just appeared on arxiv:1406.7350 in a collaboration that connect people in Sanata Barabra, Waterloo, Kocaeli, and Delft.
In a flux qubit, the energy spectrum versus magnetic flux must be single hyperbolic, but what is observed usually in practice is double lines.
Cooling down does not help to remove the second line but it helps only a little bit to reduce the gap between the two. Why is this so?
This paper explains that the reason is quasiparticle poisoning in the junction. These quasiparticles have nonequilibrium nature, which at higher temperature turns to the equilibrium one. We propose a detailed theory and exactly extracted the gap from a quasiparticle tunneling theory.
Previously I made 2 more contributions to the theory of qusiparticle tunneling, here arXiv:1211.4745 (published) and arxiv:1303.1453 (recently submitted for publication).
The new preprint is an experimental evidence to the problem.