Measuring the dynamic structure factor of a quantum gas undergoing a structural phase transition

TL;DR

This paper demonstrates a novel, real-time, non-destructive method to measure the dynamic structure factor of a quantum gas undergoing a structural phase transition, using inelastic photon scattering in an optical cavity.

Contribution

It introduces a new technique for measuring density fluctuations and collective excitations in ultracold quantum gases via cavity-enhanced inelastic scattering.

Findings

Successful real-time measurement of density fluctuations.

Observation of quasi-particle mode occupation during phase transition.

Theoretical modeling of the dissipative quantum system.

Abstract

The dynamic structure factor is a central quantity describing the physics of quantum many-body systems, capturing structure and collective excitations of a material. In condensed matter, it can be measured via inelastic neutron scattering, which is an energy-resolving probe for the density fluctuations. In ultracold atoms, a similar approach could so far not be applied due to the diluteness of the system. Here, we report on a direct, real-time and non-destructive measurement of the dynamic structure factor of a quantum gas exhibiting cavity-mediated long-range interactions. The technique relies on inelastic scattering of photons, stimulated by the enhanced vacuum field inside a high finesse optical cavity. We extract the density fluctuations, their energy and lifetime while the system undergoes a structural phase transition. We observe an occupation of the relevant quasi-particle mode on the level of a few excitations, and provide a theoretical description of this dissipative quantum many-body system.