Niobium air bridges as low-loss components for superconducting quantum hardware

Scaling up superconducting quantum processors requires a high routing density for readout and control lines, relying on low-loss interconnects to maintain design flexibility and device performance. We propose and demonstrate a universal subtractive fabrication process for air bridges based on an aluminum hard mask and niobium as the superconducting film. Using this technology, we fabricate superconducting CPW resonators incorporating multiple niobium air bridges in and across their center conductors. Through rigorous cleaning methods, we achieve mean internal quality factors in the single-photon limit exceeding $Q_{\mathrm{int}} = 8.2 \times 10^6$. Notably, the loss per air bridge remains below the detection threshold of the resonators. Due to the larger superconducting energy gap of niobium compared to conventional aluminum air bridges, our approach enables stable performance at elevated temperatures and magnetic fields, which we experimentally confirm in temperatures up to 3.9 K and in a magnetic field of up to 1.60 T. Additionally, we utilize air bridges to realize low-loss vacuum-gap capacitors and demonstrate their successful integration into transmon qubits by achieving median qubit lifetimes of $T_1 = 51.6 \,\mu\text{s}$.