Benjamin Lev
Phonons, quantized sound waves in crystals, play an integral role in the properties of materials. These lattice vibrations determine a crystal's heat capacity and thermal and electrical conductivity. By binding electrons together, they induce the superconductivity common to simple metals at low temperature. Despite their ubiquity, however, phonons are absent in simulators of quantum materials constructed of neutral atoms and light: unlike real solids, traditional optical lattices are silent because they are perfectly rigid. Adding sound to the toolbox of quantum simulation would allow the emulation of a wider range of quantum materials. Here, we create an optical lattice with phonon modes by scattering photons off a BEC coupled to a confocal optical resonator. Playing the role of an active quantum gas microscope, the multimode cavity QED system both images phonons imprinted onto the BEC and induces the phonons arising from photon-mediated, momentum-exchanging atom-atom interactions. Dynamical susceptibility measurements reveal the Goldstone mode dispersion relation of transverse-oscillating phonons. These collective excitations exhibit a speed of sound dependent on the BEC-photon coupling strength. The compliant optical lattice provides access to quantum many-body physics where phonons play a critical role, such as Peierls transitions and polarons, or where phonon-bath engineering may lead to novel nonequilibrium phases of matter.