Quantum Detection of Wavelength Division Multiplexing Optical Coherent Signals in Lossy Channels

We numerically evaluate the wavelength division multiplexing (WDM) data transmission of coherent phase-shift keying (PSK) and quadrature amplitude modulation (QAM) signals in optical fiber communication and deep-space communication channels with conventional homodyne-based(dyne-type) detections and various quantum detection strategies. We show the quantitative gap between these detection strategies and especially in the quantum-limited region where the quantum noise seriously limits the transmission rate. For an extremely weak signal input power, there is a crucial gap between the capacity limit and the transmission rates of the WDM system with dyne-type detections. We show that this gap is filled by applying a collective square root detection (SRD) only for each channel, not necessary for quantum collective decoding among WDM channels.