Suppression of quasiparticle poisoning in transmon qubits by gap engineering

Performance of several types of superconducting devices operating at ultra-low temperatures is impaired by the presence of non-equilibrium quasiparticles (QP). In particular, inelastic QP tunneling across the Josephson junctions in superconducting qubits impedes the realization of quantum error correction. Here we use the so-called gap engineering to suppress tunneling of low-energy quasiparticles in Al-based transmon qubits, a leading platform for multi-qubit circuits. By implementing potential barriers for QP, we suppressed QP tunneling across the junction and preserved charge parity for well over 10³ seconds. Suppression of QP tunneling results in reduction of the qubit energy relaxation and dephasing rates. The demonstrated approach to gap engineering can be easily implemented in all circuits with Al-based Josephson junctions.