Propagation velocity measurements of substrate phonon bursts using MKIDs for superconducting circuits

TL;DR

This study measures the propagation velocity of substrate phonon bursts in superconducting circuits using MKIDs, revealing how high-energy radiation affects error propagation and providing a model to estimate phonon velocities.

Contribution

It introduces a method using MKID arrays to measure substrate phonon burst velocities and models their propagation in superconducting circuits.

Findings

Propagation velocity decreases with increased NIS power

A simple model estimates phonon velocity and conversion efficiency

Reduced phonon flux impacts the apparent propagation speed

Abstract

High-energy bursts in superconducting quantum circuits from various radiation sources have recently become a practical concern due to induced errors and their propagation in the chip. The speed and distance of these disturbances have practical implications. We used a linear array of multiplexed MKIDs on a single silicon chip to measure the propagation velocity of a localized high-energy burst, introduced by driving a Normal metal- Insulator-Superconductor (NIS) junction. We observed a reduction in the apparent propagation velocity with NIS power, which is due to the combined effect of reduced phonon flux with distance and the existence of a minimum detectable QP density in the MKIDs. A simple theoretical model is fitted to extract the longitudinal phonon velocity in the substrate and the conversion efficiency of phonons to QPs in the superconductor.