Low-Loss Ferrite Circulator as a Tunable Chiral Quantum System

Ferrite microwave circulators allow one to control the directional flow of microwave signals and noise, and thus play a crucial role in present-day superconducting quantum technology. They are typically viewed as a black box, with their internal structure neither specified nor used as a quantum resource. In this work, we show a low-loss waveguide circulator constructed with single-crystalline yttrium iron garnet in a three-dimensional cavity, and analyze it as a multimode hybrid quantum system with coupled photonic and magnonic excitations. We show the coherent coupling of its chiral internal modes with integrated superconducting niobium cavities, and how this enables tunable nonreciprocal interactions between the intracavity photons. We also probe experimentally the effective non-Hermitian dynamics of this system and its effective nonreciprocal eigenmodes. The device platform provides a test bed for implementing nonreciprocal interactions in open-system circuit QED.