Competence Center for Micro- and Nanotechnologies

Superconducting circuits are one of the most promising realizations for quantum technologies especially for quantum computing. Corresponding research must consider both: the quantum bit (so-called qubit) itself, which is often implemented as a transmon-type qubit for large scale applications, and the qubit’s on-chip environment. Therein, coplanar waveguide resonators and coplanar transmission lines are mostly used for addressing and reading out the transmons. The coplanar resonators need to have high intrinsic quality factors which significantly depend on the material and the fabrication process. Consequently, a profound knowledge of the fabrication process is mandatory. For an increasing number of qubits on a chip and, thus, more complex circuit designs, beyond-2D integration technologies are required to suppress unwanted slotline modes due to anharmonicities and discontinuities within the circuits ground planes. Superconducting low-inductance air bridges can ensure this feature by equalizing the ground planes’ potential. The aim of this work is to describe our recent progress in wafer-scale fabrication of beyond-2D superconducting quantum circuits. We show a combined study of resonator circuits and aluminum air bridges. We discuss our optimized fabrication process for resonator circuits with intrinsic quality factors up to approx. Qi = 2,000,000 at high photon numbers as well as the implementation of air bridges into resonator circuits. Finally, we demonstrate the successful implementation of air bridges in context of open quantum systems.