Positive Operator-Valued Measure reconstruction of a beam-splitter tree   based photon-number-resolving detector
  Here we present a reconstruction of the Positive Operator-Value Measurement of a photon-number-resolving detector comprised of three 50:50 beamsplitters in a tree configuration, terminated with four single-photon avalanche detectors. The four detectors' outputs are processed by an electronic board that discriminates detected photon number states from 0 to 4 and implements a "smart counting" routine to compensate for dead time issues at high count rates. 
F. Piacentini, M. P. Levi, A. Avella, M. Lopez, S. Kueck, S. V.   Polyakov, I. P. Degiovanni, G. Brida, and M. Genovese
quant-ph