Memristively Programmable Josephson Junctions

Superconducting quantum interference devices (SQUIDs) can be used for realization of highly sensitive magnetic field sensors and qubits for quantum computers. SQUIDs rely on Josephson junctions and flux quantization. Achieving ultra-high sensitivity, low noise, and long coherence times demands precise tuning of the junction's critical current while simultaneously suppressing the sub-gap current. Nb/NbOx/Nb junctions are highly attractive due to the large gap-energy of Nb, which minimizes the sub-gap current. However, the inherent reactivity of Nb with its native oxide introduces considerable variations of the critical current. Today, Al/AlOx/Al and Nb/Al/AlOx/Al/Nb junctions show the best and reliable performance. Nevertheless, even with the application of advanced fabrication techniques like laser annealing or high-voltage electron beam lithography, the critical current variation still extends up to 15% on the wafer level. Here, we introduce an innovative solution by utilizing a memristive Nb/NbOx/Nb heterostructure, serving as a programmable Josephson junction. By resistive switching at room temperature, a filamentary weak link (metallic quantum point contact) or tunneling junction between two electrodes formed, effectively operating as a Josephson junction at cryogenic temperatures. In this proof-of-concept, we analyzed in detail and modelled the electrical and physico-chemical properties of the Nb-based memristors. Particular attention is paid to the quantum charge transport at cryogenic temperatures and the statistical variations of the critical current. We demonstrate reproducible forming and erasing of the filamentary junction, which may transcend the limitations of conventional Nb-based junctions such as stress or degradation. The fundamental transport and switching studies are complemented by advanced spectroscopic and microscopies techniques. These memristively programmable Josephson junctions allow for precise tuning of the junction properties post-fabrication through simple programming steps. This may eliminate the need for sophisticated deposition or patterning techniques and unlock the full application potential of Nb-based superconducting electronics.