A Superconducting 180{\deg} Hybrid Ring Coupler for circuit Quantum Electrodynamics

TL;DR

This paper reports the design and fabrication of superconducting 180-degree hybrid ring couplers using niobium films, suitable for circuit quantum electrodynamics experiments, with detailed performance characterization.

Contribution

It introduces a novel superconducting hybrid ring coupler design with specific frequency and bandwidth characteristics for quantum circuits.

Findings

Couplers operate effectively at ~6 GHz with -3.5 dB coupling.

Isolation of at least -15 dB achieved over 2 GHz bandwidth.

Low reflection effects confirmed via laser scanning microscopy.

Abstract

Superconducting circuit quantum electrodynamics experiments with propagating microwaves require devices acting as beam splitters. Using niobium thin films on silicon and sapphire substrates, we fabricated superconducting 180{\deg} microstrip hybrid ring couplers, acting as beam splitters with center frequencies of about 6GHz. For the magnitude of the coupling and isolation we find -3.5+/-0.5dB and at least -15dB, respectively, in a bandwidth of 2GHz. We also investigate the effect of reflections at the superconductor-normal conductor contact by means of low temperature laser scanning microscopy. Our measurements show that our hybrid rings are well suited for on-chip applications in circuit quantum electrodynamics experiments.