Compact vacuum gap transmon qubits: Selective and sensitive probes for superconductor surface losses

State-of-the-art transmon qubits rely on large capacitors which systematically improves their coherence due to reduced surface loss participation. However, this approach increases both the footprint and the parasitic cross-coupling and is ultimately limited by radiation losses – a potential roadblock for scaling up quantum processors to millions of qubits. In this work we present transmon qubits with sizes as low as 36×39μm2 with ≳100\,nm wide vacuum gap capacitors that are micro-machined from commercial silicon-on-insulator wafers and shadow evaporated with aluminum. After the release in HF vapor we achieve a vacuum participation ratio up to 99.6\% in an in-plane design that is compatible with standard coplanar circuits. Qubit relaxation time measurements for small gaps with high vacuum electric fields of up to 22\,V/m reveal a double exponential decay indicating comparably strong coupling to long-lived two-level-systems (TLS). %We also show that the fast ‚initial‘ and slow ‚residual‘ decay strongly correlates with the measured sub-single-photon and high-drive-power quality factors of lumped element vacuum gap resonators, respectively. The exceptionally high selectivity of >20\,dB to the superconductor-vacuum surface allows to precisely back out the sub-single-photon dielectric loss tangent of aluminum oxide exposed to ambient conditions of tanδ=1.5×10−4 for a thickness of 3\,nm. %assuming 3\,nm thick. %the widely used aluminum oxide exposed to ambient conditions. In terms of future scaling potential we achieve a qubit quality factor by footprint area of 20μs−2, which is on par with the highest T1 devices relying on larger geometries and expected to improve substantially for lower loss superconductors like NbTiN, TiN or Ta.