Quasiparticle effects in transmons with gap-asymmetric junctions

Seminar featuring Giampiero Marchegiani - Technology Innovation Institute, Abu Dhabi

Single-particle excitations, known as Bogoliubov quasiparticles, threaten the operation of superconducting qubits. In this presentation, we theoretically revisit and generalize the qubit-quasiparticle interaction, including the gap asymmetry in Josephson junctions, which naturally arises from the deposition of aluminum layers with different thicknesses. We develop a semi-phenomenological description of the qubit-quasiparticle coupling in terms of quasiparticle densities. The subtle interplay of generation, tunneling and relaxation mechanisms strongly influences the steady-state of nonequilibrium quasiparticles. Two substantially different regimes are identified: 1) small gap difference, where quasiparticles are mainly located at the larger gap energy in both leads and the excited state of the qubit is depleted 2) strong gap asymmetry, similar to or higher than qubit frequency, with quasiparticles trapped in the lower gap superconductor and reduced relaxation rate. The differences between the two regimes can be explored in a split-transmon, comprising a superconducting ring interrupted by two Josephson junctions, where the qubit frequency can be modulated on-chip. Our results may be relevant to the design of qubits with improved suppression of quasiparticle poisoning.

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