  The performance of various superconducting devices operating at ultra-low
temperatures is impaired by the presence of non-equilibrium quasiparticles.
Inelastic quasiparticle (QP) tunneling across Josephson junctions in
superconducting qubits results in decoherence and spurious excitations and,
notably, can trigger correlated errors that severely impede quantum error
correction. In this work, we use "gap engineering" to suppress the tunneling of
low-energy quasiparticles in Al-based transmon qubits, a leading building block
for superconducting quantum processors. By implementing potential barriers for
QP, we strongly suppress QP tunneling across the junction and preserve charge
parity for over $10^3$ seconds. The suppression of QP tunneling also results in
a reduction in the qubit energy relaxation rates. The demonstrated approach to
gap engineering can be easily implemented in all Al-based circuits with
Josephson junctions.
