Laser Cooling below the One-Photon Recoil Energy by Velocity-Selective Coherent Population Trapping

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 ⁴He atoms to a temperature of 2 {μ}K, lower than both the usual Doppler cooling limit (23 {μ}K) and the one-photon recoil energy (4 {μ}K). The corresponding de Broglie wavelength (1.4 {μ}m) is larger than the atomic-transition optical wavelength.