Experimental determination of a nonclassical Glauber-Sudarshan P   function
  A quantum state is nonclassical if its Glauber-Sudarshan P function fails to be interpreted as a probability density. This quantity is often highly singular, so that its reconstruction is a demanding task. Here we present the experimental determination of a well-behaved P function showing negativities for a single-photon-added thermal state. This is a direct visualization of the original definition of nonclassicality. The method can be useful under conditions for which many other signatures of nonclassicality would not persist. 
T. Kiesel, W. Vogel, V. Parigi, A. Zavatta, M. Bellini