High-velocity dark states in velocity-selective coherent population trapping

TLDR: Experimental and theoretical studies of high-velocity superposition dark states that to the authors' knowledge have not been previously observed of VSCPT are presented.

Abstract: Some of the most interesting and important topics in the field of optical control of atomic motion occur in the quantum domain. In this domain the motion is described in terms of de Broglie wave fields of massive particles, and the interactions between these fields and the optical fields result in manipulation of the atoms by optical pumping and Raman transitions. Perhaps the most spectacular example results from the accumulation of atoms in ‘‘dark states,’’ atomic states that cannot be excited by the light field. Some atomic states are trivially dark, that is, they cannot be excited because the light has the wrong frequency or polarization. The more interesting cases are superposition states created by coherent Raman coupling by the optical field. A very special case occurs when those states whose evolution to excitable states vanishes exactly because their external ~de Broglie wave! states are characterized by a particular momentum. Such velocity-selective coherent population trapping ~VSCPT! has been a subject of considerable interest @1,2# since its first demonstration by Aspect et al. in 1988 @3#. VSCPT is of special interest because it enables atoms to accumulate steadily in dark states, creating arbitrarily narrow peaks in the momentum distribution. This paper presents our experimental and theoretical studies of high-velocity superposition dark states that to our knowledge have not been previously observed. Any description of the quantum nature of the atomic motion, including VSCPT, requires that the energy of such motion be included in the Hamiltonian: