Magnetic trapping of an ultracold (^87)Rb -(^133)Cs atomic mixture

Abstract

This thesis reports on the realisation and characterisation of a magnetically trapped ultracold atomic mixture of (^87)Rb and (^133)Cs in the F = 1, m(_F) = -1 and F = 3, m(_F) = - 3 hyperfine states respectively. A compact two-species double magneto-optical trapping (MOT) apparatus is constructed in which a pyramid MOT acts to provide an independent flux of both atomic species for capture in the ultra-high vacuum science region of the apparatus. For the two-species science MOT in which this atom flux is captured, interspecies light assisted inelastic collisions are found to be a highly significant loss mechanism. A novel optical pressure spatial displacement technique is developed to minimise such losses, allowing near independent simultaneous loading of up to ~ 8 x 10(^8) (^87)Rb and ~ 3 x 10(^8) (^133)Cs atoms into an Ioffe-Pritchard 'baseball' magnetic trap at magnetic biasfields of 166.70(6) and 165.50(6) G respectively. At the loaded 87Rb and 133Cs atom number densities of 1.78(6) x 10(^9) and 2.53(6) X 10(^9) cm-3 respectively the magnetic trap lifetime of each atomic species is shown to be 100(10) s and independent of the presence of the second atomic species. Radio-frequency evaporative cooling trajectories for (^87)Rb and (^133)Cs of 129 s duration are separately optimised under single species magnetic trap operation to achieve phase-space densities of 6(1) x 10(^-7) and 3(1) X 10(^-4) respectively at temperatures of 7.6(1) μK and 520(10) nK.(^133)Cs Feshbach resonances at 118.06(8) and 133.4(1) G are characterised through the measurement of magnetic field dependent losses at the increased phase-space density. Implementation of simultaneous evaporative cooling following the single species trajectories is found to be ineffective below ~10 μK due to the increased thermal load imposed upon the (^133)Cs atoms as the(^87)Rb single species elastic collision cross section approaches the low energy limit. Following simultaneous evaporation to ~ 15 μK thermalisation of the mixtures axial and radial temperature components suggests a (^87)Rb-(^133)Cs interspecies elastic collision rate 3(1) and 7(1) times greater than the calculated single species (^133)Cs and (^87)Rb elastic collision rates respectively. An interspecies Feshbach resonance search is undertaken by measuring the number of atoms of each species remaining in the magnetic trap as a function of applied magnetic field following simultaneous evaporation. The absence of magnetic field dependent losses in conjunction with analysis of the measurement sensitivity demonstrates that no interspecies Feshbach resonances wider than 1 G with two-body inelastic collision rate constants greater than 5 X 10(^-10) cm(^3) s(^-1) are present over the magnetic field range 166 < B < 370 G in the trapped states. The sensitivity of this measurement is found to be highly dependent upon the magnetic field induced differential gravitational sag of the mixtures components.