Gap engineered - Asymmetric Josephson junctions for Quantum Information Processing

Superconducting qubits based on Al/AlOx/Al Josephson junctions (JJs) have been the backbone of quantum computing research, owing to their reliable fabrication via double-angle shadow evaporation. However, these conventional devices remain vulnerable to quasiparticle tunneling and correlated error bursts induced by high-energy radiation, which pose a fundamental barrier to scalable fault-tolerant quantum computation. Addressing these challenges requires new approaches to junction design and material choice.

In this work, we introduce a novel approach to explore alternative JJ materials that enable gap engineering—a technique designed to suppress quasiparticle tunneling by creating an energy barrier across the junction. Using the Quantum Cluster system at the Molecular Foundry, we combine e-beam evaporation and sputter deposition to fabricate asymmetric JJ stacks with systematically varied superconducting gaps. In particular, we demonstrate Al/AlOx/NbN junctions, where the distinct asymmetry arising from the superconducting gaps of aluminum and Niobium Nitride act as a natural platform for robust gap engineering.

In this talk, we will discuss the fabrication process, material and qubit characterization results, and the broader implications of this approach for advancing superconducting qubit performance.