Photonic communications with quadrature-amplitude modulated quantum coherent states in alternated and dual polarizations

We present the analysis of the quantum detection of coherent states used in photonic communications systems that employ the dimensions of both a) the complex amplitude in the quadrature-amplitude modulation (QAM) of order M (M-ary QAM) format, and b) the state of polarization of the quantum state. At the transmitter, a constellation of quantum composite coherent states is prepared producing a four-dimensional (4D) M-ary QAM modulation on the photonic carrier, either in alternated or in dual polarizations; the receiver implements a quantum detection strategy on the composite states constellation, to determine as precisely as possible which quantum state was sent. We employ the square root method (SRM) for the detection of the received 4D quantum coherent states constellation both: (a) in alternate polarizations, and (b) in dual polarizations, for the assessment of the photonic communications system performance in mutual information and in error probability.