  Superconducting qubits are an attractive platform for quantum computing since
they have demonstrated high-fidelity quantum gates and extensibility to modest
system sizes. Nonetheless, an outstanding challenge is stabilizing their
energy-relaxation times, which can fluctuate unpredictably in frequency and
time. Here, we use qubits as spectral and temporal probes of individual
two-level-system defects to provide direct evidence that they are responsible
for the largest fluctuations. This research lays the foundation for stabilizing
qubit performance through calibration, design, and fabrication.
