QPSK coherent state discrimination via a hybrid receiver
  We propose and experimentally demonstrate a near-optimal discrimination scheme for the quadrature phase shift keying protocol (QPSK). We show in theory that the performance of our hybrid scheme is superior to the standard scheme - heterodyne detection - for all signal amplitudes and underpin the predictions with our experimental results. Furthermore, our scheme provides the hitherto best performance in the domain of highly attenuated signals. The discrimination is composed of a quadrature measurement, a conditional displacement and a threshold detector. 
Christian R. M\"uller, Mario A. Usuga, Christoffer Wittmann, Masahiro   Takeoka, Christoph Marquardt, Ulrik L. Andersen, Ger...
quant-ph