Realization of a Quantum-Optical Spin Glass

Abstract: Spin glasses—large-scale networks of spins with deeply frustrated interactions—are canonical examples of complex matter. Although much about their structure remains uncertain, they inform the description of a wide array of complex phenomena, ranging from magnetic ordering in metals with impurities to aspects of evolution, protein folding, climate models, and combinatorial optimization. Indeed, spin glass theory forms a mathematical basis for neuromorphic computing and brain modeling. Advancing experimental insight into their structure requires repeatable control over microscopic degrees of freedom. Here, we achieve this at the atomic level using a quantum-optical system comprised of ultracold gases of Rb atoms coupled via photons resonating within a multimode optical cavity. The network of atomic ensembles scatter light from a transverse pump laser into the cavity modes to realize an unusual type of transverse-field spin glass with all-to-all connectivity. A superradiant phase transition occurs upon reaching a critical pump strength, concomitant with spin glass ordering. Spin configurations are observed in the superradiant cavity emission and reveal the emergence of replica symmetry breaking and nascent ultrametric structure as signatures of spin-glass order. The controllability provided by this new spin-glass system, potentially down to the quantum-spin-level, enables the study of spin-glass physics in novel regimes with application to quantum neural network computing.