Laser cooling below the one-photon recoil energy by velocity-selective coherent population trapping

Abstract

We present a new laser-cooling scheme based on velocity-selective optical pumping of atoms into a nonabsorbing coherent superposition of states. This method has allowed us to achieve transverse cooling of metastable He4 atoms to a temperature of 2 K, lower than both the usual Doppler cooling limit (23 K) and the one-photon recoil energy (4 K). The corresponding de Broglie wavelength (1.4 m) is larger than the atomic-transition optical wavelength. © 1988 The American Physical Society.