  Quantum error correction will be an essential ingredient in realizing
fault-tolerant quantum computing. However, most correction schemes rely on the
assumption that errors are sufficiently uncorrelated in space and time. In
superconducting qubits this assumption is drastically violated in the presence
of ionizing radiation, which creates bursts of high energy phonons in the
substrate. These phonons can break Cooper-pairs in the superconductor and,
thus, create quasiparticles over large areas, consequently reducing qubit
coherence across the quantum device in a correlated fashion. A potential
mitigation technique is to place large volumes of normal or superconducting
metal on the device, capable of reducing the phonon energy to below the
superconducting gap of the qubits. To investigate the effectiveness of this
method we fabricate a quantum device with four nominally identical
nanowire-based transmon qubits. On the device, half of the
niobium-titanium-nitride ground plane is replaced with aluminum (Al), which has
a significantly lower superconducting gap. We deterministically inject high
energy phonons into the substrate by voltage biasing a galvanically isolated
Josephson junction. In the presence of the low gap material, we find a factor
of 2-5 less degradation in the injection-dependent qubit lifetimes, and observe
that undesired excited qubit state population is mitigated by a similar factor.
We furthermore turn the Al normal with a magnetic field, finding no change in
the phonon-protection. This suggests that the efficacy of the protection in our
device is not limited by the size of the superconducting gap in the Al ground
plane. Our results provide a promising foundation for protecting
superconducting qubit processors against correlated errors from ionizing
radiation.
