Phonon-targeted quasiparticle poisoning mitigation in superconducting circuits

Superconducting devices are susceptible to chip-wide, correlated quasiparticle poisoning events mediated by non-equilibrium phonons with energies exceeding the superconducting gap. Pair-breaking phonons are primarily excited in the device substrate by the absorption of cosmic rays or particle radiation and are benign until they enter the superconductor via interfacial transmission. We evaluate a quasiparticle poisoning mitigation strategy that seeks to thermally isolate the device substrate from the superconducting thin film. We report the fabrication of superconducting resonators integrating the thermal isolation, and present device measurements from Fermilab's QUIET laboratory, an underground and radiation-controlled environment.