Delayed-choice test of complementarity with single photons
  We report an experimental test of complementarity using clock-triggered single-photon pulses emitted by an individual N-V color center in a diamond nanocrystal. The single photons are sent into a Mach-Zehnder interferometer with an output beamsplitter of adjustable reflection coefficient R. In addition, the choice of introducing or removing this beamsplitter is random and relativistically space-like separated from the entering of the photon inside the interferometer, as required for the Wheeler's delayed-choice regime. Each set value of R allows us to observe interference with visibility V and to obtain incomplete which-path information characterized by the distinguishability D. The measured values of V and D are found to obey the complementarity relation V^2 + D^2 =< 1. 
Vincent Jacques (LPQM), E. Wu (LPQM), Fr\'ed\'eric Grosshans (LPQM),   Fran\c{c}ois Treussart (LPQM), Philippe Grangier (LCF...