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

Nb/Al-AlO_x/Nb tunnel junctions with controllable critical current density J_c are fabricated using the standard selective Nb etching process. Tunnel barriers are formed in different oxygen exposure conditions (oxygen pressure P and oxidation time t), giving rise to J_c ranging from 100 A/cm² to above 2000 A/cm². J_c shows a familiar linear dependence on P × t in logarithmic scales. We calculate the energy levels of the phase- and flux-type qubits using the achievable junction parameters and show that the fabricated Nb/Al-AlO_x/Nb tunnel junctions can be used conveniently for quantum computation applications in the future.