A new joint research project (JRP) integrating metrology institutes and universities from nine countries is aimed at realization of a new generation of standards for quantum resistance metrology. The project exploits graphene's properties to simplify operation of standards without compromising the unprecedented precision delivered by semiconductor quantum Hall devices. Higher operating temperatures (above 4.2 K, and up to 8 K) and together with lower magnetic fields (below 5 T, and potentially down to 2 T) will lead to a significantly improved and cost-saving dissemination of intrinsically referenced resistance standards to all end-users relying on electrical measurements.

The Reference Step Method for calibrators can be modified for calibration also of the gain ratios on a meter. The method has earlier been evaluated in the range 100 mV-1000 V on DMM HP 3458A1 and shows that an accuracy of typically < ± 0.2 μ V/V could be obtained. We have investigated the performance of the method when calibrating ratios in the range 1 mV-100 mV on two common nV-meters. In our comparisons with the Josephson Voltage Standard the differences are within ± ± 12μ V/V at the ratio 10m V:1mV and within ± ± 1.3μ V/V at 100mV:10mV.

We report the first precision QHR measurements at SP using a graphene chip. We compare the results of a resistance calibration using GaAs based chips with the results using a graphene chip. The results agree within a few parts in 10⁹ for calibrations of 100 Ω and 10 kΩ resistors. Consistency checks indicate that the uncertainty is lower with the graphene chip, and the noise level is slightly lower. The measurements with the graphene chip were performed exclusively at 4.2 K, which simplifies the calibration procedure considerably compared with GaAs chips.

We present an active low pass filter constructed for Delta-Sigma modulated Josephson waveforms. The filter has a fifth order Butterworth characteristic with high input impedance and stable passband performance, including dc. We present results from frequency response, input impedance characteristics, passband ac-dc measurements and spectra of a Delta-Sigma modulated sine wave from a programmable Josephson voltage standard.

We present the design and electrical characteristics of an active low-pass filter. It will be used in a reference waveform source based on a programmable Josephson voltage standard with multilevel delta-sigma modulation to generate accurate and spectrally pure arbitrary waveforms. The filter has a fifth-order Butterworth characteristic with high input impedance, low distortion, high stopband attenuation, and stable passband performance, including dc.

This article presents a method for realising absolute phase and ac resistance for current shunts using only impedance ratio measurements. The method is based on three geometrically identical current shunts with different resistances, but with the same inductance, capacitance and ac resistance (change of resistance at ac compared to dc), We demonstrate how the inductance, capacitance and ac resistance can be calculated from the complex impedance ratio measurements, thereby realising absolute current shunt impedance. This method simplifies the procedure of current shunt calibration, since the same impedance ratio setup which is used to compare a shunt to a reference shunt, is used to realise the impedance of the starting reference shunt.

This paper presents a simplified method for realizing absolute phase and ac resistance for current shunts using only impedance ratio measurements. The method is based on three geometrically identical current shunts with different resistances, but with the same inductance, capacitance, and ac-dc resistance change of resistance at ac compared to dc). We demonstrate how the inductance, capacitance, and ac resistance can be calculated from the complex impedance ratio measurements, therefore realizing absolute current shunt impedance. The method gives competitive uncertainties of around 200 μΩ/Ω for amplitude and 400 μrad for phase at 1 MHz in the 1-Ω range.

We present a novel method of generating an ac voltage using a programmable Josephson voltage standard. The ac voltage is generated by Δ-Σ (Delta-Sigma) modulation of multiple Josephson levels. Compared with an ac voltage generated by stepwise approximation, the Δ-Σ method reduces the low frequency harmonics of the generated signal. We demonstrate this method by generating a 90 Hz sine wave and measuring the spectrum with a digitizer.

We present a reference system for calibration of electrical power at frequencies up to 1 MHz. The system consists of several components and we have developed methods of calibrating the components in order to ensure traceability to the SI units. The reference system is designed for steady state sinusoidal signals and gives a preliminary uncertainty of 50 μW/VA at frequencies up to 1 kHz, and 10 mW/VA at 1 MHz.

We have developed methods to characterise voltage dependence of the phase response in voltage dividers. The voltage dependence is a combination of two parts, a linear dependence and a power dependence due to voltage and temperature dependence in capacitors respectively. The standard uncertainty of the phase at 240 V and 100 kHz is estimated to be 20 μrad.

GIQS: Graphene Impedance Quantum Standard is a Joint Research Project of the European Metrology Programme for Innovation and Research (EMPIR). The project objective is to combine novel digital impedance measurement bridges with graphene-based ac quantum Hall resistance standards in a simplified cryogenic environment, to achieve simple, user-friendly quantum impedance standards suitable for primary realisation of impedance units in national metrology institutes, calibration centers, and the industry.

Josephson impedance comparison bridges have been developed at PTB and SP. The bridges are based on programmable Josephson voltage standards. We report the first ever comparison of Josephson impedance bridges performed with 1:1 and 1:10 capacitance ratio measurements up to 2 kHz.

We describe a low frequency Josephson impedance bridge and the measurement methods. The bridge is useable below 400 Hz and is based on two programmable Josephson voltage standards. Measurements of 1:1, 1:2 and 1:10 capacitance ratio have been performed and compared with ratio measurements of an inductive voltage divider bridge. © 2016 IEEE.

We describe high accuracy low-frequency ac-dc transfer determinations performed with a programmable Josephson voltage standard. The ac-dc difference characteristic of a planar multijunction thermal converter is determined indirectly by comparison via a high input impedance thermal transfer standard at 0.8 V in the frequency range 10 Hz to 110 Hz. 

We describe the work towards a setup of a low frequency 'Josephson impedance bridge' based on programmable Josephson voltage standards. Tests of ac voltage synthesis and dc voltage measurements of the two programmable Josephson standards have been performed including a direct Josephson dc voltage comparison.

Graphene quantum Hall effect (QHE) resistance standards have the potential to provide superior realizations of three key units in the new International System of Units (SI): the ohm, the ampere, and the kilogram (Kibble Balance). However, these prospects require different resistance values than practically achievable in single graphene devices (~12.9 kΩ), and they need bias currents two orders of magnitude higher than typical breakdown currents IC ~ 100 μA. Here we present experiments on quantization accuracy of a 236-element quantum Hall array (QHA), demonstrating RK/236 ≈ 109 Ω with 0.2 part-per-billion (nΩ/Ω) accuracy with IC ≥ 5 mA (~1 nΩ/Ω accuracy for IC = 8.5 mA), using epitaxial graphene on silicon carbide (epigraphene). The array accuracy, comparable to the most precise universality tests of QHE, together with the scalability and reliability of this approach, pave the road for wider use of graphene in the new SI and beyond. © 2022, The Author(s).

Here we demonstrate a stable and tunable method to alter the carrier concentration of epitaxial graphene grown on silicon carbide. This technique relies on chemical doping by an acceptor molecule. Through careful tuning one can produce chemically doped graphene quantum resistance devices which show long-term stability in ambient conditions and have performance comparable to that of GaAs quantum resistance standards. This development paves the way for controlled device fabrication of graphene quantum hall resistance standards, which can be reliably tailored to operate below 5 T and above 4 K out-of-the-box, without further adjustments from the end-user.

One of the aspirations of quantum metrology is to deliver primary standards directly to end-users thereby significantly shortening the traceability chains and enabling more accurate products. Epitaxial graphene grown on silicon carbide (epigraphene) is known to be a viable candidate for a primary realisation of a quantum Hall resistance standard, surpassing conventional semiconductor two-dimensional electron gases, such as those based on GaAs, in terms of performance at higher temperatures and lower magnetic fields. The bottleneck in the realisation of a turn-key quantum resistance standard requiring minimum user intervention has so far been the need to fine-tune the carrier density in this material to fit the constraints imposed by a simple cryo-magnetic system. Previously demonstrated methods, such as via photo-chemistry or corona discharge, require application prior to every cool-down as well as specialist knowledge and equipment. To this end we perform metrological evaluation of epigraphene with carrier density tuned by a recently reported permanent molecular doping technique. Measurements at two National Metrology Institutes confirm accurate resistance quantisation below 5n-1. Furthermore, samples show no significant drift in carrier concentration and performance on multiple thermal cycles over three years. This development paves the way for dissemination of primary resistance standards based on epigraphene

Josephson voltage standards are well established for use at dc and low-frequency ac voltages. The increasing demand i) to provide traceability for arbitrary waveforms, ii) to extend the range of use to higher frequencies, and iii) to improve the accuracy is being addressed in the project Q-WAVE, which is jointly funded by the European Union and the participating countries within the European Metrology Research Programme (EMRP). Here, sampling measurements and procedures using binary-divided 1 V and 10 V arrays are investigated. In addition, a quantum-based analogue-to-digital converter for direct measurements of arbitrary waveforms is under development. It will contain several Josephson arrays in series biased by a pulse drive based on opto-electronics, in order to increase the output voltage.

We demonstrate reversible carrier density control across the Dirac point (Δ n∼ 1013cm-2) in epitaxial graphene on SiC (SiC/G) via high electrostatic potential gating with ions produced by corona discharge. The method is attractive for applications where graphene with a fixed carrier density is needed, such as quantum metrology, and more generally as a simple method of gating 2DEGs formed at semiconductor interfaces and in topological insulators.

The paper reports an interlaboratory comparison of low impedance measurements at frequencies relevant for electrochemical impedance spectroscopy (EIS) of commercial lithium-ion cells. The comparisons cover an impedance range from 50 ΌΩ to 100 mΩ across the full complex plane in a frequency range 0.01 Hz up to 5 kHz. A first comparison covered calibration of low impedance standards by reference digital sampling impedance setups in 4-terminal and 4 terminal-pair connections. A second comparison used commercial 4-terminal EIS meters to measure the low impedance standards characterised in the first comparison. 

The paper reports a results of series of interlaboratory comparisons of low impedance measurements at frequencies relevant for electrochemical impedance spectroscopy (EIS) of commercial lithium-ion cells. Two comparisons are presented. The first, bilateral comparison has focused on low impedance standards calibration in a full complex plane using digital sampling setups. The second comparison has focused on calibration and use of commercial 4-terminal battery EIS meters. Both comparisons have covered the impedance range from 50μ℧ to 100m℧ across the full complex plane in a frequency range from 0.01Hz up to 5kHz. Finally, the paper summarizes practices identified as critical for achieving measurement compatibility among various labs. © 2023 The Authors

The paper reports an interlaboratory comparison of low impedance measurements at frequencies relevant for electrochemical impedance spectroscopy (EIS) of commercial lithium-ion cells. The comparisons cover an impedance range from 50 μΩ to 100 mΩ across the full complex plane in a frequency range 0.01 Hz up to 5 kHz. A first comparison covered calibration of low impedance standards by reference digital sampling impedance setups in 4-terminal and 4 terminal-pair connections. A second comparison used commercial 4-terminal EIS meters to measure the low impedance standards characterised in the first comparison.

This paper describes a new four terminal pair digital sampling impedance bridge designed for frequency range up to 1MHz and small impedances, such as shunts. The bridge is capable of comparing impedance standards of arbitrary ratios in a full complex plane from approximately 100 kΩ down to 50mΩ, limited by maximum achievable current 3 A. To keep low uncertainties a new multiplexer was designed and a very simple and fully automated linearity correction method based on the pair of calculable resistors was developed and validated. The paper describes the design and details of the bridge topology, basic uncertainty budget and first results of the validation. The expanded uncertainty of impedance module is about 50 μΩ/Ω at 1MHz for impedance ratios up to 1:16 and voltage drops above 10mV and the expanded uncertainty of a phase angle was about 360 μrad/MHz. Expanded uncertainty for frequency 100 kHz about 10 μΩ/Ω was reached. Typical expanded uncertainty for low impedance ratios below 1:1.1 is only 35 μΩ/Ω. Set of measurement of impedance standards of known values and a small international comparison of ac-dc and phase angle errors of current shunts were carried out to validate the bridge capabilities in wide range of impedances. The validation measurements showed the deviations of the bridge are below 35 μΩ/Ω and below 350 μrad at 1 MHz.

This paper describes progress made by the NPL, NMISA and RISE collaboration on developing the "Next Generation"Kibble balance. Delays to the availability of the full size balances have concentrated work on the smaller Demonstration balances, which were developed by NPL to provide publicity for the redefinition of the kilogram. The Demonstration balances now work in an almost identical way to the planned, full size "Next Generation"Kibble balance, using the same software and much of the same electronics. This has allowed all three groups to solve problems and make advances that will speed up the work on the "Next Generation"Kibble balance. It will also allow the investigation of wider applications of Kibble balance technology using the software and compact electronics which has been developed. 

This paper describes a method for realisation of inductance and quality factor to high frequencies by determining the frequency response of gain-and phase-error of an inductance meter using two coils made of a single copper wire. As a starting point a traceable calibration of inductance at 1 kHz is used

A new phase comparator for current shunts with frequency range to 1 MHz has been developed and a new method to determine the absolute phase angle error of current shunts is described. Preliminary expanded uncertainty of the absolute phase angle error for a reference shunt with resistance 1,5 Ω is ±200 μrad at 1 MHz. Based on the reference shunt the phase comparator is used to determine the phase angle errors of a shunt set. At 5 A, 100 kHz the new determination agrees within the expanded uncertainty ±100 μrad with our earlier realization.

We investigate the properties of the magnetocapacitance and dissipation factor of epitaxial graphene Hall bars with different electrode configurations to gain insight into the underlying physical mechanisms. The dependence of magnetocapacitance and dissipation factor on the magnetic field shows how the screening ability of the two-dimensional electron gas (2DEG) changes at the transition from the nonquantized to the quantized state. Both magnetocapacitance and dissipation factor exhibit a characteristic and correlated voltage dependence, which is attributed to the alternating contraction and expansion of the nonscreening 2DEG regions due to the alternating local electric field. Two regimes with seemingly different voltage dependencies are explained as the limiting cases of weak and strong electric fields of the same general voltage dependence. Electric fields in the plane of the 2DEG are found to cause about three orders of magnitude more ac dissipation than perpendicular electric fields. This strong directionality is attributed to the fact that the electrons are mobile in the plane of the 2DEG but are confined in the third dimension. In the quantized state, not only the screening edge of the 2DEG but also compressible puddles embedded in the bulk cause ac dissipation, as follows from the measured frequency dependence. Finally, characteristic parameters like the width of the screening edge, the threshold voltage, and the charging time of the compressible puddles are determined. .

In this work we investigate the long-term stability of epitaxial graphene (epigraphene) quantum Hall resistance standards, including single devices and an array device composed of 236 elements providing (R K/236 ≈ 109 Ω) , with R K the von Klitzing constant. All devices utilize the established technique of chemical doping via molecular dopants to achieve homogenous doping and control over carrier density. However, optimal storage conditions and the long-term stability of molecular dopants for metrological applications have not been widely studied. In this work we aim to identify simple storage techniques that use readily available and cost-effective materials which provide long-term stability for devices without the need for advanced laboratory equipment. The devices are stored in glass bottles with four different environments: ambient, oxygen absorber, silica gel desiccant, and oxygen absorber/desiccant mixture. We have tracked the carrier densities, mobilities, and quantization accuracies of eight different epigraphene quantum Hall chips for over two years. We observe the highest stability (i.e. lowest change in carrier density) for samples stored in oxygen absorber/desiccant mixture, with a relative change in carrier density below 0.01% per day and no discernable degradation of quantization accuracy at the part-per-billion level. This storage technique yields a comparable stability to the currently established best storage method of inert nitrogen atmosphere, but it is much easier to realize in practice. It is possible to further optimize the mixture of oxygen absorber/desiccant for even greater stability performance in the future. We foresee that this technique can allow for simple and stable long-term storage of polymer-encapsulated molecular doped epigraphene quantum Hall standards, removing another barrier for their wide-spread use in practical metrology. 

We report on a power standard for measurement of electrical power of sinusoidal signals based on digitzers and a phase-controlled phantom power source. The voltage channel ranges up to 1000 V at 100 kHz, and 20 V at 1 MHz, the current channel ranges up to 100 A at 100 kHz and 1 A at 1 MHz. The phase can be set arbitrarily ±180 degrees. We present the design of the system, its components and preliminary performance and measurement uncertainties.

Comparison for ultra-low DC current sources between LNE (France), BFKH (Hungary), NSAI NML (Ireland), IPQ (Portugal), RISE (Sweden), METAS (Switzerland) and TÜBİTAK UME (Turkey). To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/. The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

We present the first steps taken towards a graphene quantum Hall effect ac resistance standard at RISE. A new measurement setup has been developed including a graphene quantum Hall effect device suitable for the kilohertz range and a coaxial cryoprobe to be used together with a coaxial impedance bridge based on inductive voltage dividers. 1:1 ratio resistance measurements of the graphene device against a 12.9 kΩ ac resistance standard resulted in a linear frequency dependent (≈ 0.2(μΩ/Ω)/kHz) deviation from the quantized dc value.

The European Union funded project MICEV aims at improving the traceability of electrical and magnetic measurement at charging stations and to better assess the safety of this technology with respect to human exposure. The paper describes some limits of the instrumentation used for electrical measurements in the charging stations, and briefly presents two new calibration facilities for magnetic field meters and electric power meters. Modeling approaches for the efficiency and human exposure assessment are proposed. In the latter case, electromagnetic computational codes have been combined with dosimetric computational codes making use of highly detailed human anatomical phantoms in order to establish human exposure modeling real charging stations. Detailed results are presented for light vehicles where, according to our calculations, the concern towards human exposure is limited. Currently, the project has reached half way point (about 18 months) and will end in August 2020.