Hybrid quantum receiver for quadrature amplitude modulation coherent-state discrimination beating the classical limit

We present a quantum receiver for quadrature amplitude modulation (QAM) coherent-state discrimination with the homodyne-displacement hybrid structure. Our strategy is to carry out two successive measurements on parts of the quantum states. The homodyne result of the first measurement reveals partial information about the state and is forwarded to a displacement receiver, which finally identifies the input state by using feedback to adjust a reference field. Theoretical analysis and numerical simulation results show that for 16-QAM, the hybrid receiver could outperform the standard quantum limit using only on–off single-photon detectors with a reduced number of signal-state partitions, showing great potential toward implementing the receiver practically.