We discuss the origin of decoherence in Josephson junction qubits. We find that two level systems in the surrounding insulator cannot be the dominant source of noise in small qubits. We argue that electron traps in the Josephson barrier with large Coulomb repulsion would emit noise that agrees both in magnitude and in temperature dependence with experimental data.

• Received 5 July 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.047001