Reduction of the critical current in narrow superconducting NbN lines with sharp and rounded bends with respect to the critical current in straight lines was studied at different temperatures. We compare our experimental results with the reduction expected in the framework of the London model and the Ginsburg-Landau model. We have experimentally found that the reduction is significantly less than either model predicts. We also show that in our NbN lines the bends mostly contribute to the reduction of the critical current at temperatures well below the superconducting transition temperature.

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Forhigh-performance superconducting quantum devices based on Josephson junctions (JJs) decreasing lateral sizes is of great importance. Fabrication of sub-µm JJs is challenging due to non-flat surfaces with step heights of up to several 100 nm generated during the fabrication process. We have refined a fabrication process with significantly decreased film thicknesses, resulting in almost flat surfaces at intermediate steps during the JJ definition. In combination with a mix-&-match process, combining electron-beam lithography (EBL) and conventional photolithography, we can fabricate JJs with lateral dimensions down to 0.023 µm². We propose this refined process as an alternative to the commonly used chemical-mechanical polishing (CMP) procedure. We present transport measurements of JJs at 4.2 K that yield critical-current densities in the range from 50 to 10 4 A/cm². Our JJ process yields excellent quality parameters, R sg /R N up to ~50 and V gap up to 2.81 mV, and also allows the fabrication of high-quality sub-µm wide long JJs (LJJs) for the study of Josephson vortex behavior. The developed technique can also be used for similar multilayer processes and is very promising for fabricating sub-µm JJs for quantum devices such as SQUIDs, qubits and SIS mixers.

, T. H. (2013). Thermo-magnetic stability of superconducting films controlled by nano-morphology. Applied Physics Letters, 102 (25), 252601-1-252601-5.

We have developed a combined photolithography and electron-beam lithography fabrication process for sub-μm to μm-size Nb/Al-AlO x /Nb Josephson junctions. In order to define the junction size and protect its top electrode during anodic oxidation, we developed and used the new concept of an aluminum hard mask. Josephson junctions of sizes down to 0.5 μm 2 have been fabricated and thoroughly characterized. We found that they have a very high quality, which is witnessed by the I V curves with quality parameters V m > 50 mV and V gap = 2.8 mV at 4.2 K, as well as I c R N products of 1.75-1.93 mV obtained at lower temperatures. In order to test the usability of our fabrication process for superconducting quantum bits, we have also designed, fabricated and experimentally investigated phase qubits made of these junctions. We found a relaxation time of T 1 = 26 ns and a dephasing time of T 2 = 21 ns.