text: Like Sisyphus cooling, the cooling mechanism of gray molasses relies on a two-photon Raman-type transition between two hyperfine-split ground states mediated by an excited state. Orthogonal superpositions of these ground states constitute "bright" and "dark" states, so called since the former couples to the excited state via dipole transitions driven by the laser, and the latter is only accessible via spontaneous emission from the excited state. As neither are eigenstates of the kinetic energy operator, the dark state also evolves into the bright state with frequency proportional to the atom's external momentum. Gradients in the polarization of the molasses beam create a sinusoidal potential energy landscape for the bright state in which atoms lose kinetic energy by traveling "uphill" to potential energy maxima that coincide with circular polarizations capable of executing electric dipole transitions to the excited state. Atoms in the excited state are then optically pumped to the dark state and subsequently evolve back to the bright state to restart the cycle. Alternately, the pair of bright and dark ground states can be generated by electromagnetically-induced transparency (EIT).
  The net effect of many cycles from bright to excited to dark states is to subject atoms to Sisyphus-like cooling in the bright state and select the coldest atoms to enter the dark state and escape the cycle. The latter process constitutes velocity-selective coherent population trapping (VSCPT).
  The combination of bright and dark states thus inspires the name "gray molasses."