  The quantum-mechanical and thermodynamic properties of a 3-level molecular
cooling cycle are derived. An inadequacy of earlier models is rectified in
accounting for the spontaneous emission and absorption associated with the
coupling to the coherent driving field via an environmental reservoir. This
additional coupling need not be dissipative, and can provide a thermal driving
force - the quantum analog of classical absorption chillers. The dependence of
the maximum attainable cooling rate on temperature, at ultra-low temperatures,
is determined and shown to respect the recently-established fundamental bound
based on the second and third laws of thermodynamics.
