Compact vacuum gap transmons - selective and sensitive probes for superconductor surface losses

The large capacitor of state-of-the-art transmon qubits systematically lowers the coupling to parasitic losses localized in material interfaces, but it also lowers the achievable integration density of quantum processors, and increases unwanted radiation losses, cross-couplings and leakage. We fabricate and characterize transmons with a footprint area as low as 36 x 39 μm² and show that the interaction with the lossy dielectric interfaces is rapidly decreased by using very narrow (≥ 100 nm) capacitor vacuum gaps. We utilize commercial silicon-on-insulator wafers, where the sidewalls of suspended silicon beams are metalized by angle evaporation such that up to 99.6% of the electric field energy is distributed in vacuum and the effective dielectric constant is close to unity. Measuring the internal quality factor of lumped element resonators and the energy relaxation time of qubits hence allows to precisely probe the superconductor surface loss properties and isolate a single loss channel – two level defects in the aluminum surface oxide exposed to ambient conditions. This offers new means to systematically improve coherence times by testing appropriate superconducting materials and surface treatments.