Open Quantum Dynamics of Mesoscopic Bose-Einstein ... - Physics

7. Quantum simulations of evaporatively cooled Bose condensatesbenchmark for these treatments, and to precisely determine coherence properties, all thequantum effects must be included without approximation.7.1.2 Evaporative coolingThe production of atomic Bose condensates in recent years has relied upon the developmentof efficient atom trapping and cooling techniques. In particular, it is the amazing successof evaporative cooling techniques that has enabled condensation in atom traps. Atomevaporation cools far below recoil limit of laser cooling and reduces the temperature bymany orders of magnitude without diluting the gas. In this process, the gas is loaded into amagnetic or magneto-optical trap, perhaps at first undergoing some laser cooling[145]. Thegas will approximately obey a Bose-Einstein distribution. Atoms in the high-temperaturetail of the distribution have enough energy to escape from the trap, and do so takingaway more than the average energy per atom. The remaining atoms rethermalise throughelastic interatomic collisions, thereby repopulating the upper levels. The temperature isthus lowered, and there is always a supply of hot atoms which can take away more energy.The evaporative cooling procedure was first developed to cool hydrogen atoms[41, 82]in attempts to produce Bose condensation, but was more successfully applied to alkalimetalgases[37, 103, 143], leading to condensation in rubidium ( 87 Rb)[3] sodium ( 23 Na)[35],and lithium ( 7 Na)[16]. These alkali gases have a larger effective scattering cross sectionfor elastic collisions and so rethermalise at the faster rate necessary to overcome atomloss processes, such as three-body recombination. Evaporative cooling is now a commonprocedure, with Bose condensates being produced in approximately twenty laboratoriesworldwide at the time of writing, mainly in 87 Rb and including one in hydrogen[59].This chapter presents the results of using phase-space methods to directly simulate thequantum dynamics of evaporatively cooled condensates. The method enables calculationof the time evolution of system properties. We will review the results for several physicalsituations, which reveal interesting transient behaviours and also evidence for the spontaneousformation of additional structures, including the evidence for vortices as reportedin [46]. This thesis does not attempt to provide conclusive answers to all the issues raisedabove or to completely characterise the condensate ground state, but rather it summarisesthe progress that has been made using phase-space techniques. These first attempts ofsuch multimode quantum calculations constitute only the first steps of a comprehensive142