Evaporative Cooling

Abstract

Laser cooling leads to the production of samples of atoms with low temperature and high density. In the 1920s Bose and Einstein predicted that for sufficiently low temperature and high density, a gas of atoms undergoes a phase transition that is now called Bose-Einstein condensation (BEC—see Chapter 17). This phase transition is predicted to occur at a phase space density $p \equiv n\lambda _{dB}^3 \cong 2.612$, where n is the density of the gas and $\lambda _{dB} = h/M\bar v = h/\sqrt {3Mk_B T}$ is the deBroglie wavelength of the atoms. For ordinary gases at room temperature and pressure, ρ ∼ 10^-6, but in a practical atomic beam oven, ρ ∼ 3 × 10^-10.