Fig. 5: Utilization of well-designed tantalum airbridges in a direct coupling element.

a Optical micrograph of the plane two-qubit superconducting quantum chip coupled via the separate tantalum airbridge, with fully-capped structure incorporated over each control line Zoom-in images of the coupling for both control group and experimental group are shown in the right panel respectively. b Characterization of effective coupling strength. The qubits in both groups are initially prepared in the \(\left\vert 01\right\rangle\) state (or \(\left\vert 10\right\rangle\) state), followed by fine-tuning the qubit frequencies to achieve resonance. The red dotted line in the experimental group represents the maximum resonance position where we extract the coupling strength to be 3.9 MHz. c T₁ distribution for both control group and experimental group. Here the red and blue dotted line represent the extracted average T₁ (〈T₁〉) and median T₁ (T_1,Mdn), respectively. The standard deviation is also calculated as σ. It can be found that the coupling capacitor via airbridge seems to have little effect on qubit decoherence through comparison between control group and experiment group. d Interleaved-RB of diabatic CZ gate for the experimental group. The gate fidelity is measured at 99.2(2)% for both the experimental and the control group. e Preparation of Bell state \(\left\vert \psi \right\rangle =\left(\left\vert 01\right\rangle +\left\vert 10\right\rangle \right)/\sqrt{2}\) using CZ gate in (d) with state fidelity measured to be 99.70% via QST. f Schematic diagrams of the airbridge coupling mechanism in superconducting quantum processors. Here we consider two potential airbridge structures for coupling. Left panel: Small-distance airbridge (L_ab≤ 200 μm where L_ab is defined in Eq. (1)) realizing neighboring or next-neighboring qubit-qubit connection. Right panel: Long-distance airbridge (L_ab > 200 μm) realizing long distance qubit-qubit connection.