Fundamental Limits of Thermal-noise Lossy Bosonic Multiple Access Channel

TL;DR

This paper analyzes the capacity limits of thermal-noise bosonic multiple access channels, showing coherent states are optimal in certain regimes and squeezed states can enhance capacity under specific constraints.

Contribution

It develops an outer bound for the capacity region of the thermal-noise lossy bosonic MAC and identifies conditions where coherent and squeezed states are capacity-achieving.

Findings

Coherent states are capacity-achieving at high and low input photon numbers.

The sum rate capacity is achieved by coherent states.

Squeezed states can outperform coherent states under individual photon constraints.

Abstract

Bosonic channels describe quantum-mechanically many practical communication links such as optical, microwave, and radiofrequency. We investigate the maximum rates for the bosonic multiple access channel (MAC) in the presence of thermal noise added by the environment and when the transmitters utilize Gaussian state inputs. We develop an outer bound for the capacity region for the thermal-noise lossy bosonic MAC. We additionally find that the use of coherent states at the transmitters is capacity-achieving in the limits of high and low mean input photon numbers. Furthermore, we verify that coherent states are capacity-achieving for the sum rate of the channel. In the non-asymptotic regime, when a global mean photon-number constraint is imposed on the transmitters, coherent states are the optimal Gaussian state. Surprisingly however, the use of single-mode squeezed states can increase the capacity over that afforded by coherent state encoding when each transmitter is photon number constrained individually.