Multistability in Josephson Junction Devices

B. Muhlfelder, Warren W. Johnson

J. M. Meckbach, Michael Merker, Simon J. Buehler +7 more

For high-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 nonflat 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-and-match process, combining electron-beam lithography 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 procedure. Transport measurements of JJs, having critical-current densities ranging from 50 to 104A/cm2, are presented at 4.2 K. Our JJ process yields excellent quality parameters, Rsg/RNup to ~ 50, Vmfrom 15 to 80 mV and Vgapup 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.

I. V. Vernik, V. V. Bol'ginov, Sergey V. Bakurskiy +4 more

We investigate a Magnetic Josephson Junction (MJJ) - a superconducting device with ferromagnetic barrier for a scalable high-density cryogenic memory compatible with energy-efficient single flux quantum (SFQ) circuits. The superconductor-insulator-superconductor-ferromagnet-superconductor (SIS'FS) MJJs are analyzed both experimentally and theoretically. We found that the properties of SIS'FS junctions fall into two distinct classes based on the thickness of S'layer. We fabricate Nb-Al/AlOx-Nb-PdFe-Nb SIS'FS MJJs using a co-processing approach with a combination of HYPRES and ISSP fabrication processes. The resultant SIS'FS structure with thin superconducting S'-layer is substantially affected by the ferromagnetic layer as a whole. We fabricate these type of junctions to reach the device compatibility with conventional SIS junctions used for superconducting SFQ electronics to ensure a seamless integration of MJJ-based circuits and SIS JJ-based ultra-fast digital SFQ circuits. We report experimental results for MJJs, demonstrating their applicability for superconducting memory and digital circuits. These MJJs exhibitIcRnproduct only ~ 30% lower than that of conventional SIS junctions co-produced in the same fabrication. Analytical calculations for these SIS'FS structures are in a good agreement with the experiment. We discuss application of MJJ devices for memory and programmable logic circuits.

Paul A. Warburton

I. N. Askerzade, Ali Bozbey, Mehmet Cantürk

Alexander Stehli

Superconducting qubits are one of the most promising candidates for near term quantum applications. In this work we established a subtractive method to build Josephson junctions, the centerpiece of almost all superconducting quantum circuits. Using this technique we built a quantum simulator of the multistate Landau-Zener model. In time-resolved measurements we studied the transient dynamics of this system for different initial states.