Superconducting phase qubits with shadow-evaporated Josephson junctions**Project supported by the National Basic Research Program of China (Grant Nos. 2014CB921202, 2015CB921104, and 2016YFA0300601) and the National Natural Science Foundation of China (Grant Nos. 91321208 and 11674380).

Abstract

We develop a fabrication process for the superconducting phase qubits in which Josephson junctions for both the qubit and superconducting quantum interference device (SQUID) detector are prepared by shadow evaporation with a suspended bridge. Al junctions with areas as small as are fabricated for the qubit, in which the number of the decoherence-causing two-level systems (TLS) residing in the tunnel barrier and proportional to the junction area are greatly reduced. The measured energy spectrum shows no avoided crossing arising from coherent TLS in the experimentally reachable flux bias range of the phase qubit, which demonstrates the energy relaxation time T1 and dephasing time on the order of 100 ns and 50 ns, respectively. We discuss several possible origins of decoherence from incoherent or weakly-coupled coherent TLS and further improvements of the qubit performance.