  Although Josephson junction qubits show great promise for quantum computing,
the origin of dominant decoherence mechanisms remains unknown. We report Rabi
oscillations for an improved phase qubit, and show that their coherence
amplitude is significantly degraded by spurious microwave resonators. These
resonators arise from changes in the junction critical current produced by
two-level states in the tunnel barrier. The discovery of these high frequency
resonators impacts the future of all Josephson qubits as well as existing
Josephson technologies. We predict that removing or reducing these resonators
through materials research will improve the coherence of all Josephson qubits.
