Fig. 4: Investigation of tantalum airbridges in a tunable coupling superconducting quantum processor.

a Optical micrograph of the 13-qubit flip-chip processor. Here, the top chip is primarily used for qubit layer containing both Josephson junctions and shunt capacitors, while the bottom chip comprises readout resonators and control lines. Tantalum airbridges with fully-capped structure are incorporated over each control line to minimize crosstalk. b SEM images of the tantalum airbridges with fully-capped structure in (a). c A box plot of T₁ results for all thirteen qubits, with a median T₁ exceeding 100 μs for most qubits. Here, the box plot can illustrate the distribution of the data, with an interquartile range (IQR) of 1.5, indicating the range within which the middle 50% of the data lies. Meanwhile, the box represents the 25th percentile to the 75th percentile, and the whiskers extend to the minimum and maximum values, excluding any outliers. d Characterization of T₁ measurement varied with qubit frequency for Q₁₃. The inset illustrates the histogram with a median T₁ of 109 μs. e A representative single T₁ measurement result marked in (d). f Measurement of flux crosstalk (left panel) and microwave crosstalk (right panel) with tantalum airbridges utilizing the fully-capped structure. g Distance dependence of the flux crosstalk β_z (top panel) and the microwave crosstalk Λ_XY (bottom panel). The inset provides definitions for the distance d relevant to both types of crosstalk. Specifically, d^(z) represents the shortest distance from the target qubit (i.e., the target SQUID) to the flux line, while d^(xy) is defined as the center separation between the target qubit and the source qubit.