Bose-Einstein Condensation

Abstract

Bose-Einstein condensation (BEC) refers to a prediction of quantum statistical mechanics (Bose [1], Einstein [2]) where an ideal gas of identical bosons undergoes a phase transition when the thermal de Broglie wavelength exceeds the mean spacing between the particles. Under these conditions, bosons are stimulated by the presence of other bosons in the lowest energy state to occupy that state as well, resulting in a macroscopic occupation of a single quantum state. The condensate that forms constitutes a macroscopic quantum-mechanical object. BEC was first observed in 1995, seventy years after the initial predictions, and resulted in the award of 2001 Nobel Prize in Physics to Cornell, Ketterle and Weiman. The experimental observation of BEC was achieved in a dilute gas of alkali atoms in a magnetic trap. The first experiments used 87Rb atoms [3], 23Na [4], 7Li [5], and H [6] more recently metastable He has been condensed [7]. The list of BEC atoms now includes molecular systems such as Rb2 [8], Li2 [9] and Cs2 [10]. In order to cool the atoms to the required temperature (~200 nK) and densities (1013–1014 cm–3) for the observation of BEC a combination of optical cooling and evaporative cooling were employed. Early experiments used magnetic traps but now optical dipole traps are also common. Condensates containing up to 5x109 atoms have been achieved for atoms with a positive scattering length (repulsive interaction), but small condensates have also been achieved with only a few hundred atoms. In recent years Fermi degenerate gases have been produced [11], but we will not discuss these in this chapter.