An optical lattice with sound

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) [1][2][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 stiff [4]. 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 shortrange, 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 [5] to exotic "fractonic" topological defects [6] in the quantum regime.

Dipolar physics: A review of experiments with magnetic quantum gases

“…More recently, a continuous U (1) translational symmetry was created in system of multimode light coupled to a BEC. The breaking of this symmetry resulted in a supersolid with vibrations exhibiting a Goldstone dispersion of acoustic phonon excitations [498]. We will now discuss the simultaneous quest for a supersolid supporting crystal phonon excitations like a real solid in a dipolar gas.…”

Holographic supersolids

“…The questions of whether such a system could exist, how the superflow could appear in a solid and where a supersolid could be experimentally observed have occupied the minds of several physicists in the past seventy years [9][10][11][12][13] since the original observation by O. Penrose and Onsager [14]. Despite the tremendous theoretical progress, supersolid systems have eluded clear detection for the longest time, and it was in recent times only that they have finally been observed in laboratories: historically, supersolid behavior has been first looked for in solid helium [15][16][17][18][19][20], while in the last few years advancements in the field of ultracold atom gases provided some clear signatures of it [21][22][23]. Nonetheless, a complete understanding of this phenomenon is still lacking.…”

Holographic Supersolids

“…The questions of whether such a system could exist, how the superflow could appear in a solid and where a supersolid could be experimentally observed have occupied the minds of several physicists in the past seventy years [9][10][11][12][13] since the original observation by Penrose and Onsager [14]. Despite the tremendous theoretical progress, supersolid systems have eluded clear detection for the longest time, and it was in recent times only that they have finally been observed in laboratories: historically, supersolid behavior has been first looked for in solid helium [15][16][17][18][19][20], while in the last few years advancements in the field of ultracold atom gases provided some other clear signatures [21][22][23]. Nonetheless, a complete understanding of this phenomenon is still lacking.…”