  Nitrogen vacancy (NV) centers in diamond have attracted considerable recent
interest for use in quantum sensing, promising increased sensitivity for
applications ranging from geophysics to biomedicine. Conventional sensing
schemes involve monitoring the change in red fluorescence from the NV center
under green laser and microwave illumination. Due to the strong fluorescence
background from emission in the NV triplet state and low relative contrast of
any change in output, sensitivity is severely restricted by a high optical shot
noise level. Here, we propose a means to avoid this issue, by using the change
in green pump absorption through the diamond as part of a semiconductor
external cavity laser run close to lasing threshold. We show theoretical
sensitivity to magnetic field on the pT/sqrt(Hz) level is possible using a
diamond with an optimal density of NV centers. We discuss the physical
requirements and limitations of the method, particularly the role of amplified
spontaneous emission near threshold and explore realistic implementations using
current technology.
