Superconducting Nb Nanobridges for Reduced Footprint and Efficient Next-Generation Electronics

We optimized a process to reliably fabricate thin Nb nanobridge weak links having a physical size comparable with Nb coherence length ξ(4.2K) ∼16 nm, controlled degraded superconductivity with respect to the electrodes, and excellent edge roughness. We then investigated the feasibility to use these nanobridges as the Josephson element for reduced footprints and efficient next-generation single flux quantum (SFQ) logic electronics. First of all, we demonstrated that in such thin Nb nanobridges, there is no thermal hysteresis in the current–voltage characteristics (IVC) that instead is usually observed in other weak links and prevents their use in SFQ electronics. We fitted the experimental IVCs of nanobridges with the resistively shunted junction model implemented with piecewise linear current–phase relation (CPR) finding a very good agreement with data. This allowed us to infer the CPR parameters and evaluate the product of critical current and normal resistance, <italic>I<sub>c</sub>R<sub>n</sub></italic> ∼mV, at varying temperatures. Using these data, we simulated the generation of voltage pulses at varying CPRs and verified that they still have a quantized area equal to the magnetic flux quantum Φ<sub>0</sub> and the product <italic>I<sub>c</sub>R<sub>n</sub></italic> allows for speed of operation <italic>I<sub>c</sub> R<sub>N</sub></italic> / Φ<sub>0</sub> <inline-formula><tex-math notation="LaTeX">$\gg $</tex-math></inline-formula>100 GHz. Moreover, their critical current <italic>I<sub>c</sub></italic> <inline-formula><tex-math notation="LaTeX">$\approx $</tex-math></inline-formula>100 <inline-formula><tex-math notation="LaTeX">$\mu $</tex-math></inline-formula>A, comparable with that of tunnel Josephson junctions (JJs) used in SFQ electronics, is orders of magnitude larger than thermal current noise <italic>I</italic><sub>TN</sub> = (2π / Φ<sub>0</sub>) <italic>k<sub>B</sub> T</italic> <inline-formula><tex-math notation="LaTeX">$\approx $</tex-math></inline-formula> 0.2 <inline-formula><tex-math notation="LaTeX">$\mu $</tex-math></inline-formula>A at temperature <italic>T</italic> = 4.2 K, for stable and, at the same time, efficient operation with energy per switch of only <italic>E<sub>J</sub></italic> ≈ <italic>I<sub>C</sub></italic>Φ<sub>0</sub> <inline-formula><tex-math notation="LaTeX">$\mathbin{\hbox{$\buildrel<\over {\smash{\scriptstyle \sim}\vphantom{_x}}$}} $</tex-math></inline-formula> 1 aJ. To assess the potential use of these nanobridges in SFQ logic electronics with a large number of elements, we used an open-source simulation software (JSim) to simulate the behavior of a standard DC-to-SFQ converter circuit. From the simulation made by implementing the CPR inferred from experimental data, we observed that the circuit behaves exactly as intended. Our results strongly suggest that these nanobridges can be used to develop large-scale SFQ electronics with several advantages over tunnel JJs. The reduced footprint, just one-third or less than standard tunnel JJs, and simplified fabrication process, with only 2 steps involved against typically ∼20 for tunnel JJs, could allow for a better fabrication tolerance, higher control on operation parameters, higher circuit density, and easier integration with other technology platforms. These characteristics could be very appealing also to replace tunnel JJs in quantum technology devices like transmon qubits and superconducting parametric amplifiers.