  We review progress at UCSB on understanding the physics of decoherence in
superconducting qubits. Although many decoherence mechanisms were studied and
fixed in the last 5 years, the most important ones are two-level state defects
in amorphous dielectrics, non-equilibrium quasiparticles generated from stray
infrared light, and radiation to slotline modes. With improved design, the
performance of integrated circuit transmons using the Xmon design are now close
to world record performance: these devices have the advantage of retaining
coherence when scaled up to 9 qubits.
