Measurement and modification of two-level tunneling states in amorphous aluminum oxide

Low energy excitations modeled as two-level tunneling systems (TLSs)—a significant source of decoherence in qubits built around superconducting Josephson junctions—are found nearly universally in amorphous dielectrics. Although TLS-free amorphous dielectric materials would be a technological boon, to date there is only one such material system: amorphous silicon thin films vapor deposited onto substrates held at elevated temperatures. The question arises whether TLSs can be eliminated in other, technologically pertinent amorphous dielectrics. To this end we have measured the internal friction of 100–300nm sputter-deposited aluminum oxide films, at substrate temperatures up to 500°C. This measurement directly yields the tunneling strength parameter from the TLS model, and we find a factor of five reduction in tunneling strength for films deposited at high temperatures.