UCSB final report for the CSQ program: Review of decoherence and   materials physics for superconducting qubits
  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. 
John M. Martinis, A. Megrant
quant-ph cond-mat.mes-hall cond-mat.supr-con