Investigating the impact of air-bridge and Josephson-junction fabrication order on transmon performance in circuit QED

Free-standing air bridges are key components of superconducting quantum processors. They are used to suppress odd modes of propagation in the coplanar-waveguide transmission lines of resonators and control lines, as well as to create cross-overs where transmission lines unavoidably intersect. Conventionally, air bridges are fabricated after the Josephson junctions of qubits, subjecting the junctions to more fabrication steps than strictly necessary (resist layers, lithography, temperature variations, etc.). Therefore, it may be advantageous to fabricate the qubit Josephson junctions last in order to minimize the impact of fabrication steps on the yield, coherence, and frequency targeting of superconducting qubits. We present an investigation of the impact of changing fabrication order on transmon performance metrics.