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State-of-the-art transmon qubits rely on large capacitors whichsystematically improves their coherence due to reduced surface lossparticipation. However, this approach increases both the footprint and theparasitic cross-coupling and is ultimately limited by radiation losses - apotential roadblock for scaling up quantum processors to millions of qubits. Inthis work we present transmon qubits with sizes as low as 36$ \times $39$\mu$m$^2$ with $\gtrsim$100 nm wide vacuum gap capacitors that aremicro-machined from commercial silicon-on-insulator wafers and shadowevaporated with aluminum. After the release in HF vapor we achieve a vacuumparticipation ratio up to 99.6\% in an in-plane design that is compatible withstandard coplanar circuits. Qubit relaxation time measurements for small gapswith high vacuum electric fields of up to 22 V/m reveal a double exponentialdecay indicating comparably strong coupling to long-lived two-level-systems(TLS). The exceptionally high selectivity of $>$20 dB to thesuperconductor-vacuum surface allows to precisely back out thesub-single-photon dielectric loss tangent of aluminum oxide exposed to ambientconditions. In terms of future scaling potential we achieve a qubit qualityfactor by footprint area of $20 \mu \mathrm{s}^{-2}$, which is on par with thehighest $T_1$ devices relying on larger geometries and expected to improvesubstantially for lower loss superconductors like NbTiN, TiN or Ta.

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