Suppression of quasiparticle poisoning in superconducting circuits by local ion bombardment of Aluminum electrodes*

Tunneling of non-equilibrium quasiparticles across Josephson junctions is a significant source of dephasing in superconducting qubits. The rate of quasiparticle tunneling can be reduced by local enhancement of the superconducting energy gap Δ in the junction electrodes. Aluminum offers a unique opportunity for gap engineering, because Δ in Al films significantly increases with disorder. We observed an increase of Δ by 0.3 – 0.5K in thin Al films treated with a focused beam of 25 keV Ne ions using a Zeiss Orion Plus microscope and applied this technique for local modification of Al films in prefabricated transmon qubits. We will discuss the results of measurements of the coherence time and thermal occupation in the qubits before and after ion bombardment.