Loophole-free Bell test for one atom and less than one photon
  We consider the entanglement between two internal states of a single atom and two photon number states describing either the vaccum or a single photon and thus containing, on average, less than one photon. We show that this intriguing entanglement can be characterized through substantial violations of a Bell inequality by performing homodyne detections on the optical mode. We present the experimental challenges that need to be overcome to pave the way towards a loophole-free Bell test in this setup. 
N. Sangouard, J.-D. Bancal, N. Gisin, W. Rosenfeld, P. Sekatski, M.   Weber, and H. Weinfurter
quant-ph