  Quantum error correction can preserve quantum information in the presence of
local errors, but correlated errors are fatal. For superconducting qubits,
high-energy particle impacts from background radioactivity produce energetic
phonons that travel throughout the substrate and create excitations above the
superconducting ground state, known as quasiparticles, which can poison all
qubits on the chip. We use normal metal reservoirs on the chip back side to
downconvert phonons to low energies where they can no longer poison qubits. We
introduce a pump-probe scheme involving controlled injection of pair-breaking
phonons into the qubit chips. We examine quasiparticle poisoning on chips with
and without back-side metallization and demonstrate a reduction in the flux of
pair-breaking phonons by over a factor of 20. We use a Ramsey interferometer
scheme to simultaneously monitor quasiparticle parity on three qubits for each
chip and observe a two-order of magnitude reduction in correlated poisoning due
to background radiation.
