Mitigation of quasiparticle poisoning in superconducting qubits using normal metal backside metallization

The absorption of high-energy particles, such as gammas and cosmic ray muons, in superconducting qubit chips generates pair-breaking phonons that can travel long distances and generate quasiparticles over significant areas of a device. The quasiparticle poisoning from such events can lead to correlated errors between distant qubits, resulting in the breakdown of error-correction schemes, such as the surface code. Therefore, it is critical to develop strategies for mitigating such quasiparticle poisoning to protect large qubit systems from such errors. We have fabricated devices with normal metal phonon absorbers on the opposite face of the chip from an array of charge-sensitive transmon qubits. We present measurements of devices with and without this backside metallization. We demonstrate the effectiveness of the phonon absorbers on the mitigation of quasiparticle poisoning by measuring the qubit behavior in the presence of direct quasiparticle injection by biasing on-chip Josephson tunnel junctions in both pulsed and continuous modes.