  Using a density matrix approach, we study the simplest systems that display
both gain and feedback: clusters of 2 to 5 atoms, one of which is pumped. The
other atoms supply feedback through multiple scattering of light. We show that,
if the atoms are in each other's near-field, the system exhibits large gain
narrowing and spectral mode redistribution. The observed phenomena are more
pronounced if the feedback is enhanced. Our system is to our knowledge the
simplest exactly solvable microscopic system which shows the approach to laser
oscillation.
