Vacuum-gap transmon qubits realized using flip-chip technology

Significant progress has been made in building large-scale superconducting quantum processors based on flip-chip technology. In this work, we use flip-chip technology to realize a modified transmon qubit, denoted as the “flipmon,” whose large shunt capacitor is replaced by a vacuum-gap parallel plate capacitor. We place one of the qubit pads and a single Josephson junction on the bottom chip and the other pad on the top chip, which is galvanically connected with the junction through an indium bump. The electric field energy participation ratio can arrive at nearly 53% in air when the vacuum-gap is about 5 $μm$⁠, thus potentially leading to a lower dielectric loss. The coherence times of the flipmons are obtained in the range of 30–60 $μ$s, which are comparable with that of conventional transmons with similar fabrication processes. The electric field simulation indicates that the metal-air interface's energy participation ratio increases significantly and may dominate the flipmon's decoherence. This suggests that more careful surface treatment needs to be considered. No evidence shows that the indium bumps inside the flipmons cause significant decoherence. With well-designed geometry and good surface treatment, the coherence of the flipmons can be further improved.