Density Matrix From Photon Number Tomography
  We provide a simple analytic relation which connects the density operator of the radiation field with the number probabilities. The problem of experimentally "sampling" a general matrix elements is studied, and the deleterious effects of nonunit quantum efficiency in the detection process are analyzed showing how they can be reduced by using the squeezing technique. The obtained result is particulary useful for intracavity field reconstruction states. 
Stefano Mancini, Paolo Tombesi and Vladimir I.Manko