Suppression of Phonon-mediated Quasiparticle Poisoning by Silicon Micromachining

Phonon-mediated quasiparticle poisoning associated with particle impacts results in correlated relaxation errors in superconducting qubits that share a common substrate. We describe approaches to suppress these correlated errors based on substrate modification by micromachining. In one approach, we use deep reactive ion etching (deep RIE) from the backside of the substrate to incorporate an array of scattering centers that prevent ballistic phonon propagation. In a separate approach, we use deep RIE to define phonon bottlenecks or moats that decouple individual qubits acoustically from neighboring devices. We use direct quasiparticle injection to characterize the suppression of quasiparticle poisoning in devices incorporating these mitigation approaches, and we compare the rate of correlated errors in these devices to that seen in baseline devices with no mitigation.