NbSi nanowire quantum-phase-slip circuits: dc supercurrent blockade, microwave measurements and thermal analysis

TL;DR

This paper investigates quantum phase-slip phenomena in NbSi nanowire circuits through microwave experiments and thermal analysis, highlighting challenges in observing dual Shapiro steps due to elevated electron temperatures and Johnson noise.

Contribution

It provides a detailed experimental and theoretical study of QPS in NbSi nanowires, including a thermal model and design considerations for future quantum current standards.

Findings

No dual Shapiro steps observed due to thermal noise

Elevated electron temperature caused by resistors washed out expected signals

A numerical model informs future device design and measurement strategies

Abstract

We present a detailed report of microwave irradiation of ultra-narrow superconducting nanowires. In our nanofabricated circuits containing a superconducting NbSi nanowire, a dc blockade of current flow was observed at low temperatures below a critical voltage Vc, a strong indicator of the existence of quantum phase-slip (QPS) in the nanowire. We describe the results of applying microwaves to these samples, using a range of frequencies and both continuous-wave and pulsed drive, in order to search for dual Shapiro steps which would constitute an unambiguous demonstration of quantum phase-slip. We observed no steps, and our subsequent thermal analysis suggests that the electron temperature in the series CrO resistors was significantly elevated above the substrate temperature, resulting in sufficient Johnson noise to wash out the steps. To understand the system and inform future work, we have constructed a numerical model of the dynamics of the circuit for dc and ac bias (both continuous wave and pulsed drive signals) in the presence of Johnson noise. Using this model, we outline important design considerations for device and measurement parameters which should be used in any future experiment to enable the observation of dual Shapiro steps at experimentally accessible temperatures and thus lead to the development of a QPS-based quantum current standard.