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IEEE Transactions on Applied Superconductivity

Microwave Inductive Readout of EBL Nanobridge SQUIDs

Tom Godfrey, George Long, John C. Gallop, David Cox, Elias Polychroniou, Jie Chen, Edward J. Romans, Ling Hao

Published May 28, 2020• 3 citations
Publisher

Abstract

Niobium-based nanobridge superconducting quantum interference devices (SQUIDs) have shown very low noise performance and high-frequency operation. We describe how we are bringing together these two aspects of nanobridge SQUIDs with the aim of realizing single spin-flip detection using microwave inductive readout techniques, where the nanobridge SQUID is integrated into a superconducting coplanar waveguide resonator. Using electron beam lithography (EBL) is ideal for fabricating nanobridge junctions and has the advantage of being easily scalable compared to using a focused ion beam. In this article, we demonstrate that EBL is suitable for fabricating nanobridge junction SQUIDs, and they exhibit very comparable performance to previous focused ion beam milled SQUIDs. Our integrated devices show potential to be used for flux-tunable sensors.

DOI
10.1109/tasc.2020.2998374

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Niobium-based nanobridge superconducting quantum interference devices (SQUIDs) have shown very low noise performance and high-frequency operation. We describe how we are bringing together these two aspects of nanobridge SQUIDs with the aim of realizing single spin-flip detection using microwave inductive readout techniques, where the nanobridge SQUID is integrated into a superconducting coplanar waveguide resonator. Using electron beam lithography (EBL) is ideal for fabricating nanobridge junctions and has the advantage of being easily scalable compared to using a focused ion beam. In this article, we demonstrate that EBL is suitable for fabricating nanobridge junction SQUIDs, and they exhibit very comparable performance to previous focused ion beam milled SQUIDs. Our integrated devices show potential to be used for flux-tunable sensors.
Niobium nanobridge SQUIDs have shown exceptional noise performance with potential applications in quantum information processing, weak signal detection and single spin detection where the nanobridge geometry should enable efficient electromagnetic coupling to implanted spins. Combining such devices with dispersive microwave readout circuitry allows the spin sensitivity to be further improved by overcoming the standard thermal limit. Here we report on the fabrication and dispersive microwave readout of an array of niobium nanobridge rf SQUIDs incorporated into a superconducting resonator, including the optimization of the nanobridge fabrication process by electron beam lithography. We show the measured flux-tuneability of the resonance is in good agreement with theory, and we also discuss how the nonlinearity of the weak-link in the resonator structure allows for the mediation of parametric effects to enhance performance.
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DC SQUIDs with outer loop size ∼10 μmx 10 μm have been fabricated with nanobridge Josephson junctions in thin Niobium films (25, 100 and 200 nm) using either electron beam or focused ion beam techniques. Such SQUIDs are ideally suited to incorporate into the centre strip of a superconducting coplanar waveguide. The current-voltage characteristics and magnetic field modulation of the SQUIDs are compared for the two methods. The temperature dependence of the critical current is quite linear for the e-beam devices whilst the focused ion beam devices fit an exponential. Here we also investigate trimming methods to reduce the critical currents of the nanojunctions fabricated in the thicker films, in order to improve the SQUID parameters at lower temperatures.
We report on the low-frequency noise performance of niobium dc superconducting quantum interference devices (SQUIDs), which contain nanobridges fabricated by focused ion beam lithography as the active Josephson elements. The devices have feature sizes down to 70 nm. We have measured devices of different loop sizes in two readout configurations: nano-scale loop sizes in small signal mode using a series SQUID array as a low-temperature pre-amplifier, and larger micron-scale loop sizes in a conventional flux-locked loop. We investigate the different contributions to the low frequency noise and report on electrical measurements made in applied magnetic fields of up to 0.5 T (in-plane) and 0.1 T (perpendicular to the plane) at operating temperatures around 7 K. We compare the measurements with the existing theories of noise in a dc SQUID.
Nonlinear microwave circuits are key elements for many groundbreaking research directions and technologies, such as quantum computation and quantum sensing. The majority of microwave circuits with Josephson nonlinearities to date are based on aluminum thin films; therefore, they are severely restricted in their operation range regarding temperatures and external magnetic fields. Here, we present the realization of superconducting niobium microwave resonators with integrated, three-dimensional (3D) nanobridge-based superconducting quantum interference devices. The 3D nanobridges (constriction weak links) are monolithically patterned into prefabricated microwave LCLCLC circuits using neon focused-ion-beam milling, and the resulting quantum interference circuits show frequency tunabilities, flux responsivities, and Kerr nonlinearities on par with comparable aluminum nanobridge devices, but with the perspective of a much larger operation parameter regime. Our results demonstrate that neon focused-ion-beam milling is a promising method for fabricating 3D constriction junctions with flexible parameters and reveal great potential for application of the resulting microwave circuits in hybrid systems with, e.g., magnons and spin ensembles or in flux-mediated optomechanics.
A superconducting quantum interference device (SQUID) miniaturized into the nanoscale is promising in the inductive detection of a single electron spin. A nano-SQUID with a strong spin coupling coefficient, a low flux noise, and a wide working magnetic field range is highly desired in a single spin resonance measurement. Nano-SQUIDs with Dayem bridge junctions excel in a high working field range and in the direct coupling from spins to the bridge. However, the common planar structure of nano-SQUIDs is known for problems such as a shallow flux modulation depth and a troublesome hysteresis in current–voltage curves. Here, we developed a fabrication process for creating three-dimensional (3-D) niobium (Nb) nano-SQUIDs with nanobridge junctions that can be tuned independently. Characterization of the device shows up to 45.9% modulation depth with a reversible current–voltage curve. Owning to the large modulation depth, the measured flux noise is as low as 0.34 μΦ 0 /Hz 1/2 . The working field range of the SQUID is greater than 0.5 T parallel to the SQUID plane. We believe that 3-D Nb nano-SQUIDs provide a promising step toward effective single-spin inductive detection.