Abstract

There are two experimentally tested laser cooling methods that lead to final velocities below the recoil velocity from a single photon, v_rec = ħk/M: velocity-selective coherent population trapping (VSCPT), introduced in 1988,¹ and Raman cooling, first demonstrated in 1992 with sodium atoms.² While VSCPT requires a specific type of atomic transition, Raman cooling can be applied to any three-level system with a split ground state and one excited state (Fig. 1). The method consists of a sequence of velocity-selective Raman pulses, tuned successively closer to resonance, which selectively transfer atoms from |1〉 to |2〉. Each of these pulses is followed by a repumping pulse, which resonantly excites atoms from |2〉 to |3〉, from where they irreversibly return to the |1〉 state by spontaneous emission. Appropriate choice of pulse shape, length, and detuning leads to accumulation of atoms in a narrow velocity region around v = 0. No fundamental lower limit is known for the final temperature.

© 1994 IEEE