Quantum Limits on the Capacity of Multispan Links with Phase-sensitive Amplification

TL;DR

This paper investigates the fundamental quantum limits on the capacity of long-distance fiber optic links with phase-sensitive amplification, showing that quantum advantages are minimal and diminish over extended spans.

Contribution

It derives the ultimate quantum limits on multispan fiber links with phase-sensitive amplifiers, highlighting the limited quantum advantage over classical detection methods.

Findings

Quantum limits are close to classical capacity for long links.

Quantum advantage diminishes as link length increases.

Phase-sensitive amplification offers minimal capacity improvement.

Abstract

Long-distance fiber communication stands as a cornerstone of modern technology. One of the underlying principles, preventing signal levels from diminishing below the detectability threshold, is optical amplification. In particular, phase-sensitive amplifiers offer a promising solution as ideally they do not introduce any excess additive noise. Since such devices in principle operate at the quantum noise level, a natural question is whether one can further improve the capacity of amplified links using principles of quantum mechanics as it offers a much broader scope of signal modulations and detection schemes. We derive ultimate limits determined by the laws of quantum mechanics on the capacity of multispan links with phase sensitive amplification. We show that the quantum advantage over the standard approach based on optical quadrature detection is small and vanishes for long links.