  The performance of superconducting qubits is degraded by a poorly
characterized set of energy sources breaking the Cooper pairs responsible for
superconductivity, creating a condition often called "quasiparticle poisoning."
Recently, a superconductor with one of the lowest average quasiparticle
densities ever measured exhibited quasiparticles primarily produced in bursts
which decreased in rate with time after cooldown. Similarly, several cryogenic
calorimeters used to search for dark matter have also observed an unknown
source of low-energy phonon bursts that decrease in rate with time after
cooldown. Here, we show that a silicon crystal glued to its holder exhibits a
rate of low-energy phonon events that is more than two orders of magnitude
larger than in a functionally identical crystal suspended from its holder in a
low-stress state. The excess phonon event rate in the glued crystal decreases
with time since cooldown, consistent with a source of phonon bursts which
contributes to quasiparticle poisoning in quantum circuits and the low-energy
events observed in cryogenic calorimeters. We argue that relaxation of
thermally induced stress between the glue and crystal is the source of these
events, and conclude that stress relaxation contributes to quasiparticle
poisoning in superconducting qubits and the athermal phonon background in a
broad class of rare-event searches.
