Statistical characterization of discrete amplitude-modulated coherent states at telecom wavelengths by means of an up-conversion-based photon-number-resolving detector

The successful implementation of quantum communication protocols relies on the proper encoding of information in the degrees of freedom of the employed optical states. Particular interest is devoted to amplitude-phase-shift keying coherent states, which can provide robust solutions in satellite communication systems and guarantee high values of channel capacity. In this work, we implement a sum-frequency-based photon-number-resolving detector, capable of revealing discrete amplitude modulation of coherent states produced at telecom wavelengths. The detection is performed in the visible spectral range and in the photon-number-resolving domain, thus encouraging the use of more complex alphabets in which both amplitude and phase vary.