Source of residual nonequilibrium quasiparticles in superconducting qubits
In plain words
At the millikelvin temperatures where qubits run, essentially no electron pairs should be broken, yet broken pairs (quasiparticles) are found at densities vastly above thermal expectation, and they cause errors. Which sources remain once stray light and radiation are blocked is unknown.
Precise statement
In Al-based qubits at $T \sim 20\,\mathrm{mK}$, the thermal quasiparticle fraction $x_{qp} \sim \sqrt{2 \pi T/\Delta}\,\exp(-\Delta/T)$ is below $10^{-40}$, while measured $x_{qp}$ ranges from $\sim 10^{-11}$ to $10^{-5}$. Known sources are stray infrared photons, ionizing radiation (gamma rays, cosmic-ray muons) and phonon bursts from stress relaxation (Mannila et al. 2022). With infrared shielding, radiation shielding and gap engineering in place, determine the residual $x_{qp}$ and charge-parity switching rate and identify the remaining generation mechanism.
What would settle it
A device in which each candidate source is independently suppressed or enhanced (shielding, underground operation, stress control, controlled irradiation) and the residual parity-switching rate is accounted for quantitatively.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
With multilayer infrared shielding the parity-switching rate fell by four orders of magnitude to $0.069\ \mathrm{Hz}$, an inferred $x_{\mathrm{qp}} \sim 1.9 \times 10^{-11}$ (Lin et al., arXiv 2606.07339, 2026); the residual source in shielded devices is unidentified.
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