Impact of ionizing radiation on superconducting qubit coherence (2001.09190v2)

Abstract: The practical viability of any qubit technology stands on long coherence times and high-fidelity operations, with the superconducting qubit modality being a leading example. However, superconducting qubit coherence is impacted by broken Cooper pairs, referred to as quasiparticles, with a density that is empirically observed to be orders of magnitude greater than the value predicted for thermal equilibrium by the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. Previous work has shown that infrared photons significantly increase the quasiparticle density, yet even in the best isolated systems, it still remains higher than expected, suggesting that another generation mechanism exists. In this Letter, we provide evidence that ionizing radiation from environmental radioactive materials and cosmic rays contributes to this observed difference, leading to an elevated quasiparticle density that would ultimately limit superconducting qubits of the type measured here to coherence times in the millisecond regime. We further demonstrate that introducing radiation shielding reduces the flux of ionizing radiation and positively correlates with increased coherence time. Albeit a small effect for today's qubits, reducing or otherwise mitigating the impact of ionizing radiation will be critical for realizing fault-tolerant superconducting quantum computers.

Impact of Ionizing Radiation on Superconducting Qubit Coherence

This paper investigates the deleterious effects of ionizing radiation on the coherence of superconducting qubits, a leading candidate for quantum computing technologies. The paper primarily focuses on understanding the link between environmental ionizing radiation and quasiparticle-induced decoherence in superconducting qubits, an issue that impacts the fidelity and scalability of quantum operations.

Background and Motivation

Superconducting qubits are at the forefront of the race towards practical quantum computing due to their compatibility with existing semiconductor technologies and their potential for scalability. However, coherence times—an essential parameter for quantum computation—are limited by various decoherence mechanisms, including quasiparticle poisoning. The density of quasiparticles observed experimentally in superconducting qubits significantly exceeds predictions based on equilibrium Bardeen-Cooper-Schrieffer (BCS) theory. Previous studies have demonstrated the contributions of infrared photons to quasiparticle productions, but even well-isolated systems exhibit higher-than-expected quasiparticle densities, prompting the hypothesis of alternative generation mechanisms.

Methodology

To elucidate the influence of ionizing radiation, the authors conducted experiments using state-of-the-art transmon qubits. They subjected these qubits to ionizing radiation from a copper-64 ($^64$Cu) source and measured the resultant quasiparticle density and its impact on qubit coherence. They complemented these experiments with radiation transport simulations to quantify radiation-induced power densities.

The experiments involved systematic measurement of the energy-relaxation rates ($T_1$) of qubits under varying radiation conditions. This empirical data was compared to theoretical models to estimate the correlation between radiation intensity and quasiparticle generation.

Key Findings

1. Radiation-Induced Quasiparticle Density: The paper provided quantitative evidence linking ionizing radiation levels in laboratory environments to increased quasiparticle densities in superconducting materials. This correlation suggests that ionizing radiation is a significant factor in the previously unexplained excess quasiparticle presence.
2. Impact on Qubit Coherence Times: The authors found that the introduction of radiation shielding improved coherence times ($T_1$) by reducing the flux of ionizing radiation, affirming the potential of radiation mitigation techniques.
3. Energy-Relaxation Rate Due to Quasiparticles: The paper estimates a lower bound on the energy-relaxation rate solely due to atmospheric radiation, predicting it could limit coherence times to several milliseconds in the absence of other noise sources.

Implications and Future Directions

The findings underscore the necessity to incorporate radiation shielding and enhanced material design in the development of fault-tolerant quantum processors. Achieving the millisecond coherence times needed for scalable quantum computation necessitates minimizing quasiparticle interference, particularly from environmental radiation.

The paper suggests several avenues for future work:

• Exploring alternative qubit designs that are inherently less susceptible to quasiparticle impacts.
• Investigating the effects of deeper underground or specially shielded environments to further mitigate radiation effects.
• Developing improved materials and fabrication techniques that intrinsically reduce the effect of quasiparticles.

In conclusion, the paper makes a compelling case for considering environmental radiation in the design and deployment of superconducting qubit-based technologies. While this paper provides vital insights, continued research is essential for overcoming the decoherence challenges posed by ionizing radiation, thereby advancing the quest for fault-tolerant, scalable quantum computing systems.