Nonreciprocal quantum matter
Matteo Brunelli (Collège de France)
Nonreciprocal interactions, in which an agent A attracts another agent B while B repels A, lead to exciting phenomena in classical systems, such as novel phase transitions and time-crystal behavior. Whether such phenomena occur in quantum many-body systems remains to a large extent an open question. In this talk, I will discuss how nonreciprocal interactions can be engineered in interacting quantum systems. I will present a model of two ensembles of driven quantum spins featuring attraction-repulsion type interactions, analogous to predator-prey dynamics. The model can be realized with two atomic ensembles coupled via chiral waveguides. In the thermodynamic limit, nonreciprocal interactions result in a nonreciprocal phase transition into time-crystalline states, associated with spontaneous breaking of parity-time symmetry. Continuous monitoring of the waveguide’s output field also induces a time-crystal state in finite-size systems, where parity-time symmetry is spontaneously broken, and the corresponding quantum trajectories reveal quantum traveling-wave states. I will then show that nonreciprocal interactions arise in a model of superradiant laser in which a fraction of the atoms are not driven, resulting in frequency shift and spectral broadening of the emitted radiation. This represents a fundamental limitation for the operation of super radiant lasers ultra-stable frequency reference.