  Identifying, quantifying, and suppressing decoherence mechanisms in qubits
are important steps towards the goal of engineering a quantum computer or
simulator. Superconducting circuits offer flexibility in qubit design; however,
their performance is adversely affected by quasiparticles (broken Cooper
pairs). Developing a quasiparticle mitigation strategy compatible with
scalable, high-coherence devices is therefore highly desirable. Here we
experimentally demonstrate how to control quasiparticle generation by
downsizing the qubit, capping it with a metallic cover, and equipping it with
suitable quasiparticle traps. Using a flip-chip design, we shape the
electromagnetic environment of the qubit above the superconducting gap,
inhibiting quasiparticle poisoning. Our findings support the hypothesis that
quasiparticle generation is dominated by the breaking of Cooper pairs at the
junction, as a result of photon absorption by the antenna-like qubit structure.
We achieve record low charge-parity switching rate (<1Hz). Our aluminium
devices also display improved stability with respect to discrete charging
events.
