Guglielmo La Magna

Phonon down-conversion by normal metals for superconducting devices

  1. Guglielmo La Magna,
  2. and Gianluigi Catelani
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.
arxiv pdf

Modeling Phonon-mediated Quasiparticle Poisoning in Superconducting Qubit Arrays

  1. Eric Yelton,
  2. Clayton P. Larson,
  3. Vito Iaia,
  4. Kenneth Dodge,
  5. Guglielmo La Magna,
  6. Paul G. Baity,
  7. Ivan V. Pechenezhskiy,
  8. Robert McDermott,
  9. Noah Kurinsky,
  10. Gianluigi Catelani,
  11. and Britton L. T. Plourde
Correlated errors caused by ionizing radiation impacting superconducting qubit chips are problematic for quantum error correction. Such impacts generate quasiparticle (QP) excitations
in the qubit electrodes, which temporarily reduce qubit coherence significantly. The many energetic phonons produced by a particle impact travel efficiently throughout the device substrate and generate quasiparticles with high probability, thus causing errors on a large fraction of the qubits in an array simultaneously. We describe a comprehensive strategy for the numerical simulation of the phonon and quasiparticle dynamics in the aftermath of an impact. We compare the simulations with experimental measurements of phonon-mediated QP poisoning and demonstrate that our modeling captures the spatial and temporal footprint of the QP poisoning for various configurations of phonon downconversion structures. We thus present a path forward for the operation of superconducting quantum processors in the presence of ionizing radiation.
arxiv pdf