Roton-type mode softening in a quantum gas with cavity-mediated long-range interactions

TL;DR

This paper demonstrates how cavity-mediated long-range interactions in a Bose-Einstein condensate cause a roton-like mode softening, leading to a superfluid to supersolid phase transition, confirmed by spectroscopic measurements and ab initio calculations.

Contribution

It provides experimental evidence of roton mode softening in a quantum gas with cavity-mediated long-range interactions, advancing quantum simulation of such systems.

Findings

Observation of mode softening at finite momentum

Agreement between measured spectrum and ab initio calculations

Detection of diverging susceptibility at phase transition

Abstract

Long-range interactions in quantum gases are predicted to give rise to an excitation spectrum of roton character, similar to that observed in superfluid helium. We investigate the excitation spectrum of a Bose-Einstein condensate with cavity-mediated long-range interactions, which couple all particles to each other. Increasing the strength of the interaction leads to a softening of an excitation mode at a finite momentum, preceding a superfluid to supersolid phase transition. We study the mode softening spectroscopically across the phase transition using a variant of Bragg spectroscopy. The measured spectrum is in very good agreement with ab initio calculations and, at the phase transition, a diverging susceptibility is observed. The work paves the way towards quantum simulation of long-range interacting many-body systems.