Improved Coherence in Optically Defined Niobium Tri-layer Josephson Junction Qubits

While higher-critical-temperature Josephson junctions have been widely replaced by low-loss aluminum junctions for sensitive quantum circuitry, aluminum-based quantum circuits are still limited by quasiparticles to operation at temperatures below 0.1 Kelvin due to their low superconducting temperature. With significant recent interest in the exploration of diverse materials that can push these boundaries, we revisit niobium tri-layer junctions as the core components in qubits. Combining recent tri-layer junction advancements including sidewall-passivating spacer structures, heat annealed barriers and selective etching to reduce dielectric material, we apply material advancements from modern superconducting qubits to fabricate all-niobium transmons using only optical lithography. We characterize devices at various frequencies in the microwave range, measuring coherence times corresponding to qubit quality factors above 10⁵: much higher than previously demonstrated with niobium junctions, and much closer to the loss rates seen in conventional aluminum junction qubits. We find that the higher superconducting gap energy also results in reduced quasiparticle sensitivity around 0.1 – 0.2 K, where aluminum junction performance starts to deteriorate. This low-loss junction process is readily applied to standard niobium optical-based foundry processes, opening new pathways for integration and scalability, and paves the way for higher temperature and higher frequency quantum devices.