Weak link nanobridges as single flux quantum elements

Abstract

This paper investigates the feasibility of using weak link nanobridges as Josephson junction elements for the purpose of creating Josephson circuits. We demonstrate the development of a single-step electron beam lithography procedure to fabricate niobium nanobridges with dimensions down to . The single-step process facilitates fabrication that is scalable to complex circuits that require many junctions. We measure the IV-characteristics (IVC) of the nanobridges between temperatures of and and find agreement with numerical simulations and the analytical resistively shunted junction (RSJ) model. Furthermore, we investigate the behaviour of the nanobridges under rf irradiation and observe the characteristic microwave-induced Shapiro steps. Our simulated IVC under rf irradiation using both the RSJ model and circuit simulator are in agreement with the experimental data. As a potential use of nanobridges in circuits requiring many junctions, we investigate the theoretical performance of a nanobridge-based Josephson comparator circuit using .

Existence of Shapiro Steps in the Dissipative Regime in Superconducting Weak Links

“…Typical IVCs measured without rf irradiation are shown in Figure 2 taken at different temperatures. The behaviour is qualitatively similar to those observed previously by the authors [28] and elsewhere [20,22,25] showing that both I c and I r are temperature dependent and that the hysteresis disappears above the crossover temperature T H .…”

“…The Josephson effect can also be demonstrated through the observation of microwave-induced Shapiro steps [27]. We have previously observed Shapiro steps in WLs operated in the non-hysteretic regime [28] and they have also been found in long nanowires when driven into the 'phase slip center' regime [29,30].…”

Modelling of a Two-Signal SFQ Detection Scheme for the Readout of Superconducting Nanowire Single Photon Detectors

“…2. We have also observed the characteristic frequency-dependent voltage steps (Shapiro steps) as a result of coupling of a microwave current into a single nanobridge (not shown) [9]. These two properties demonstrate that our nanobridge devices exhibit the Josephson effect and can be used as elements in more complex SFQ circuits.…”

“…Low-temperature measurements (sub-9 K) were performed with the SQUID devices exhibiting characteristic Josephson behaviour of − Φ modulation. Additional measurements of single nanobridges demonstrate further evidence of Josephson behaviour with characteristic frequency-dependent voltage steps when subject to microwave radiation [9]. We also model an SFQ circuit known as a 'coincidence buffer' in Josephson circuit simulator JSIM and propose its potential use as a time-resolving two-photon coincidence detector.…”

Bridging the Gap Between Nanowires and Josephson Junctions: A Superconducting Device Based on Controlled Fluxon Transfer

“…These limitations leave a gap in the family of superconducting electronics for a device that offers the robust interfacing capabilities of a traditional nanowire, but with a more moderated response that allows for modulation of the output, analogous (but not identical) to incrementing in a JJ. Past approaches to creating such a device have focused on Dayem bridge weak-links 17,18 , where the dimensions of a nanowire in relation to the material's coherence length allow for preservation of phase coherence over a temporary phase-slip center. However, there have been very few experimental demonstrations of these devices in real circuit operations, and the primary goal has largely been to achieve true Josephson behavior in a nanowire rather than to demonstrate a device with intermediate characteristics of both technologies.…”