  The presence of quasiparticles in superconducting qubits emerges as an
intrinsic constraint on their coherence. While it is difficult to prevent the
generation of quasiparticles, keeping them away from active elements of the
qubit provides a viable way of improving the device performance. Here we
develop theoretically and validate experimentally a model for the effect of a
single small trap on the dynamics of the excess quasiparticles injected in a
transmon-type qubit. The model allows one to evaluate the time it takes to
evacuate the injected quasiparticles from the transmon as a function of trap
parameters. With the increase of the trap size, this time decreases
monotonically, saturating at the level determined by the quasiparticles
diffusion constant and the qubit geometry. We determine the characteristic trap
size needed for the relaxation time to approach that saturation value.
