Small-Footprint Superconducting Qubits with Nb/Si/Nb Fin Capacitors

The large footprint of a transmon is a critical constraint in the fabrication of large-scale superconducting quantum processors. Reducing the size of a conventional planar shunt capacitor increases microwave loss due to the enhanced participation of lossy air interfaces. Using a parallel-plate capacitor shifts the field away from air interfaces to the metal-dielectric interface, thereby enabling a better compromise between coherence and size. In this work, we use cryogenic deep etching to fabricate a silicon fin dielectric. Following the deposition of niobium, the Nb/Si/Nb fin stack realizes a low-loss and small-footprint parallel-plate capacitor. We confirm its low-loss characteristics by building lumped-element resonators with internal quality factors of (1.83 ± 0.31) ×10⁶ in the single-photon regime. We further realize a fin-based transmon with an order-of-magnitude reduction in footprint compared to conventional planar designs, while preserving comparable coherence times. The Nb/Si/Nb fin capacitors thus enable the fabrication of high-density and high-coherence superconducting quantum circuits.