The total amount of information that two parties can share is determined by the physical properties of the channel they are communicating over. Typically in optical communication, lasers are used to produce coherent states of light, and information can be encoded into either the phase, amplitude, or both. While the limits of information transfer over typical channels such as lossy optical fiber have been extensively studied, such limits in channels that also induce phase noise are not well understood, even though they are more realistic models for certain situations. When information is encoded into the phase of light, these channels severely degrade the amount of information that can be transmitted, especially when using conventional techniques of encoding and decoding, i.e. modulation and measurement.
This restriction of conventional approaches to optical communication lead us to propose and demonstrate methods for optimized communication strategies over such a phase-noise channel. The two key ingredients are finding particular physical states of light that can help shield information from the noise while keeping it easily extractable, and a novel coherent measurement based on counting single photons. These ingredients are optimized together for a specific noise channel in order to maximize the total amount of information transfer. This approach of a joint optimization allows for an increase in the amount of information that can be transmitted compared to conventional methods, even though the fundamental limits for the channel are not well known.
Optimized communication strategies with binary coherent states over phase noise channels
- T. DiMario, L. Kunz, K. Banaszek & F. E. Becerra
npj Quantum Information 5, 65 (2019)