Time dependence of the rate of catastrophic bursts of globally suppressed relaxation time in a 5-qubit superconducting processor.

We report the observation of intriguing time dependence in the incidence of bursts of globally suppressed qubit relaxation times on the Starmon-5 processor (available via the Quantum Inspire platform [1]), presumably caused by cosmic-ray impacts. Despite the low qubit count, we use time-domain filters to clearly discern real bursts from false positives caused by baseline qubit relaxation and readout error. Immediately following cooldown, we detect real bursts lasting ~1.5 ms, at steady rates of 1/84 and 1/60 s^-1 with and without a lead shield, respectively, consistent with a cosmic-ray origin. Curiously, a few weeks following cooldown, the burst rate suddenly increases fivefold and remains so over a ~12-hour window. During this window, a fraction of bursts show significantly longer recovery time and also the baseline qubit relaxation time is reduced. Following this window, the burst rate decreases over ~12 hours to a value two orders of magnitude lower than immediately following cooldown. We have reliably observed this effect over multiple thermal cycles to 4 K and 80 K.

[1] quantum-inspire.com