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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 $df/dT$ 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 intrated devices show potential to be used for flux-tunable sensors.

Since the carrier density and resistivity of graphene are heavily dependent on the Fermi level, Josephson junctions with graphene as the weak link can have their I – V properties easily tuned by the gate voltage. Most of the previous work on superconductor–graphene–superconductor (SGS) junctions and superconducting quantum interference devices (SQUIDs) were based on mechanically exfoliated graphene, which is not compatible with large scale production. Here, we show that SGS junctions and dc SQUIDs can be easily fabricated from chemical vapor deposition (CVD) graphene and exhibit good electronic properties. The SGS junctions can work without any hysteresis in their electrical characteristics from 1.5 K down to a base temperature of 320 mK, and the critical current can be effectively tuned by the gate voltage by up to an order of magnitude. As a result, dc SQUIDs made up of these junctions can have their critical current tuned by both the magnetic field and the gate voltage.

This paper outlines how demands on electromagnetic metrology have developed over recent years with the continuing reduction in the typical length scale of electronic components and circuits. In addition, novel materials, especially 2-D self-supporting structures such as graphene, have properties quite unlike the conventional systems. At the nanoscale radical changes occur in techniques required and even in the physical quantities measured. This paper focusses on the example of a near-field scanning microwave microscope and its application to nano-electromechanical system resonators to illustrate these changes and challenge.

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 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.

Superconducting QUantum Interference Devices (SQUIDs) based on nanobridge junctions have shown increasing promise for single particle detection. This paper describes the development of the fabrication of improved and reproducible nanobridge junctions fabricated by focused ion beam (FIB) milling from niobium thin films. Although the very low noise properties of nanobridge SQUIDs are well known, the nature of the milling process is little understood at the level of local superconducting properties. In this paper, we report the results for nanobridge Josephson devices and SQUIDs, which we believe are the first to be made by Xenon (Xe) FIB milling. Temperature-dependent current–voltage behavior, microwave-induced Shapiro steps, and SQUID response to magnetic fields have been measured. We make preliminary comparisons with nominally identical devices milled from Nb thin films using either Xe or Ga ions.

Josephson junctions and SQUIDs with graphene or other 2D materials as the weak link between superconductors have become a hot topic of research in recent years, with respect to both fundamental physics and potential applications. We have previously reported ultra-wide Josephson junctions (up to 80 {\mu}m wide) based on CVD graphene where the critical current was found to be uniformly distributed in the direction perpendicular to the current. In this paper, we demonstrate that the unusually large Josephson penetration depth {\lambda}_J that this corresponds to is enabled by the unique geometric structure of Josephson junctions based on 2D materials. We derive a new expression for the Josephson penetration depth of such junctions and verify our assumptions by numerical simulations., Comment: 15 pages, 4 figures

We discuss steps toward the readout of the position of a nanoelectromagnetic system (NEMS) beam resonator using a Nb nanosuperconducting quantum interference device (nanoSQUID). We describe our fabrication procedure for coupling the nanoSQUID and a suspended Al-coated Si3N4 NEMS resonator together by a combination of focused ion beam lithography and nanomanipulation. We discuss typical electrical characteristics of the integrated devices, and independent postfabrication atomic force microscope nanoindentation measurements of the elastic properties of the integrated resonator to estimate its natural frequencies of vibration. We compare and discuss the response of a nanoSQUID with current-carrying and superconducting screening (noncurrent carrying) modes of operation of the resonator

The macroscopic quantum nature of SQUID devices has been amply described in earlier chapters in this book. Here we deal with the coming together of two recent streams of solid state device development, namely superconductivity and nanoscience, together with the applications which are growing out of this conjunction. As the size of superconducting devices shrinks below the micrometre scale, better performance is promised, new applications reveal themselves but new challenges in fabrication also emerge at this scale. Different Josephson junction types are required and a move from 2D to 3D structures is apparent. This chapter summarises progress achieved so far and points towards future possible developments.

Over the past decade, nanoscale superconducting quantum interference devices (nanoSQUIDs) have rapidly risen from nowhere to forge a new sphere of applications of these macroscopic quantum devices. New fabrication techniques have enabled these advances. In this Perspective, we highlight another recent major development in this area-the demonstration of a three-axis nanoSQUID magnetometer, which enables the vector magnetization of a nanoscale magnetic particle to be measured in the presence of an applied magnetic field. We illustrate the technological demands and developments that have driven the development of nanoSQUIDs and make suggestions for future directions for applications.

In this work we present combined Kelvin probe force microscopy and Raman spectroscopy studies of supported and suspended structures formed out of chemical vapor deposition (CVD) grown graphene. Work function of both suspended and supported graphene was -4.81 ± 0.06eV and -4.92 ± 0.06eV respectively. By G and 2D modes correlation we showed, that CVD graphene was influenced by biaxial strain. Increased contact potential difference (CPD) on the suspended graphene in comparison with the areas of the supported graphene was the sign of increased strain (from 0.05% to ~ 0.12%) rather than decreased doping (p-doping decreased from ~ 5.5 × 1012cm-2 to ~ 4.5 × 1012cm-2).

The self-supporting monolayer material which is graphene has excited enormous interest over the ten years since its discovery due to its remarkable electrical, mechanical thermal and chemical properties. In this paper we describe our work to develop chemical vapour deposition methods to grow monolayer graphene on copper foil substrates and the subsequent transfer process. Raman microscopy, scanning electron microscopy and atomic force microscopy (AFM) are used to examine the quality of the transferred material. To demonstrate the process we describe transfer onto patterned SiO2/Si substrates which forms freely suspended graphene with focus on circular wells forming graphene drums. These show interesting mechanical properties which are being explored as nanomechanical resonators. UK NMS Programme, the EU EMRP (European Metrology Research Programme) projects MetNEMS and GraphOhm

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.

This paper outlines how demands on electromagnetic metrology have developed over recent years with the continuing reduction in the typical length scale of electronic components and circuits. In addition novel materials, especially 2D self-supporting structures such as graphene, have properties quite unlike conventional systems. At the nanoscale radical changes occur in techniques required and even in the physical quantities measured. This paper focusses on the example of a nearfield scanning microwave microscope and its application to nanoelectromechanical system resonators to illustrate these changes and challenges. UK NMS Program; EPSRC

Nano-SQUIDs which can be the readout for inductive superconducting transition edge detectors (ISTED) were fabricated on monolayer niobium (Nb) film by focus ion beam (FIB) milling. The dimension of the nanobridges which formed the junctions was less than 60 nm and the diameter of the loop was $20\ \mu \text{m}$ , The properties of this nano-SQUID were investigated. The design of ISTED was introduced and the transition temperature $(T_{\text{c}})$ of the absorbing layer was adjusted by both the thickness and the proportion Si doping of Nb.

We use a contactless microwave dielectric resonator gas sensing platform to study the adsorption dynamics of NO2 gas present in air onto a graphene surface. The use of microwaves removes the need for metal contacts that would otherwise be necessary for traditional conductivity measurements, and therefore allows non-invasive determination of NO2 concentrations to sub parts per million. As a result, gas-metal interactions and localised graphene doping in the vicinity of metal contacts are eliminated, with the advantage that only graphene-gas adsorbate interactions are responsible for the measured signal. We show that the sensor response for all considered concentrations can be described using a surface coverage dependent Langmuir model. We demonstrate that the possible variation of the NO2 binding energy, which is frequently considered as the main parameter, plays only a secondary role compared to the rising adsorption energy barrier with increasing NO2 coverage. The continuous distribution of the properties of the graphene adsorption sites used in the theoretical model is supported by our Kelvin probe and Raman surface analysis. Our results demonstrate that the non-invasive microwave method is a promising alternative platform for gas sensing. Moreover it provides valuable insights towards the understanding of the microscopic processes occurring in graphene based gas sensors, which is a key factor in the realization of reproducible and optimized device properties.

Epitaxial graphene is a promising route to wafer scale production of electronic graphene devices. Chemical vapor deposition of graphene on silicon carbide offers epitaxial growth with layer control, but is subject to significant spatial and wafer-to-wafer variability. We use terahertz time-domain spectroscopy and micro four-point probes to analyze the spatial variations of quasi-free-standing bi-layer graphene grown on 4 inch silicon carbide (SiC) wafers, and find significant variations in electrical properties across large regions, which are even reproduced across graphene on different SiC wafers cut from the same ingot. The DC sheet conductivity of epitaxial graphene was found to vary more than one order of magnitude across a 4 inch SiC wafer. To determine the origin of the variations, we compare different optical and scanning probe microscopies with the electrical measurements from nano- to millimeter scale and identify three distinct qualities of graphene, which can be attributed to the microstructure of the SiC surface.

We report here recent work at the U.K. National Physical Laboratory on developing nanoscale SQUIDs using Dayem bridge Josephson junctions. The advantages are simplicity of fabrication, exceptional low-noise performance, toward the quantum limit, and a range of novel applications. Focused ion beam patterned Nb SQUID, possessing exceptionally low noise (∼200 nΦ0/Hz1/2 above 1 kHz), and operating above 4.2 K can be applied to measurement of nanoscale magnetic objects or coupled to nanoelectromechanical resonators, as well as single particle detection of photons, protons, and ions. The limited operating temperature range may be extended by exposing the Dayem bridges to carefully controlled ion beam implantation, leading to nonreversible changes in junction transition temperature. The work reported here was supported in part by the EMRP projects ‘MetNEMS’ NEW-08 and ‘BioQUART’SIB-06. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

Conducting graphene-based hybrids have attracted considerable attention in recent years for their scientific and technological significance in many applications. In this work, conductive graphene hybrid films, consisting of a metallic network fully encapsulated between monolayer graphene and quartz–glass substrate, were fabricated and characterized for their electromagnetic interference shielding capabilities. Experimental results show that by integration with a metallic network the sheet resistance of graphene was significantly suppressed from 813.27 to 5.53 Ω/sq with an optical transmittance at 91%. Consequently, the microwave shielding effectiveness (SE) exceeded 23.60 dB at the Ku-band and 13.48 dB at the Ka-band. The maximum SE value was 28.91 dB at 12 GHz. Compared with the SE of pristine monolayer graphene (3.46 dB), the SE of graphene hybrid film was enhanced by 25.45 dB (99.7% energy attenuation). At 94% optical transmittance, the sheet resistance was 20.67 Ω/sq and the maximum SE value was 20.86...

An evanescent field microwave microscope has been developed at the National Physical Laboratory. This instrument has multiple applications and has been developed to allow traceable measurements of local complex permittivity, unlike most other microwave scanning microscopes. In this paper we describe basic operation of the microscope and show measurements on graphene samples produced at Imperial College. The microscope obtains images by raster scanning of a wire probe in ‘contact mode’. Of particular interest to the graphene community is the possibility of being able to scan over large areas (up to 4×4 mm2), and to be able to measure actual values of surface resistance without a requirement for metal contacts. As an ultrathin semimetal, a graphene layer being placed in the evanescent field of the probe is expected to behave like a lossy dielectric material, its microwave loss tangent is proportional to its conductivity. Employing a high Q dual mode re-entrant cavity as host resonator and a spherical metal probe of 180 μm diameter, we found that spatial variations of the conductivity of graphene can be clearly resolved.

Due to the urgent demand for portable electronics, enormous research efforts are devoted for the development of flexible, wearable and lightweight supercapacitors. Herein, free-standing, binder-free and high-performance metal @ carbon sphere/graphene film electrodes (Fe@C/G and Co@C/G), consisting of β-cyclodextrins (β-CDs) as carbon sources, iron and cobalt as metal materials, and graphene sheets as “binders”, have been synthesized and characterized. The resultant Fe@C/G and Co@C/G film electrodes exhibit maximum areal specific capacitances of 2.19 F cm−2 and 0.80 F cm−2 at 0.5 mA cm−2 (438 F g−1 and 160 F g−1), respectively. Furthermore, a flexible asymmetric all-solid-state supercapacitor (Fe@C/G//Co@C/G) is assembled and it displays a series of intriguing features including high volumetric specific capacitance of about 13.3 F cm−3 at 0.5 mA cm−2, excellent volumetric energy density of up to 5.99 mWh cm−3 at a volumetric power density of 0.013 W cm−3, and good cycle stability of 87.3% capacitance retention after 10,000 charge/discharge cycles. Even after 500 bending/unbending cycles, still about 80.5% of its initial capacitance is retained. Such results well demonstrate its strong potential for the practical applications of flexible energy storage devices.

NanoSQUIDs with Dayem nanobridge weak-link junctions were fabricated on single layer niobium film by focus ion beam(FIB) milling. The dimension of the nanobridge is less than 60 nm and the loop scale is from 400×400 nm to 2.5×20 μm. The working temperature range for this device was researched. The voltage-flux transfer properties were measured at different working temperatures.

We show that a microwave near-field coaxial resonator system allows mechanical cantilever excitation on a scale much shorter than the microwave wavelength. Thermal noise is observed in the unexcited system, enabling room temperature displacement sensitivity of ∼70 fm/Hz 1/2 . The measured force between near-field probe and cantilever varies with separation, in excellent agreement with theory. Uniquely, optical excitation and read-out lasers are also included. The dependence of mechanical resonator quality factor on ambient air pressure has been accurately measured. We have demonstrate passive cantilever mode cooling from 300 K to 80 K by frequency detuning the microwave resonator and propose pulsed cooling operation to enable several high-sensitivity applications.

NanoSQUID sensors (Superconducting Quantum Interference Devices with nanoscale dimensions) are regarded as a useful tool for sensitive magnetic measurements and have achieved a low noise level. The nanoSQUIDs comprising nanobridge junctions are fabricated by photolithography followed by focused ion beam patterning. While previously such self-shunted nanobridge junctions have shown hysteretic characteristics at 4.2 K, we propose an approach to increase the operational temperature in order to reduce the critical current of the device. We report the design of a readout system, comprising all necessary components to achieve a low noise readout, which is limited by the nanoSQUID noise. It consists of a variable temperature (5-10 K) liquid helium probe stick, a coil system and a SQUID preamplifier. The orthogonal coil system enables particle magnetization up to 100 mT in parallel to and a flux bias of up to 28 mT perpendicular to the nanoSQUID, to set the working point. The nanoSQUID achieves a low white noise level of 0.23 μΦ0/√Hz including a readout contribution of 0.10 μΦ0/√Hz.

An important requirement across a range of sensitive detectors is to determine accurately the energy deposited by the impact of a particle in a small volume. The particle may be anything from a visible photon through to an X-ray or massive charged particle. We have been developing nanobridge Josephson junctions based SQUIDs and nanoSQUID devices covering the entire range of particle detection energies from 1eV to MeV. In this paper we discuss some developments in nanobridge Josephson junctions fabrication using focussed ion beam (FIB) and how these developments impact future applications. We focus on tuning of the transition temperature of a superconducting thin-film absorber, with the aim to match the absorber Tc to the working temperature range of the SQUID and also on using a new Xe FIB to improve Josephson junction and superconducting film quality.

Great progress has been made over the past 50 years in replacing artefact definitions of the SI base units by those based on fundamental constants. However it is important to recognise that the development of the SI is not ‘complete’ once the base units are all defined in this way. New physics and technologies can always be expected to emerge and these will require developments and modifications to the SI. At the present time a dominant trend seems to be that measurements are changing from dealing with average properties of ensembles to metrology on single entities. This is already apparent for single atoms, ions or photon detection and measurement but can be expected to extend into the technologically important areas of single spins, single THz photons and even phonons in the future. Another significant development may be expected towards measurement of quite different physical quantities from those currently required, such as spin current, decoherence time etc. The paper addresses some of these issues, as well as the influence of quantum mechanics on precise measurement, which will also increase as metrology addresses both single entities and the nano scale.

Magnetic sensors having high spatial and stray field resolutions are key elements in many biomedical applications. One promising magnetic detector is a microsized Hall sensor. We present our first results in realising a measurement system based on a Hall sensor made of an asymmetric 2DEG InAs/GaSb-based heterostructure. The work aims to investigate magnetotransport properties of such Hall sensors and optimize their performance. In particular, we focus on examining noise characteristics of the sensor as it allows us to determine and improve the device sensitivity. We show that in investigated devices a magnetic field sensitivity of better than 0.5 mu T/Hz1/2 (corresponding to a magnetization detection threshold of 2times105 muB/Hz1/2) should be readily achievable at room temperature and at a frequency of around 3 kHz.

The ability to reduce SQUID dimensions into the sub-micrometer or nanometer regime points the way towards novel applications, particularly in emerging fields such as quantum information processing, single-photon/particle detection, and experimental studies of nano-scale entities such as Bose-Einstein condensates. We report here on our ongoing work combining traditional thin-film and photolithographic fabrication processes with computer-aided-design software and focused ion beam milling to realize sub-micrometer superconducting structures. Their magnetic field sensitivity, noise behavior, spatial resolution, and prospects for magnetic spin detection are discussed.

As metrology extends toward the nanoscale, a number of potential applications and new challenges arise. By combining photolithography with focused ion beam and/or electron beam methods, superconducting quantum interference devices (SQUIDs) with loop dimensions down to 200 nm and superconducting bridge dimensions of the order 80 nm have been produced. These SQUIDs have a range of potential applications. As an illustration, we describe a method for characterizing the effective area and the magnetic penetration depth of a structured superconducting thin film in the extreme limit, where the superconducting penetration depth A is much greater than the film thickness and is comparable with the lateral dimensions of the device

As metrology extends toward the nanoscale, a number of potential applications and new challenges arise. By combining photolithography with focused ion beam and/or electron beam methods, superconducting quantum interference devices (SQUIDs) with loop dimensions down to 200 nm and superconducting bridge dimensions of the order 80 nm have been produced. These SQUIDs have a range of potential applications. As an illustration, we describe a method for characterizing the effective area and the magnetic penetration depth of a structured superconducting thin film in the extreme limit, where the superconducting penetration depth $\lambda$ is much greater than the film thickness and is comparable with the lateral dimensions of the device.

This paper extends a preliminary proposal and description of a superconductivity-based phase conjugating antenna for operation in the microwave range. The complementary properties of high-temperature superconductors and other oxide functional materials are emphasised. The potential applications of such an antenna are very wide. We consider the requirements for the Josephson junction which is at the heart of the phase conjugation operation, especially issues of the use of the internal Josephson oscillation or of an external signal to provide the local oscillator requirement. Other design requirements are also explored and the necessary substrate properties are set out which enable suitable junction properties, as well as phase shifter and interconnection functions.

Single and two dimensional carbon materials represent a major development in advanced materials of the past two decades, with potential major disruptive applications across ICT, high frequency electronics. The microwave interactions of these materials has only much more recently been investigated. In this paper we present details of a very sensitive non-invasive method to characterize the electrical properties of graphene at the large scale, also the investigation of carbon nanotubes as high frequency interconnects and graphene nano-electromechanical resonators driven and detected by a microwave resonator near-field technique. We speculate on future microwave and reduced dimension carbon applications.

The effect of a bilayer area on the electronic response to environmental gating of a monolayer graphene Hall bar device is investigated using room temperature magnetotransport and scanning Kelvin probe microscopy measurements in a controlled environment. The device is tuned through the charge neutrality point with n-p-n-junctions formed. Scanning Kelvin probe measurements show that the work function of the monolayer graphene decreases more than that of the bilayer area however magnetotransport measurements show a larger change in carrier concentration for bilayer graphene with environmental gating. Interface scattering at the boundary between the monolayer and bilayer regions also affects device response with field-dependent suppression of the conductivity observed near the charge neutrality point. Simultaneous electronic and environmental scanning Kelvin probe measurements are used to build nano-scale maps of the work function of the device surface revealing the areas of greatest work function change with environmental gating. Crown Copyright (C) 2015 Published by Elsevier Ltd. All rights reserved. Funding Agencies|UK NMS Programme; EU; EMRP the participating countries within EURAMET; European Union; European Union [604391]; UK Department for Business, Innovation and Skills (BIS) in Sensor networks: data to knowledge [IRD/2013/09]; graphene flagship

The exfoliation of oxygenated functional groups from 60 W hydrogen plasma treated graphene oxide films was investigated using X-ray diffractometric (XRD), Raman spectroscopic and atomic force microscopic techniques. The interlayer spacing of the graphene oxide sheets was found from the XRD pattern to decrease from 0.88 to 0.35 nm after plasma treatment. The reduced intensity ratio of the D and G peaks of the Raman spectra indicates a decrease in the crystallite size of the sp 2 domains due to plasma treatment. Atomic force microscope showed the continuous morphology of the plasma treated film. The electrical properties of plasma treated samples spin-coated on silicon were studied using Van Der Pauw and non contacting microwave techniques. The sheet resistivity determined from Van der Pauw measurements was $$1.62\,{\text{M}}\Omega /{\text{sq}}$$ , yielding the value of $$3.1\,{\text{S}}\,{\text{m}}^{ - 1}$$ for the bulk conductivity. The charge mobility of $$3.8\,{\text{m}}^{2} \,{\text{V}}^{ - 1} \,{\text{s}}^{ - 1}$$ has been determined from Hall measurement technique.

Graphene is a remarkable material, which is yet to make the transition from unique laboratory phenomenon to useful industrial material. One missing element in the development process is a quick method of quality control of the electrical properties of graphene which may be applied in, or close to, the graphene growth process on an industrial scale. In this study, the authors describe a non-contact method using microwave resonance which potentially solves this problem. They describe the technique, consider its limitations and accuracy and suggest how the method may have future take up. UK NMS Programme, the EU EMRP project ‘GraphOhm’ and ‘MetNEMS’. The EMRP (European Metrology Research Programme)

A complete characterization of high temperature superconducting films, as a function of microwave power, must track changes of the surface resistance and the penetration depth. A key problem has always been tracking power-dependent nonlinearities associated with a change in penetration depth. In this paper we report a technique that exploits the generic response of resonators to a microwave impulse. We show that a transient response is observed which has a frequency equal to the difference between the driving frequency and the resonant frequency of the resonator. Moreover, we demonstrate how this transient response can be used to determine the nonlinear properties of superconducting thin films.

The past 35 years have seen the development of an unexpected plethora of quantum electrical standards based on just two fundamental constants, e and h . First came a voltage standard based on the Josephson a.c. effect, in terms of which most maintained primary standards of voltage are defined. This was followed a decade later by the quantized Hall effect, based on the von Klitzing constant which allows the ohm to be maintained very precisely. It became clear 20 years ago that there is also a possible quantum current standard. This third standard has yet to play a full part in practical electrical metrology. However, recent developments suggest that there are many different possible manifestations in which such a current standard might be realized. The three quantum standards, taken together, define the quantum electrical triangle of standards which would allow the units to be realized in terms of different combinations of e and h . We summarize the very different physics behind the three standards, reviewing the present state of development in all three. Implications for the future are also considered, especially relating to ultra-low temperature, nanoscale and truly quantum mechanical versions of the standards.

Detectors with sufficient sensitivity to resolve the energy of single photons and particles combined with rapid response times are becoming increasingly important. We describe work in progress on a recently-proposed type of sensor, which we have named the Inductive Superconducting Transition Edge Detector (ISTED). We have theoretically estimated the energy sensitivity of such a sensor, in the case where the heat absorber is incorporated within the sensitive area of a nano-scale SQUID which acts as an inductive readout device. Recent results on the experimental characterization of prototype devices in the temperature range 5-8 K are presented, and ongoing work which is intended to achieve smaller device dimensions together with lower operating temperatures is described.

We report the development of a novel form of bolometric detector, which may have widespread application to particle detection and energy resolved measurement, and may be applicable to massive particles, photons or even phonons. The basis for bolometric operation is the response of coupled microwave dielectric resonators, one of which has an almost temperature independent resonant frequency while the other, much smaller, resonator has strongly temperature-dependent permittivity.

The dynamic range, ratio balance error, and current resolution have been evaluated for an upgraded prototype high-temperature superconductor (HTS) dc current comparator, which has been previously developed and demonstrated for the noninvasive sensing of charged particle beams. The addition of a custom-designed 2-SQUID gradiometer coupled to an S-shaped current concentrator is expected to provide an improvement in the flux sensitivity, as well as a reduction in the stray coupling of external fields. In the longer term, when the limiting factors have been fully identified, the technique may be considered for potential applications in fundamental metrology, for example, in a physical determination of the faraday by an ion-collection method.