Fano-enhanced low-loss on-chip superconducting microwave circulator

TL;DR

This paper presents an improved on-chip superconducting microwave circulator that uses Fano interference to achieve high fidelity and low loss, relaxing fabrication tolerances compared to previous designs.

Contribution

The authors introduce capacitive couplings to enhance Fano interference, improving device performance and fabrication robustness of superconducting microwave circulators.

Findings

Circulation fidelity above 97% achieved.

On-resonance insertion loss of 0.2 dB demonstrated.

Isolation of 18 dB and power reflectance of -15 dB measured.

Abstract

Ferrite-free circulators that are passive and readily integratable on a chip are highly sought-after in quantum technologies based on superconducting circuits. In our previous work, we implemented such a circulator using a three-Josephson-junction loop that exhibited unambiguous nonreciprocity and signal circulation, but required junction energies to be within $1\%$ of design values. This tolerance is tighter than standard junction fabrication methods provide, so we propose and demonstrate a design improvement that relaxes the required junction fabrication precision, allowing for higher device performance and fabrication yield. Specifically, we introduce large direct capacitive couplings between the waveguides to create strong Fano scattering interference. We measure enhanced `circulation fidelity' above $97\%$, with optimised on-resonance insertion loss of $0.2$~dB, isolation of $18$~dB, and power reflectance of $-15$~dB, in good agreement with model calculations.