  Nonequilibrium quasiparticles represent a significant source of decoherence
in superconducting quantum circuits. Here we investigate the mechanism of
quasiparticle poisoning in devices subjected to local quasiparticle injection.
We find that quasiparticle poisoning is dominated by the propagation of
pair-breaking phonons across the chip. We characterize the energy dependence of
the timescale for quasiparticle poisoning. Finally, we observe that
incorporation of extensive normal metal quasiparticle traps leads to a more
than order of magnitude reduction in quasiparticle loss for a given injected
quasiparticle power.
