Past quantum state analysis of the photon number evolution in a cavity
  A quantum system can be monitored through repeated interactions with meters, followed by their detection. The state of the system at time t is thus conditioned on the information obtained until that time. More insight in the state dynamics is provided, however, by the past quantum state (PQS) [S. Gammelmark et al. Phys. Rev. Lett. 111, 160401 (2013)]. It relies on all aspects of the system evolution which are recorded in the past and in the future of t. Using PQS analysis for the quantum non-demolition photon number counting in a cavity, we can reveal information hidden in the standard approach and resolve a wider range of number states. This experiment demonstrates the strong potential of PQS analysis. 
Th\'eo Rybarczyk (LKB (CdF)), Stefan Gerlich (LKB (CdF)), Bruno   Peaudecerf (LKB (CdF)), Mariane Penasa (LKB (CdF)), Brian ...
quant-ph