Low-loss ferrite waveguide circulator for non-reciprocal circuit QED

In a quantum information processing network, it is highly desirable to have low-loss directional transmission channels with circulators/isolators to control the information flow while maintaining high fidelity. Moreover, such directional channels can be used to mediate non-reciprocal interactions between qubits or cavities to stabilize entanglement or novel many-body states. Here, using single crystalline yttrium iron garnet (YIG) in a waveguide-based package, we realize a microwave circulator with insertion loss of ~1% and isolation of >20 dB in the quantum regime (at single-photon power and 20 mK temperature). The circulator can be converted into a multi-mode cavity, which enables detailed study of hybridized magnon-photon spectra and the internal loss of various modes. The circulator operates at a relatively low magnetic field of 25 mT (significantly below saturation magnetization of YIG), allowing for direct coupling to transmons qubits and superconducting cavities under proper magnetic shielding. We will describe our ongoing progress towards a 3D circuit QED platform with non-reciprocal qubit-qubit interactions.