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We study decoherence in superconducting qubits due to quasiparticle tunnelingwhich is enhanced by two known deviations from the equilibrium BCS theory. Thefirst process corresponds to tunneling of an already existing quasiparticleacross the junction. The quasiparticle density is increased, e.g., because ofan effective quasiparticle doping of the system. The second process isquasiparticle tunneling by breaking of a Cooper pair. This can happen attypical energies of superconducting qubits if there is an extendedquasiparticle density inside the gap. We calculate the induced energy decay andpure dephasing rates in typical qubit designs. Assuming the lowest reportedvalue of the non-equilibrium quasiparticle density in Aluminum, we find for thepersistent-current flux qubit decay times of the order of recent measurements.Using the typical sub-gap density of states in Niobium we also reproduceobserved decay times in the corresponding Niobium flux qubits.

Fragility of multi-junction flux qubits against quasiparticle tunneling