Superconducting Nb nanobridges for reduced footprint and efficient next generation electronics

Collins, J., Rose, C. and Casaburi, A. (2023) Superconducting Nb nanobridges for reduced footprint and efficient next generation electronics. IEEE Transactions on Applied Superconductivity, 33(1), 1800208. (doi: 10.1109/TASC.2022.3218895)

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We optimised a process to reliably fabricate thin Nb nanobridge weak links having 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 Josephson element for reduced footprint 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 (RSJ) 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, $I_{c}R_{n}\sim mV$ IcRn, at varying of the temperature. Using this data, we simulated the generation of voltage pulses at varying of CPR and verified that they still have a quantized area equal to the magnetic flux quantum $\Phi_{0}$ and the product $I_{c}R_{n}$ allows for speed of operation $I_{c}R_{N} \,/\,\Phi_{0}\gg\,100$ GHz. Moreover, their critical current $I_{c}\approx \,100$ μA, comparable with that of tunnel Josephson junctions (JJs) used in SFQ electronics, is orders of magnitude larger than thermal current noise $I_{TN}=(2\pi\,/\,\Phi_{0})\,k_{B}\,T\,\approx\,0.2$ μA at temperature T = 4.2 K, for stable and, at the same time, efficient operation with energy per switch of only $E_{J}\approx I_{c}\Phi_{0}\lesssim 1$ aJ. To assess potential use of these nanobridges in SFQ logic electronics with large number of elements, we used an open-source simulation software (JSim) to simulate the behaviour 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 suggests that these nanobridges can be used to develop large scale SFQ electronics with several advantages over tunnel JJs. The reduced footprint, just 1/3 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.

Item Type:Articles
Additional Information:This work was supported by the Control Interface for Quantum Integrated Technology Array -CIQUITA UKRI Project (EP/T025743/1).
Glasgow Author(s) Enlighten ID:Collins, Mr Jonathan and Rose, Mr Calum and Casaburi, Dr Alessandro
Authors: Collins, J., Rose, C., and Casaburi, A.
College/School:College of Science and Engineering > School of Engineering > Electronics and Nanoscale Engineering
Journal Name:IEEE Transactions on Applied Superconductivity
ISSN (Online):1558-2515
Published Online:04 November 2022
Copyright Holders:Copyright © 2022 IEEE
First Published:First published in IEEE Transactions on Applied Superconductivity 33(1): 1800208
Publisher Policy:Reproduced with the permission of the Publisher

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
308853Control Interface for QUantum Integrated Technology ArraysMartin WeidesEngineering and Physical Sciences Research Council (EPSRC)EP/T025743/1ENG - Electronics & Nanoscale Engineering