Photon blockade with a four-level quantum emitter coupled to a   photonic-crystal nanocavity
  We study the photon blockade phenomenon in a nanocavity containing a single four-level quantum emitter. By numerically simulating the second-order autocorrelation function of the intra-cavity field with realistic parameters achievable in a state-of-the-art photonic-crystal nanocavity, we show that in the strongly coupled regime the resulting photon blockade is significantly better than that achievable with a two-level emitter. We introduce an intuitive picture of the photon blockade with a four-level emitter that explains the performance difference between the two-level and the four-level emitter schemes, as well as why -- in contrast to a cavity containing a two-level atom -- signatures of photon blockade appear and should be experimentally observable even outside the strong coupling regime when a four-level emitter is used. Finally, we show that the emitter-cavity coupling achievable in a nanocavity can overcome the non-ideal spacing of optical transitions in realistic four-level emitters that has so far prevented experimental realization of this photon blockade scheme. 
M. Bajcsy, A. Majumdar, A. Rundquist and J. Vu\v{c}kovi\'c
quant-ph