Light amplification without stimulated emission: Beyond the standard quantum limit to the laser linewidth

The standard quantum limit to the linewidth of a laser for which the gain m edium can be adiabatically eliminated is l(0) = K/2 (n) over bar Here K is the intensity damping rate and (n) over bar the mean photon number. This co ntains equal contributions from the loss and gain processes, so that simple arguments which attribute the linewidth wholly to phase noise from spontan eous gain are wrong. I show that an unstimulated gain process actually intr oduces no phase noise, so that the ultimate quantum limit to the linewidth comes from the loss alone and is equal to l(ult) = K/4 (n) over bar. I inve stigate a number of physical gain mechanisms which attempt to achieve gain without phase noise: a linear atom-field coupling with a finite interaction time, a nonlinear atom-field coupling, and adiabatic photon transfer using a counterintuitive pulse sequence. The first at best reaches the standard limit l(0), the second reaches 3/4l(0), and the third reaches the ultimate limit of l(ult)= 1/2l(0). [S1050-2947(99)03711-7].