Superconducting qubits are a leading platform for scalable quantum computing. The transmon qubit, consisting of a Josephson junction (JJ) shunted by a coplanar capacitor, is widely adopted due to its robustness against charge noise. However, its scalability is limited by the large footprint and dielectric losses at material interfaces. The merged-element transmon (MET) was introduced to enable a more compact architecture, yet in current implementations, although the JJ area can be reduced to ~3 µm², the coplanar capacitor still occupies ~100 µm². Moreover, its planar geometry generates widely distributed electric fields that couple to interfacial two-level systems (TLS), contributing to energy relaxation and decoherence. To address these limitations, we replace the coplanar shunt and coupling capacitors with an Al/AlOx/Al parallel-plate capacitor. Its three-dimensional geometry confines the electric field within the dielectric, suppressing coupling to interfacial TLS and reducing dielectric loss. By employing an ultrathin AlOx layer (on the order of several nanometers), the capacitor footprint can be reduced to ~µm²—comparable to the JJ—thereby enabling a significantly more compact qubit layout. This study establishes a scalable pathway toward compact, low-loss MET architectures, advancing the integration density and coherence performance of superconducting qubit systems.
Project is currently funded by: Federal