# Strong optical coupling through superfluid Brillouin lasing

## TL;DR

This paper demonstrates ultralow threshold Brillouin lasing and strong phonon-mediated optical coupling in superfluid helium, enabling advanced applications in sensing, quantum information, and fluid dynamics exploration.

## Contribution

It introduces a novel optically-induced surface deformation mechanism in liquids that surpasses electrostrictive interactions, enabling strong optical coupling and lasing in superfluid helium.

## Key findings

- Achieved ultralow threshold Brillouin lasing in superfluid helium.
- Demonstrated strong phonon-mediated optical coupling.
- Proposed superfluid Brillouin gyroscope for precise superfluid circulation measurement.

## Abstract

Brillouin scattering has applications ranging from signal processing, sensing and microscopy, to quantum information and fundamental science. Most of these applications rely on the electrostrictive interaction between light and phonons. Here we show that in liquids optically-induced surface deformations can provide an alternative and far stronger interaction. This allows the demonstration of ultralow threshold Brillouin lasing and strong phonon-mediated optical coupling for the first time. This form of strong coupling is a key capability for Brillouin-reconfigurable optical switches and circuits, for photonic quantum interfaces, and to generate synthetic electromagnetic fields. While applicable to liquids quite generally, our demonstration uses superfluid helium. Configured as a Brillouin gyroscope this provides the prospect of measuring superfluid circulation with unprecedented precision, and to explore the rich physics of quantum fluid dynamics, from quantized vorticity to quantum turbulence.