Parity switching and decoherence by quasiparticles in single-junction transmons

Transmons are at present among the most coherent superconducting qubits, reaching quality factors of order $10^6$ both in 3D and 2D architectures. These high quality factors enable detailed investigations of decoherence mechanisms. An intrinsic decoherence process originates from the coupling between the qubit degree of freedom and the quasiparticles that tunnel across Josephson junctions. In a transmon, tunneling of a single quasiparticle is associated with a change in parity. I will discuss the theory of the parity switching rate in single-junction transmons, compare it with recent measurements, and consider the role of parity switching in limiting the coherence time.