# Fluctuating Forces Induced by Non Equilibrium and Coherent Light Flow

## TL;DR

This paper introduces a novel type of fluctuation-induced radiation force caused by mesoscopic coherent light fluctuations in random media, modeled via a hydrodynamic Langevin approach, with potential applications in sensing and quantum technologies.

## Contribution

It presents a new example of fluctuation-induced forces from coherent light in random media, modeled through a hydrodynamic Langevin framework with a tunable conductance parameter.

## Key findings

- Coherent light fluctuations can be described by a hydrodynamic Langevin approach.
- The fluctuation strength depends on a single dimensionless conductance g.
- Enhanced fluctuation-induced forces are achievable in multiple scattering setups.

## Abstract

Casimir physics covers a wealth of phenomena where forces between macroscopic objects are induced by long range fluctuations of either classical or quantum origin. Fluctuations of the quantum electrodynamic vacuum epitomize this type of physics, but such fluctuation induced forces arise in a wide range of systems. Here we present a surprisingly never anticipated example of fluctuation induced radiation forces, stemming from spatially long ranged mesoscopic coherent fluctuations of light propagating in random media. Quite remarkably, spatially coherent light fluctuations can be thoroughly described by a hydrodynamic Langevin approach, where a properly tailored noise accounts for mesoscopic coherent effects. The light flow depends on two parameters only, the diffusion coefficient $D$ and the mobility $\sigma$, otherwise related by a Einstein relation. The mapping we present between coherent light flow and out of equilibrium hydrodynamics is easily generalisable to a large class of quantum or classical wave problems. A clear asset of this type of approach is in its dependence upon two parameters only thus making it a candidate to efficient machine learning algorithms. Moreover, the strength of these coherent fluctuating forces depends on a single and easily tunable dimensionless conductance $g$ -- analog of electronic conductance -- which encapsulates both the geometry and the scattering properties of the random medium. Hence coherent multiple light scattering offers setups where fluctuation induced forces are significantly enhanced compared to other known situations. The scarcity of measurable non equilibrium phenomena makes the present proposal particularly relevant to experimental inspections and applications, e.g. a wide variety of sensors in soft condensed matter, biophysics and quantum technologies.