Vacuum-gap transmon qubits realized using flip-chip technology

TL;DR

This paper demonstrates a novel vacuum-gap transmon qubit, called flipmon, using flip-chip technology, achieving comparable coherence times to traditional transmons and highlighting the importance of surface treatment for further improvements.

Contribution

It introduces the flipmon, a vacuum-gap transmon with a large vacuum capacitor and integrated chip design, advancing scalable superconducting qubit fabrication.

Findings

Coherence times of 30-60 microseconds for flipmons.

Electric field participation ratio can reach 53% in air.

Surface treatment impacts decoherence due to metal-air interface.

Abstract

Significant progress has been made in building large-scale superconducting quantum processors based on flip-chip technology. In this work, we use the flip-chip technology to realize a modified transmon qubit, donated as the "flipmon", whose large shunt capacitor is replaced by a vacuum-gap parallel plate capacitor. To further reduce the qubit footprint, 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 single Josephson junction through an indium bump. The electric field participation ratio can arrive at nearly 53% in air when the vacuum-gap is about 5 microns, and thus potentially leading to a lower dielectric loss. The coherence times of the flipmons are measured in the range of 30-60 microseconds, which are comparable with that of traditional transmons with similar fabrication processes. The electric field simulation indicates that the metal-air interface's participation ratio increases significantly and may dominate the qubit'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.