  In recent works, it has been shown rather generally that the time-reversed
process of lasing at threshold realizes a coherent perfect absorber (CPA). In a
CPA, a lossy medium in an optical cavity with a specific degree of dissipation,
equal in modulus to the gain of the lasing medium, can perfectly absorb
coherent optical waves that are the time-reversed counterpart of the lasing
field. Here the time-reversed process of lasing is considered in details for a
homogeneously-broadened two-level medium in an optical cavity, and the
conditions for CPA are derived. It is shown that, owing to the dispersive
properties of the two-level medium, exact time-reversal symmetry is broken and
the frequency of the field at which CPA occurs is generally different than the
one of the lasing mode. Moreover, at a large cooperation parameter the
observation of CPA in the presence of bistability requires to operate in the
upper branch of the hysteresis cycle.
