Heralded amplification for precision measurements with spin ensembles
  We propose a simple heralded amplification scheme for small rotations of the collective spin of an ensemble of particles. Our protocol makes use of two basic primitives for quantum memories, namely partial mapping of light onto an ensemble, and conversion of a collective spin excitation into light. The proposed scheme should be realizable with current technology, with potential applications to atomic clocks and magnetometry. 
Nicolas Brunner, Eugene S. Polzik, Christoph Simon
quant-ph