Improving Superconducting Resonators in Magnetic Fields by Reduced Field Focussing and Engineered Flux Screening

Abstract

We experimentally investigate superconducting coplanar waveguide resonators in external magnetic fields and present two strategies to reduce field-induced dissipation channels and resonance frequency shifts. One of our approaches is to significantly reduce the superconducting ground-plane areas, which leads to reduced magnetic field-focussing and thus to lower effective magnetic fields inside the waveguide cavity. By this measure, the field-induced losses can be reduced by more than one order of magnitude in mT out-of-plane magnetic fields. When these resonators are additionally coupled inductively instead of capacitively to the microwave feedlines, an intrinsic closed superconducting loop is effectively shielding the heart of the resonator from magnetic fields by means of flux conservation. In total, we achieve a reduction of the field-induced resonance frequency shift by up to two orders of magnitude. We combine systematic parameter variations on the experimental side with numerical magnetic field calculations to explain the effects of our approaches and to support our conclusions. The presented results are relevant for all areas, where high-performance superconducting resonators need to be operated in magnetic fields, e.g. for quantum hybrid devices with superconducting circuits or electron spin resonance detectors based on coplanar waveguide cavities.

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“…Significant flux focusing from superconducting ground planes was recently investigated theoretically and experimentally in Ref. [35], where simulations gave F ≈ 27.5. The extent of flux focusing is specific to the device geometry; for example, features designed to trap flux can have large effects on F.…”

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“…Moreover, shielding may render devices useless in quantum sensing applications in which the purpose of the device is to sense the external environment. This has sparked research on further modifications to the device design and on understanding the effects of a magnetic field on different architectures of quantum circuits, including transmon qubits 156 and superconducting resonators 151,157 , which are integral components to readout.…”

“…However, Chiaro et al 160 showed that, without careful design, these features can increase the density of and subsequently losses from parasitic two-level fluctuators, thought to primarily form at surfaces and interfaces. Moreover, coplanar waveguide resonators were recently found to be more robust to external magnetic fields when the superconducting ground plane area is reduced, which lowers the effective magnetic field inside the cavity, and by coupling the resonator inductively instead of capacitively to the microwave feedline, shielding the feedline 157 .…”

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