Classical Information-Carrying Capacity of Bosonic Channels

The lossy bosonic channel provides a quantum model for optical communication systems that rely on fiber or free-space propagation. For the pure-loss case, we have shown that the classical information-carrying capacity of this channel is achievable with single-use coherent-state encoding. For the more general thermal-noise channel, the Holevo information of single-use coherent-state encoding is a lower bound on the channel capacity, which we have shown would equal that capacity if our recent conjecture about that channel's minimum output entropy were correct. We have studied multiple-access bosonic communications, in which two or more senders communicate to a common receiver over a shared propagation medium. We showed that single-use coherent-state encoding with the optimum measurement achieves the sum-rate capacity but does not realize the single-user capacity, and we quantified the capacity region that is lost when coherent (heterodyne or homodyne) detection is employed instead of the optimum measurement. We are now considering the classical information-carrying capacity of the degraded bosonic broadcast channel, in which a single sender communicates to two or more receivers through a lossless 1:M coupler whose auxiliary inputs are in their vacuum states.