Development of nanobridge based Josephson junction electronics for the readout of superconducting photon sensor arrays

Collins, Jonathan A. (2021) Development of nanobridge based Josephson junction electronics for the readout of superconducting photon sensor arrays. PhD thesis, University of Glasgow.

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Superconducting nanowire single photon detectors (SNSPDs) have been shown to have unrivalled performance for single photon detection. However, scaling up to a large scale detection array which maintains the single pixel performance has proven to be challenging. Several solutions have been demonstrated, but no single readout scheme has to date proven to be capable of meeting all of the required specifications.

One promising solution is for a low temperature digital signal processing scheme based on the generation and transmission of voltage pulses with an area corresponding to a single flux quantum (SFQ), V ≈2.07 mV ·ps. Conventional SFQ technology use superconducting tunnel junctions (STJ) as the active element, which require a multistep fabrication process which has so far proven difficult to integrate with SNSPD fabrication. To overcome this we propose the use of bridge-type weak links as the active element, known as nanobridges, to replace the STJ. Such a nanobridge based SFQ scheme could be integrated with an SNSPD detector array on-chip.

In this work the feasibility of using nanobridges as active weak link elements is investigated. To do this, a fabrication process was developed which can be used to produce nanobridges with the physical dimensions required to exhibit the Josephson effect, in a scalable and reproducible process. The electrical properties of nanobridges have been measured and show that the switching parameters can be reproduced in a scalable way. Finally, evidence of the Josephson effect in nanobridges is presented and used to derive the current-phase relationship (CPR) in nanobridges. It was found that the CPR of the nanobridge is multivalued, and deviates from the standard sinusoidal relationship. This CPR was then used to simulate SFQ circuitry where nanobridges are used as the active element, showing that nanobridges can be used in SFQ circuitry.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: T Technology > T Technology (General)
Colleges/Schools: College of Science and Engineering > School of Engineering > James Watt Nanofabrication Centre
Supervisor's Name: Casaburi, Dr. Alessandro
Date of Award: 2021
Depositing User: Theses Team
Unique ID: glathesis:2021-82550
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 03 Nov 2021 14:53
Last Modified: 04 Nov 2021 09:01
Thesis DOI: 10.5525/gla.thesis.82550

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