An optical lattice with sound

Yudan Guo; Ronen M Kroeze; Brendan P Marsh; Sarang Gopalakrishnan; Jonathan Keeling; Benjamin L Lev

doi:10.1038/s41586-021-03945-x

An optical lattice with sound

Nature. 2021 Nov;599(7884):211-215. doi: 10.1038/s41586-021-03945-x. Epub 2021 Nov 10.

Authors

Yudan Guo^{1

2}, Ronen M Kroeze^{1

2}, Brendan P Marsh^{2

3}, Sarang Gopalakrishnan⁴, Jonathan Keeling⁵, Benjamin L Lev^{6

7

8}

Affiliations

¹ Department of Physics, Stanford University, Stanford, CA, USA.
² E. L. Ginzton Laboratory, Stanford University, Stanford, CA, USA.
³ Department of Applied Physics, Stanford University, Stanford, CA, USA.
⁴ Department of Physics, The Pennsylvania State University, University Park, PA, USA.
⁵ SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews, UK.
⁶ Department of Physics, Stanford University, Stanford, CA, USA. benlev@stanford.edu.
⁷ E. L. Ginzton Laboratory, Stanford University, Stanford, CA, USA. benlev@stanford.edu.
⁸ Department of Applied Physics, Stanford University, Stanford, CA, USA. benlev@stanford.edu.

PMID: 34759361
DOI: 10.1038/s41586-021-03945-x

Abstract

Quantized sound waves-phonons-govern the elastic response of crystalline materials, and also play an integral part in determining their thermodynamic properties and electrical response (for example, by binding electrons into superconducting Cooper pairs)^1-3. 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 stiff⁴. Optical-lattice realizations 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 crystallization 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 transitions⁵ to exotic 'fractonic' topological defects⁶ in the quantum regime.