Decoherence in Josephson Qubits from Junction Resonances
Raymond W. Simmonds, Kristine Lang, Dustin A. Hite, David P. Pappas, John M. Martinis
We present experimental data that demonstrates a large reduction in the coherence of a Josephson phase qubit from spurious microwave resonances. These resonances are believed to arise from resonant fluctuators in the tunnel junction that couple to the junction critical current. We present a model that connects the subgap current-voltage characteristics with the magnitude of the rsonances, as well as the magnitude and density of 1/f critical-current fluctuations. This mechanism is argued to be a dominant source of decoherence in all present Josephson qubits. Improvements in the coherence of Josephson qubits will require materials research directed at reducing critical current fluctuations in tunnel junctions.