Fabrication of Low-Loss Niobium Air Bridges for Superconducting Qubit Applications

As superconducting quantum processors scale up, the demand for high-density routing and thus efficient interconnects becomes increasingly critical. Air bridges, which allow for signal crossings in coplanar waveguides, must minimize losses to ensure qubit performance is not compromised. In this talk, we demonstrate a subtractive fabrication process with an aluminum hard mask and a niobium air bridge layer. Rigorous cleaning techniques enable us to achieve mean internal quality factors exceeding Q_int = 7.9·10⁶ in the single-photon limit, with no detectable loss per air bridge in resonators with 27 daisy-chained bridges. Additionally, we make use of air bridges as low-loss vacuum gap capacitors. By integrating them into transmon qubits, we observe median qubit lifetimes exceeding T₁ = 50 μs. These results demonstrate the potential of niobium air bridges as a low-loss component for scalable superconducting qubit systems. By utilizing a fully subtractive process without lift-off, we ensure greater robustness, positioning this approach as a scalable method for fabricating superconducting systems.