Reducing the linewidth of an atom laser by feedback

A continuous atom laser will almost certainly have a linewidth dominated by the effect of the atomic interaction energy, which turns fluctuations in t he condensate atom number into fluctuations in the condensate frequency. Th ese correlated fluctuations mean that information about the atom number cou ld be used to reduce the frequency fluctuations, by controlling a spatially uniform potential. We show that feedback based on a physically reasonable quantum nondemolition measurement of the atom number of the condensate in s itu can reduce the linewidth enormously.