Sangouard, N., Bancal, J. D., Gisin, N., Rosenfeld, W., Sekatski, P., Weber, M., & Weinfurter, H. (2011). Loophole-free Bell test with one atom and less than one photon on average. Physical Review A, 84(5):. doi:10.1103/PhysRevA.84.052122.