Quasiparticles in superconducting qubits - theory & experiments

Superconducting qubits based on Josephson junctions are a promising platform for quantum computation, reaching quality factors of over one million. On one hand, such high quality factors enable the implementation of quantum error correction; on the other, they make possible the investigation of decoherence mechanisms with high accuracy. An intrinsic decoherence process originates from the coupling between the qubit degree of freedom and the quasiparticles that tunnel across Josephson junctions. In this talk I will review briefly the general theory of quasiparticle effects, valid both for equilibrium and non-equilibrium quasiparticles, and discuss recent experiments with transmon and fluxonium qubits. In a transmon, tunneling of a single quasiparticle is associated with a change in parity; I will compare the theory of the parity-switching rate with recent measurements and comment on the implications for the dephasing rate. In qubits that can be tuned by magnetic flux, such as the fluxonium, the quasiparticle-induced decoherence rate depends on the flux; this provides a way to differentiate quasiparticle tunneling from other sources of decoherence. Taking advantage of this flux dependence, we have recently solved a 40-year-old puzzle in the physics of Josephson junctions.