Examining phonon mediated quasiparticle poisoning in superconducting qubits through device suspension

Correlated errors caused by nonequilibrium quasiparticles in superconducting qubits remains a concern in efforts to achieve fault tolerant quantum computing. These errors can be mediated by phonon propagation in the substrate, but it remains uncertain where phonons principally generate the responsible quasiparticles in the qubit. Here, we explore this question by suspending the Josephson junction in transmon qubits above the silicon substrate, decoupling the junction itself from the bulk substrate. We compare quasiparticle tunneling rates across multiple transmons with suspended and non-suspended junctions to investigate the significance of the junction-substrate interface relative to components such as the qubit paddles or leads where phonons may also generate quasiparticles.