An optical lattice with sound

TL;DR

This paper demonstrates the creation of an optical lattice with phonon modes using a Bose-Einstein condensate coupled to a multimode optical resonator, enabling the study of elasticity and sound in quantum solids.

Contribution

It introduces a novel optical lattice system that supports phonons, combining cavity QED with BECs to simulate dynamical degrees of freedom absent in traditional optical lattices.

Findings

Phonon dispersion relation measured via dynamical susceptibility.

Sound speed depends on BEC-photon coupling strength.

System enables exploration of elasticity and quantum melting in quantum solids.

Abstract

Quantised sound waves -- phonons -- govern the elastic response of crystalline materials, and also play an integral part in determining their thermodynamic properties and electrical response (e.g., by binding electrons into superconducting Cooper pairs). The physics of lattice phonons and elasticity is absent in simulators of quantum solids constructed of neutral atoms in periodic light potentials: unlike real solids, traditional optical lattices are silent because they are infinitely stiff. Optical-lattice realisations of crystals therefore lack some of the central dynamical degrees of freedom that determine the low-temperature properties of real materials. Here, we create an optical lattice with phonon modes using a Bose-Einstein condensate (BEC) coupled to a confocal optical resonator. Playing the role of an active quantum gas microscope, the multimode cavity QED system both images the phonons and induces the crystallisation that supports phonons via short-range, photon-mediated atom-atom interactions. Dynamical susceptibility measurements reveal the phonon dispersion relation, showing that these collective excitations exhibit a sound speed dependent on the BEC-photon coupling strength. Our results pave the way for exploring the rich physics of elasticity in quantum solids, ranging from quantum melting transitions to exotic ``fractonic'' topological defects in the quantum regime.