Optimum detection for extracting maximum information from symmetric qubit sets

A class of optimum detection strategies was demonstrated for extracting the maximum information from sets of equiprobable real symmetric qubit states of a single photon. The detections with at least three outputs were found to suffice for optimum extraction of information regardless of the number of signal elements. Optimum measurement consisted of three symmetric state vectors with length less than unity in ternary or trine signals. It was found that the difference between the measured value for the mutual information and the theoretical limit was due to the leakage of the wrong polarization through polarizing beamsplitters and imperfect contrast.