  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.
