  We theoretically investigate an ensemble of three-level room-temperature
atoms in a two-mode optical cavity, focusing on the case of counterpropagating
light fields. We find that in the limit of large detunings the problem admits
relatively simple and intuitive solutions. When a classical field drives one of
the transitions, the system becomes equivalent to an ensemble of two-level
atoms with suppressed Doppler broadening. In contrast, when the same transition
is driven with a single photon, the system collectively behaves like a single
two-level atom. This latter result is particularly relevant in the context of
quantum nonlinear optics.
