Group Seminar at MPQ: Quantum many-body physics in a confocal cavity

April 25, 2023

Ronen Kroeze, Stanford
Group Seminar at MPQ lecture hall and zoom
Tuesday, April 25th, 09:00am (MEZ)


Optical cavity QED provides a versatile platform with which to explore quantum many-body physics in driven-dissipative systems. Multimode cavities are particularly useful for exploring beyond mean-field physics. In particular, confocal cavities host both local as well as non-local photon-mediated interactions. First, we demonstrate how the local interaction can be used to create an optical lattice with sound. Traditional optical lattices are silent because they are infinitely stiff, and hence lack the physics of lattice phonons and elasticity that play an integral part in determining materials’ thermodynamic properties and electrical response. We reveal the phonon dispersion relation through dynamical susceptibility measurements, showing that these collective excitations exhibit a sound speed dependent on the BEC–photon coupling strength. These results pave the way for exploring the rich physics of elasticity in quantum solids, ranging from quantum liquid crystalline structure formation to Peierls instabilities. Second, we employ the non-local, sign-changing interaction to program fully connected interaction graphs between multiple BECs located inside the cavity. This realizes an anisotropic XY-model using the density wave phase of each BEC as quasi-spin degree of freedom.  The effective model hosts ferromagnetic and spin glass phases. By using the confocal cavity as an active quantum gas microscope, where light simultaneously mediates the interaction and images the spin state, we microscopically study these magnetic phases, focusing on the replica symmetry breaking that underlies spin glasses.

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