Coherence in a transmon qubit with epitaxial tunnel junctions

We developed transmon qubits based on epitaxial tunnel junctions and interdigitated capacitors. This multileveled qubit, patterned by use of all-optical lithography, is a step towards scalable qubits with a high integration density. The relaxation time T-1 is 0.72-0.86 mu s and the ensemble dephasing time T-2* is slightly larger than T-1. The dephasing time T-2 (1.36 mu s) is nearly energy-relaxation-limited. Qubit spectroscopy yields weaker level splitting than observed in qubits with amorphous barriers in equivalent-size junctions. The qubit's inferred microwave loss closely matches the weighted losses of the individual elements (junction, wiring dielectric, and interdigitated capacitor), determined by independent resonator measurements. (C) 2011 American Institute of Physics. [doi:10.1063/1.3672000]