Field locked to Fock state by quantum feedback with single photon   corrections
  Fock states with photon numbers n up to 7 are prepared on demand in a microwave superconducting cavity by a quantum feedback procedure which reverses decoherence-induced quantum jumps. Circular Rydberg atoms are used as quantum non-demolition sensors or as single photon emitter/absorber actuators. The quantum nature of these actuators matches the correction of single-photon quantum jumps due to relaxation. The flexibility of this method is suited to the generation of arbitrary sequences of Fock states. 
X. Zhou, I. Dotsenko, B. Peaudecerf, T. Rybarczyk, C. Sayrin, S.   Gleyzes, J.M. Raimond, M. Brune, and S. Haroche
quant-ph