Nb/Al-AlOx/Nb junctions with controllable critical current density for qubit application

Nb/Al-AlO_x/Nb tunnel junctions with controllablecritical current density J_c are fabricated using thestandard selective Nb etching process. Tunnel barriers are formed indifferent oxygen exposure conditions (oxygen pressure P andoxidation time t), giving rise to J_c ranging from100~A/cm² to above 2000 A/cm². J_c shows a familiarlinear dependence on P×t in logarithmic scales. We calculatethe energy levels of the phase- and flux-type qubits using theachievable junction parameters and show that the fabricatedNb/Al-AlO_x/Nb tunnel junctions can be used conveniently forquantum computation applications in the future.