Reducing phonon backgrounds for quantum computing and cryogenic calorimeters

Superconducting qubits suffer from decoherence caused by phonons in the substrate with energies higher than the superconducting bandgap. While radiation backgrounds and cosmic rays produce large bursts of athermal phonons that causes catastrophic whole-chip failures, other sources, such as stress in films and chip packaging, produce lower energy phonon bursts more frequently. Cryogenic calorimetric sensors have achieved similar sensitivities to these phonon events. Specifically, in transition edge sensor (TES) based dark matter detectors, these events are the limiting background for low-mass dark matter searches. 

There is a strong synergy between detector R&D and quantum computing. I will discuss our efforts to reduce the phonon background by reducing stress in our detectors. The same ideas apply to quantum computing; in addition, qubits can be cloaked from phonons with suspending structures and phonon-reflective coatings. We use superconducting resonators to study the effectiveness of these methods.