Four-state discrimination via a homodyne-Kennedy hybrid receiver

We present a novel quantum receiver for the discrimination of a coherent state alphabet consisting of four phase covariant coherent states equally spaced in phase by pi/2. Our receiver is a hybrid system composed of a homodyne and a modified Kennedy receiver, which is an optimization of the original Kennedy receiver such that it reaches smaller error probability for the discrimination of two states. In our detection scheme, the arriving signal is equally divided into two parts; one part is sent to the homodyne detector and the other to the modified Kennedy detector. The output of the homodyne detector discriminates pairs of states and thus reduces the possible results from four states to two states. This information is forwarded to the modified Kennedy receiver, which is then carrying out the discrimination of the two remaining states.