesses, i.e. Majorana spin-flip losses in a QMT and a small trap volume of an ODT. After evaporative cooling one can simply transfer the ultracold sample (or BEC) to a pure ODT by switching off the QMT completely. An experimental advantage over all-optical cooling methods (see e.g. Refs. [46]) is the much lower ODT power needed for the hybrid trap. The hybrid trap has been successfully applied in several experiments, for 87Rb [3,713], 85Rb [14], 133Cs [8] and 23Na [10]. In all these previous hybrid traps the symmetry axis of the QMT is placed vertically, while the ODT is in the horizontal plane, and forced evaporative cooling in the QMT is done by RF radiation. For 87Rb all experiments are done for the F = 1, mF = 1 hyperfine substate. Here we report on the realization of BEC of 87Rb in the F = 2, mF = 2 hyperfine substate, in a hybrid trap in which the axial axes of both the QMT and ODT cross under a small angle in the horizontal plane, and forced evaporative cooling in the QMT is done by microwave (MW) radiation. Our choice for the F = 2, mF = 2 state is primarily motivated by the suppression of inelastic collisions for an ultracold mixture of Rb and metastable triplet helium [15,16], similar to the case of other mixtures with Rb [1721]. It also provides a stronger confinement than the F = 1, mF = 1 hyperfine substate, as in the QMT the peak density scales with the magnetic moment to the third power. Furthermore, our orientation of the QMT with respect to the ODT allows for a four times stronger axial confinement, providing an additional enhancement of the peak density. The use of MW radiation for evaporative cooling is also motivated by the application of an atomic mixture, as MW-induced forced evaporative cooling is species-selective. Several groups have reported on the unwanted appearance of atoms in the F = 2, mF = 1 state during the MW-induced forced evaporation in harmonic magnetic traps [1924]. We have performed Stern-Gerlach imaging to investigate the spin purity of our sample. This paper is organized as follows. In Section 2, we introduce the hybrid trap, and derive a simple analytic formula for the density profile. In Section 3, we describe our experiment and, in Section 4, we give our experimental results, discussing the alignment and loading of the hybrid trap, evaporative cooling towards BEC, the spin purity, and transfer to a pure ODT. Finally, we conclude and give an outlook in Section 5. 2 Hybrid trap In the hybrid trap, as realized by Lin et al. [3], a single beam ODT is aligned below the QMT center (see Fig. 1a), For temperatures much smaller than the trap depth, the trapping potential can be approximated by: U (x, y, z) = U0eff + in which the radial confinement (x, z) is dominated by the ODT, and the axial confinement (y) by the QMT. The radial trap frequency is given by: where U0 = 2P C/(w2) is the ODT trap depth. The ef 0 fective trap depth U0eff is equal to U0 only for B = Blev, while U0eff < U0 for B < Blev due to gravity. Expanding the trapping potential around y = 0 gives the axial trapping frequency of the hybrid trap: Fig. 1. Schematics of our hybrid trap configuration (QMT and ODT), (a) showing the offset z0 in the y-z plane and (b) showing the angles between the QMT axis, ODT beam and absorption imaging beam in the x-y (horizontal) plane. such that the trap minimum of the combined magnetic and 4B optical trap is at a finite magnetic field, and atoms do not y = m |z0| , (4) suffer Majorana spin-flip losses. After RF- or MW-induced forced evaporative cooling in the QMT the magnetic field which depends on z0 and typically is much larger than the ggrraaddiieenntt Bofletvhe QmMg/T,iswrhaemrepethdedvoewrtnictaol gthraedlieevnittactoimon- eavxeianlfofrreaqusmenaclyl gorfadthieentpounrethOeDorTd,eroaOfD1TG=/cm.2TUh0e/rmefyoR2re, pensates gravity. Here m is the mass, g is the gravitational in the hybrid trap it is much easier to obtain a BEC than acceleration and = gF mF B is the magnetic moment of in a pure single beam ODT, even for a weak gradient [25]. the atom, where gF is the Lande factor of the hyperfine The density distribution n(r) = n0 exp [U (r)/kBT ] state F , mF is the quantum number of the Zeeman state, in the hybrid trap for temperatures much lower than the and B is the Bohr magneton. Lowering the power in the trap depth is given by: ODT beam allows further (one-dimensional) evaporative cooling in the hybrid trap, in which the hot atoms can B escape mainly downwards. An extensive analysis of the n(x, y, z) = n0 exp kBT 4y2 + z02 (5) hybrid trap, in particular the transfer from the QMT to twheeshuymbmridartirzaept,hies gmivaeinn iinngSreecdtiieonnts2, wofitrheftehreenaciem[3t]o.Hperroe- exp kBBTz0 exp mr22kxB2T+ z2 , vide a simple analytic expression of the density profile in the hybrid trap. and from the condition N = n(r)dr one finds for the The trapping potential of the hybrid trap is given by: peak density: x2 + 4y2 + (z z0)2 exp 2 where the first term is the QMT potential, the second term the ODT potential, and the third term the gravitational potential. In our case, the symmetry (strong) axis of the QMT and the ODT beam are along the y-axis, the z-axis is the vertical direction (see Fig. 1a). Here B is the mag (...truncated)