Nonlinear mechanisms in Al and Ti superconducting travelling-wave parametric amplifiers

TL;DR

This paper investigates the nonlinear mechanisms in superconducting Al and Ti TWPAs, characterizing their nonlinearities and demonstrating high gain over narrow bandwidths, with implications for various superconducting devices.

Contribution

It introduces a method to characterize superconducting nonlinearities based on dissipative-reactive ratios and response times, applied to Al and Ti TWPAs.

Findings

Measured nonlinearities align with non-equilibrium heating predictions.

Achieved over 20 dB peak power gain in TWPAs.

Identified potential for broader application in superconducting devices.

Abstract

The underlying nonlinear mechanisms behind the operation of travelling-wave parametric amplifiers (TWPAs) are important in determining their performance in terms of added noise, maximum gain, and bandwidth. We describe a method of characterising the underlying nonlinearity of a superconducting material in terms of its dissipative-reactive ratio and the response time of the underlying microscopic processes. We describe and calculate the different behaviour arising from the equilibrium supercurrent nonlinearity, which has low dissipation and fast response time, and the non-equilibrium heating nonlinearity, which has high dissipation and slow response time. We have fabricated TWPAs based on Al and Ti, and characterised their nonlinearities using our analysis. For both Al and Ti, the measured dissipative-reactive ratios and response times are quantitatively similar to predictions for the non-equilibrium heating nonlinearity. We were able to obtain more than 20 dB of peak power gain, although only over a narrow bandwidth of a few kilohertz. Our method of characterising the underlying nonlinearities could also be useful in the understanding and design of other superconducting nonlinear devices such as parametric up-converters, kinetic inductance Fourier transform spectrometers, and resonator parametric amplifiers.