Phonon down-conversion by normal metals for superconducting devices
arXiv:2607.13870
The paper investigates how a normal‑metal film can down‑convert high‑energy, pair‑breaking phonons to mitigate quasiparticle poisoning in superconducting devices, using analytical and numerical solutions of kinetic equations to evaluate the conversion efficiency.
Abstract
Thanks to low dissipation, superconducting devices are promising for a number of applications, such as detectors and implementations of quantum computation. However, their working can be adversely impacted by quasiparticles, which is why so-called quasiparticle poisoning mechanisms and their mitigation are under intense investigation. Here we focus on one poisoning mechanism, namely pair-breaking phonons, and its mitigation through down-conversion by a normal-metal film - the process in which scattering of high-energy phonons by electrons lowers the energy of the former below the pair-breaking threshold. To study the down-conversion, we introduce a model based on kinetic equations, which we solve both analytically (approximately) and numerically in the steady state. We use the solution the estimate a properly-defined down-conversion efficiency which depends on material parameters (such as the strength of electron-phonon interaction and the phonon transmission coefficient at interfaces) and film and substrate thicknesses. Interestingly, we find that the efficiency is nearly optimal over a finite range of metal thicknesses, with the minimum near-optimal thickness being typically of the order of a micron.
20 pages, 9 figures