Quantum Detection Theory for the Free-Space Channel,” The Interplanetary Network

The fundamental performance limits of optical communications over the free-space channel are developed using quantum theory, and presented in terms of con-cepts familiar to communications engineers. The compact Dirac notation generally employed in quantum mechanics is deflned, and key concepts necessary for under-standing quantum projection measurements are reviewed. A derivation that pro-vides signiflcant insights into the quantum measurement performed by the optimum receiver is developed by interpreting the familiar technique of photon counting in terms of quantum projection operators. The performance of the optimum quantum receiver for on{ofi keying and optical binary phase-shift-keying (BPSK) modula-tion is treated flrst as a noise-free (or pure-state) problem, then extended to include the efiects of background radiation. The performance of the optimum quantum re-ceiver is compared to that of classical optical receivers employing photon-counting and coherent detection techniques, and it is shown to be exponentially better in most cases. I