Reduction of correlated errors in superconducting qubits using normal metal back-side metallization

The impact of high-energy particles, such as gamma rays and cosmic ray muons, in superconducting qubit chips generates pair-breaking phonons that can travel long distances generating excitations above the superconducting ground state, known as quasiparticles, leading to correlated errors across many qubits. These correlated errors between distant qubits are fatal to error-correction schemes, such as the surface code. Therefore, it is critical to develop strategies for mitigating quasiparticle poisoning to protect large qubit systems from such errors. We have fabricated devices with normal metal reservoirs for phonon downconversion on the opposite face of the chip from an array of charge-sensitive transmon qubits. We present measurements of devices with and without this back-side metallization. We utilize a pump-probe injection technique of pair-breaking phonons in the device to examine the influence of back-side metallization compared to the control device. We demonstrate the effectiveness of the phonon downconversion by measuring a factor of 20 decrease in the flux of injected pair-breaking phonons. In addition, we observe a two-order of magnitude reduction in correlated poisoning due to background radiation.