Estimation of a classical parameter with gaussian probes: magnetometry   with collective atomic spins
  We present a theory for the estimation of a classical magnetic field by an atomic sample with a gaussian distribution of collective spin components. By incorporating the magnetic field and the probing laser field as quantum variables with gaussian distributions on equal footing with the atoms, we obtain a very versatile description which is readily adapted to include probing with squeezed light, dissipation and loss and additional measurement capabilities on the atomic system. 
Klaus Molmer (1 and 2) and Lars Bojer Madsen (2) ((1) Danish National   Research Foundation Center for Quantum Optics. (2) D...