Difficult to say this problem is solved yet - the jury is still out, but I've done a good deal of work on identifying what the mechanism of a defect in superconducting phase qubits may be.
TL;DR: Two level system defects are a 20 year old unidentified noise source that can be described by an oxygen spatially delocalising in an amorphous portion of the underlying circuit.
Comments
Difficult to say this problem is solved yet - the jury is still out, but I've done a good deal of work on identifying what the mechanism of a defect in superconducting phase qubits may be.
TL;DR: Two level system defects are a 20 year old unidentified noise source that can be described by an oxygen spatially delocalising in an amorphous portion of the underlying circuit.
See http://dx.doi.org/10.1103/PhysRevLett.110.077002 and http://dx.doi.org/10.1088/1367-2630/17/2/023017.