Quantum backreaction effect in optical solitons

TL;DR

This paper investigates the quantum backreaction effects on optical solitons by quantizing fluctuations around classical solutions, revealing unstable modes that cause a measurable decrease in photon number and impact future optical experiments.

Contribution

It introduces a quantum field theoretic approach to analyze backreaction in optical solitons, going beyond the classical approximation with an analytical calculation of the effect.

Findings

Unstable modes grow during soliton propagation.

Backreaction causes a quadratic decrease in photon number.

Unstable modes dominate when initial pulse is near classical soliton.

Abstract

Optical solitons classically are stationary solutions of the nonlinear Schr\"odinger equation. We perform a quantum field theoretic treatment by quantising a linearised fluctuation field around the classical soliton solution which can be seen as providing a background spacetime for the field. The linearised fluctuation modifies the soliton background, which is often neglected, reminiscent of the nondepleted-pump approximation. Going beyond this approximation and by using a number-conserving Bogoliubov approach, we find unstable modes that grow as the soliton propagates. Eventually, these unstable modes induce a considerable (backreaction) effect in the soliton. We calculate the backreaction in the classical field fully analytically in the leading second order. The result is a quadratic local decrease of the soliton photon number in propagation due to the backreaction effect of the unstable mode. Provided the initial pulse is close to the classical soliton solution, the unstable mode contributions always become dominant. We also consider practical scenarios for observing this quantum-induced soliton distortion, in the spectral domain. The backreaction, which we expect to be present in bright and dark, discrete and continuous solitons and other nonlinear pulses plays an important role in future optical analogue gravity experiments, for soliton lasers, and optical communications.