# Saving superconducting quantum processors from qubit decay and correlated errors generated by gamma and cosmic rays

**Authors:** John M. Martinis  
**arXiv:** [2012.06137](https://arxiv.org/abs/2012.06137)  
**Date:** 2020-12-11  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2012.06137v3/

## Key points

- Effects of cosmic rays and background gamma ray radiation on superconducting qubits in quantum computing systems
- Impact of these sources on qubit state decay and quantum error correction
- Introduction of a quantitative model describing phonon down-conversion to quasiparticles
- Prediction that a large portion of radiation energy is converted into phonons, breaking Cooper pairs and reducing qubit energy decay time
- Detrimental characteristics of quasiparticles for complex quantum computations
- Proposal to redesign quantum processors by channeling phonon energy away from qubits using thick films of normal metal or low-gap superconductors
- Aim to reduce initial quasiparticle density by a factor of 100 for substantial improvement over previous experiments with thin films
- Discussion on complementary research about radiation physics and electron-hole pair breaking in silicon crystals as part of the down-conversion process
- Importance of focusing on redesigning quantum processors for advancing error correction capabilities in quantum computing systems

## Summary

In this paper, the author explores the effects of cosmic rays and background gamma ray radiation on superconducting qubits in quantum computing systems. Previous studies have observed these sources of pulsed energy but their impact has been relatively small compared to other decoherence mechanisms. However, the author argues that on a per-event basis, these effects can be substantial and correlated in both area and time, posing a significant challenge to quantum error correction. To address this issue, a quantitative model is introduced that describes the process of phonon down-conversion to quasiparticles which leads to qubit state decay. The model predicts that a large portion of radiation energy is converted into phonons with 57% breaking Cooper pairs and significantly reducing qubit energy decay time. These quasiparticles have characteristics such as large spatial extent and long duration which make them detrimental to complex quantum computations. To mitigate their impact, the author proposes redesigning quantum processors by channeling phonon energy away from the qubits using thick films of normal metal or low-gap superconductors. This redesign aims to reduce initial quasiparticle density by a factor of 100, offering a substantial improvement over previous experiments with thin films. Additionally, complementary research on radiation physics and electron-hole pair breaking in silicon crystals as part of the down-conversion process is discussed. While strategies like using low-radiation materials or operating underground may seem appealing for mitigation purposes, they may not effectively reduce error rates to an acceptable level. Therefore, focusing on redesigning quantum processors is crucial for advancing error correction capabilities in quantum computing systems.