Optimization of Quantum Information Processing at Mutual Information Criterion

A model of quantum noisy channel with input encoding by a classical random vector is described. An equation of optimality is derived to de-termine a complete set of wave functions describing quantum decodings based on quasi-measurements maximizing the classical amount of trans-mitted information. A solution of this equation is found for the Gaussian multimode case with input Gaussian distribution. It is described by the overcomplete family of coherent vectors describing an optimal quasimea-surement of the canonical annihilation amplitudes in the output Hilbert space. It is found that the optimal decoding in this case realizes the maxi-mum amount I = Sp l