Optimized communication strategies with coherent states over phase noise channels

Optimized communication graphic illustration

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.

Reference information:

Optimized communication strategies with binary coherent states over phase noise channels

  1. T. DiMario, L. Kunz, K. Banaszek & F. E. Becerra

npj Quantum Information 5, 65 (2019)

Becerra’s group publishes article on robust measurements at low powers by counting photons

Dr. Elohim Becerra Chavez’s research group recently published Robust Measurement for the Discrimination of Binary Coherent States in Physical Review Letters.

Optical communication uses light to encode and transmit information over long distances, such as in optical fibers with losses, and requires reliable detection schemes to read out information from low levels of light. However, noise and imperfections in real-world devices and detectors severely affect the measurement fidelity and ultimately limits the amount of information that we can communicate. Recent work in the quantum-optics group led by Elohim Becerra at UNM demonstrated an optimized measurement capable of overcoming these imperfections by counting the number of photons in pulses of light carrying information in their optical phase. This measurement reads off the information contained in the light by first combining the light pulse with a reference pulse to compare their relative phase, and then counting the number of photons in the combined pulse. Previous work in ultra-sensitive measurements has focused instead on detecting no light vs. any amount of light in the combined pulse, and their sensitivities have been limited by noise and imperfections. This novel measurement scheme allows for a high degree of robustness to these imperfections while reaching high measurement sensitivities. Moreover, due to its simplicity, this measurement is inherently compatible with high-bandwidth communication technologies to accommodate the high rates of information transfer in today’s optical communication networks.

This work was published in the July 2 issue of Physical Review Letters:

Robust Measurement for the Discrimination of Binary Coherent States
M. T. DiMario and F. E. Becerra
Phys. Rev. Lett. 121, 023603 (2018)

The full article can be found online.

Becerra’s group publishes article on multiple coherent states in Journal of Optical Society of America B

Dr. Elohim Becerra Chavez’s research group recently published Implementation of a single-shot receiver for quaternary phase-shift keyed coherent states in Journal of Optical Society of America B

Measurement strategies for multiple coherent states based on single-shot measurements with photon counting can be useful for high bandwidth communications with high spectral efficiency. The quantum-optics group led by Elohim Becerra at UNM investigated implementations of optimized multi-state discrimination strategies based on single-shot measurements extending previous work to include realistic situations with noise and imperfections, which impact the achievable performance of the measurement. The implementation with noise and imperfections allows us to identify the experimental requirements to outperform the sensitivity limit of an ideal heterodyne measurement and can guide future demonstrations of these measurements with high efficiency single-photon detectors surpassing the heterodyne limit.

The full article can be found online.