The Superconducting Low-inductance Undulatory Galvanometer (SLUG) is a linear, phase-sensitive, quantum-limited amplifier that can amplify ultra-weak microwave signals at frequencies up to 10 GHz or potentially higher, such as the output from cavity-based searches for the axion, a dark matter candidate. The SLUG amplifier differs from the closely related Superconducting Quantum Interference Device (SQUID) amplifier in that the input signal is directly injected into the SLUG rather than being inductively coupled through a multiturn input coil. The direct injection greatly simplifies the SLUG's microwave circuit integration and modeling, whilst enabling potentially higher frequency operation. Here we present the design and modeling of a SLUG amplifier that can be realised from a single-layer of Nb, with nanobridge weak-links fabricated using either electron-beam (EBL) or focussed ion beam (FIB) lithography as the Josephson elements. We discuss the input and output impedance, the forward transimpedance, and the potential power gain assuming a matched source and load. We also discuss the necessary impedance matching of the SLUG to the characteristic 50 Omega on-chip microwave environment.

Superconducting circuits play a crucial role in the advancement of quantum computing and quantum sensing. Typically such circuits require the presence of a non-linear element, where the engineered anharmonicity (Kerr factor) and resonant linewidth determine the potential applications of the circuit. In this work we have fabricated Nb-based CPW resonators embedded with a DC SQUID incorporating Nb nanobridges as the weak links. We use two-tone spectroscopy to study the non-linear behaviour of the device at 15 mK up to a field of 2.48 mT. Under the application of a blue-detuned pump, the device shows a decrease in the resonant frequency which is used to estimate the Kerr factor. We further apply a red-detuned pump to go beyond the bifurcation threshold and observe the appearance of an additional idler mode with net gain. The gain of the device was maximized by further decreasing the pump frequency, showing a maximum amplification of 15 dB. Finally, we show agreement between the Kerr non-linear oscillator model and the measured transmission spectrum, and highlight further design modifications to improve the gain and bandwidth of such devices.

Recently the exploration of nonlinear microwave response in superconducting circuits has been extended to Nb nanobridges that can operate at higher magnetic fields, frequencies, and temperatures than their conventional Al counterparts. Nb nanobridge-based Superconducting Quantum Interference Devices (SQUIDs) have shown potential applications in quantum information processing, weak signal detection, and single spin detection. Combining such non-linear elements with circuit Quantum Electrodynamic (QED) elements like coplanar waveguide (CPW) resonators allows the study of the nonlinearity of such systems. In this work we report on the design, fabrication and dispersive microwave readout of a Nb DC nanoSQUID embedded in a superconducting quarter-wavelength resonator. The measured flux-tuneability of the resonance shows a modulation of a few MHz and fits simulations, giving a critical current of 330 mu A and a screening parameter beta(L) of 4.07. Following this we present results showing Kerr-type detuning with a pump tone applied to the device.

The Superconducting Low-inductance Undulatory Galvanometer (SLUG) amplifier is an ultra-low noise rf amplifier that is simple to operate and can be designed for a wide range of microwave frequencies. A variant of the dc SQUID rf amplifier, the SLUG differs in that the rf signal is directly injected in the SQUID loop, allowing high frequency operation. Here, we realize a planar SLUG amplifier constructed with nanobridge weak-link junctions that is fabricated from a single layer of niobium. Impedance models for the SLUG gain element are presented in conjunction with a discussion of appropriate impedance matching techniques. Experimental characterization of a proof-of-concept SLUG amplifier designed with a single-stub tuning circuit show modulation of the SLUG operation frequency in line with biasing of the SLUG, a well-matched input network and forward gain of 8 dB.

NanoSQUIDS incorporating nanobridges as the Josephson element are desirable since their small dimensions mean they can be situated in close proximity to very small, highly localised magnetic moments for high spin sensitivity. Key to achieving the necessary dimensions are the lithographic technologies used to fabricate the nanobridges. Here we present characterisation of Nb nanoSQUIDs incorporating nanobridges fabricated by three different techniques: neon focused ion beam (FIB) lithography, gallium FIB lithography and e-beam lithography (EBL). We discuss the critical currents and voltage-current characteristics achieved by each technique at various temperatures for a range of nanobridge widths, and discuss the corresponding nanoSQUID voltage modulation and potential applications.

SQUID-embedded superconducting resonators are of great interest due to their potential for coupling highly scalable superconducting circuits with quantum memories based on solid-state spin ensembles. Such an application requires a high-Q, frequency-tunable resonator that is both resilient to magnetic field and able to operate at millikelvin temperatures. These requirements motivate the use of a higher Hc metal such as niobium; however, the challenge then becomes to sufficiently reduce the operating temperature. We address this by presenting a monolithic Nb nanoSQUID-embedded resonator, where neon focused-ion-beam fabrication of the nanoSQUID results in a device displaying frequency tunability at T=16 mK. In order to assess the applicability of the device for coupling to small spin clusters, we characterize the flux sensitivity as a function of microwave drive power and externally applied magnetic field and find that the noise is dominated by dielectric noise in the resonator. Finally, we discuss improvements to the device design that can dramatically improve the flux sensitivity, which highlights the promise of Nb SQUID-embedded resonators for hybrid superconductor-spin applications.

The Superconducting Low-inductance Undulatory Galvonometer (SLUG) microwave amplifier is distinct from a conventional SQUID amplifier in that the signal to be amplified is directly injected into the SQUID loop, allowing high frequency operation. Here, we discuss a planar SLUG amplifier constructed with nanobridge weak-links fabricated from a single niobium layer. DC characterisation of the SLUG gain element is presented for two nanobridge fabrication techniques. Measurements show that junctions fabricated from Ne focused-ion-beam, and from 100 keV electron-beam lithography, have significantly different DC characteristics with critical currents on the order of 100 μA and 1mA respectively. Both variants exhibit maximum forward transfer functions greater than 1.5 mV/Φ0, which is larger than those reported in Nb SIS-junction SLUGs. Theoretical modelling has shown that an amplifier containing a planar, nanobridge SLUG as a gain element is expected to exhibit gain exceeding 15 dB with large instantaneous bandwidth, and can be designed to operate well in excess of 20 GHz.

Nitrogen-vacancy (NV) pair in diamond has been widely investigated due to its profound quantum properties even at relatively higher temperature compared with those materials having to satisfy strict cryogenic conditions to display quantum states. Spin properties of NV center in diamond can be tuned by external electromagnetic radiations, which leads to its application in detecting microwave signals. Here in this work, density functional theory (DFT) method has been used to analyze electronic properties of diamond with NV centers with the aim of utilizing it for detecting very weak microwave radiations from electron cryotron radiations, to ultimately determine neutrino mass.

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.

Microwave absorption in radar stealth technology is faced with challenges in terms of its effectiveness in low-frequency regions. Herein, we report a new laser-based method for producing an ultrawideband metamaterial-based microwave absorber with a highly uniform sheet resistance and negative magnetic permeability at resonant frequencies, which results in a wide bandwidth in the L- to S-band. Control of the electrical sheet resistance uniformity has been achieved with less than 5% deviation at 400 Ω sq−1 and 6% deviation at 120 Ω sq−1, resulting in a microwave absorption coefficient between 97.2% and 97.7% within a 1.56–18.3 GHz bandwidth for incident angles of 0°–40°, and there is no need for providing energy or an electrical power source during the operation. Porous N- and S-doped turbostratic graphene 2D patterns with embedded magnetic nanoparticles were produced simultaneously on a polyethylene terephthalate substrate via laser direct writing. The proposed low-frequency, wideband, wide-incident-angle, and high-electromagnetic-absorption microwave absorber can potentially be used in aviation, electromagnetic interference (EMI) suppression, and 5G applications.

In 2021 the Quantum Sensors for the Hidden Sector (QSHS) collaboration was founded in the UK and received funding to develop and demonstrate quantum devices with the potential to detect hidden sector particles in the \muμeV to 100 \muμeV mass window. The collaboration has been developing a range of devices. It is building a high-field, low-temperature facility at the University of Sheffield to characterise and test the devices in a haloscope geometry. This paper introduces the collaboration’s motivation, aims, and progress.

Josephson junctions epitomize one of the most profound macroscopic quantum aspects of superconductivity, while at the same time enabling a wider range of applications than any other devices of superconducting electronics. In the past two decades, ferromagnetic barriers further enriched the junctions' physical properties range and promise application of junctions with such barriers in superconducting memories and quantum computing. All of the devices described in this section employ the Josephson effects and many also rely on combining this macroscopic quantum property with another fundamental quantum principle exemplified in superconductors, that is magnetic flux quantization. The first is superconducting logic gates which have been realized in a number of different forms, the front runner for widespread application at the time of writing being single flux quantum logic in several energy-efficient variants. The second major application device which combines both Josephson effects and flux quantization is the SQUID.

Metrology is the science of precision measurement of physical quantities and has developed a close relationship with superconductivity and superconducting materials, beginning with the discoveries of magnetic flux quantization and the Josephson effects in 1961 and 1962. Programmable Josephson voltage standard systems were first proposed and developed at NIST and are nowadays fully demonstrated in numerous metrology measurements and operated in liquid-cryogen-free refrigerators. A Josephson array device with growing applications to ac voltage metrology has emerged in recent years: the pulse-driven Josephson arbitrary waveform synthesizer. The use of superconductors in solenoids allows current densities to be much higher than for conventional conductors due to zero Joule heating. The Josephson effect has been applied to temperature measurement through the detection of Johnson noise using various superconducting junction-based technologies. The most recent application of Josephson technologies to temperature metrology involves the use of quantum-based reference noise-voltage waveforms synthesized by arrays of Josephson junctions.

In 2021 the Quantum Sensors for the Hidden Sector (QSHS) collaboration was founded in the UK. We received funding as part of the Quantum Technologies for Fundamental Physics programme to develop and demonstrate quantum devices with the potential to detect hidden-sector particles such as axions and dark photons in the ~1μeV/c^2 to ~100μeV/c^2 mass window. Our collaboration is formed from a diverse community of condensed matter physicists, quantum technologists, theorists and (astro) particle physics experimentalists. We are designing and fabricating a range of “quantum devices” and constructing a high-field, low-temperature facility at the University of Sheffield to characterise and test the devices in an axion haloscope geometry, initially using an rf cavity. This poster was presented at the 17th Patras workshop on axions, WIMPs and WISPs on the 8th August 2022 in Mainz, Germany.

We describe a novel coupled microwave dielectric resonator system (an unusual form of parametric amplifier), which is under development, aimed at realizing a high-resolution single X-ray photon energy detector. Unlike other high-resolution detectors, it operates at a relatively high cryogenic temperature in the range of 7-30 K. Construction of the cooler system and thermal characterization system is completed. All aspects of the novel calorimeter performance have been evaluated, including demonstration of single-particle detection, the required rapid thermal time constant of a coupled resonator, a high d f/dT response, and promising development of a low-noise microwave detection system.

Conductive thin films are an essential component of many electronic devices. Measuring their conductivity accurately is necessary for quality control and process monitoring. We compare conductivity measurements on films for flexible electronics using three different techniques: four-point probe, microwave resonator and terahertz time-domain spectroscopy. Multiple samples were examined, facilitating the comparison of the three techniques. Sheet resistance values at DC, microwave and terahertz frequencies were obtained and were found to be in close agreement.

A thin split ring resonator (SRR) structure with different SRR orientations is designed and simulated in this paper for X band and Ku band radar cross section (RCS) reduction. The chessboard structure was consisted by two types of SRR arrays with opposite reflection phases, where reflected incident waves from both SRR arrays are cancelled by themselves. For better absorption performance, SRRs are printed by reduced graphene oxide (rGO) rather than traditional metallic materials. The sheet resistance of SRRs have been optimized in this paper for better matching with free space, which enhanced the absorption of incident wave, also increased the bandwidth significantly. Both of the unit cell SRR, $4 \times 4$ and $8 \times 8$ chessboard structures are analyzed quantitatively.

SQUIDs (Superconducting Quantum Interference Devices) are macroscopic quantum devices capable of detecting and measuring a wide variety of physical parameters with unprecedented sensitivity. SQUIDs based on nanobridge weak links have shown increasing promise for quantum information and quantum sensing applications such as single spin detection. Focussed ion beam etched nanobridges have properties which can enhance nanoSQUID device performance but are often limited in terms of their non-hysteretic operating temperature range. Here we describe measurements of FIB-milled nanobridges, as single weak links or in nanoSQUIDs, made using either Ga, Xe or Ne ion beam sources. Their properties as a function of temperature, bias current, magnetic field and microwave power are measured and modelled according to a range of superconductivity models, as a means for improved understanding of the associated nanobridge parameters. We further propose techniques to extend the non-hysteretic operating temperature range of the devices.

A coupled microwave dielectric resonator system is under development, aimed at realizing a high resolution single X-ray photon energy detector, operating at relatively high cryogenic temperature, around 4K. Construction of the cooler system and thermal characterisation system is completed, with a demonstrated base temperature of 2.2 K. All aspects of the novel calorimeter performance have been evaluated, including demonstration of the required rapid thermal time constant of a coupled resonator, a high df/dT response and promising development of a low noise microwave detection system.

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.