Nick Fletcher

National Physical Laboratory | NPL

The redefinition of the SI base units ampere and kilogram in 2019 formalized the use of the quantum Hall effect (QHE) to provide resistance traceability (the SI ohm) from the fundamental constants h and e. Traditionally, realization of the ohm via the QHE has required large complex liquid helium cryostats (including a high field superconducting mag...

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, s...

This report gives the result of a bilateral comparison of resistance between the NPLI (India) and the BIPM carried out in 2012-2013. Two 1 Ω and two 10 kΩ travelling standards belonging to the BIPM were used. The comparison was carried out with an 'A-B-A' pattern of measurements. The standards were measured first at the BIPM for a period of about t...

This paper details developments in the use of room temperature current comparators operating with low-frequency (<1 Hz) ac for precision resistance metrology. We consider the possible benefits of this technique over traditional reversed-dc operation. We describe the construction of high-sensitivity current comparators suitable for such low frequenc...

This report gives the result of a bilateral comparison of resistance between the NIMT (Thailand) and the BIPM carried out in 2015. Two 1 Ω and two 10 kΩ travelling standards belonging to the BIPM were used. The comparison was carried out with an 'A-B-A' pattern of measurements; the standards were measured first at the BIPM for a period of about one...

This report gives the result of a bilateral comparison of resistance between the BIM (Bulgaria) and the BIPM carried out in 2013. Two 1 Ω and two 10 kΩ travelling standards belonging to the BIPM were used. The comparison was carried out with an 'A-B-A' pattern of measurements; the standards were measured first at the BIPM for a period of about one...

This report gives the result of a bilateral comparison of resistance between the NSAI NML (Ireland) and the BIPM carried out in 2014-2015. Two 1 Ω and two 10 kΩ travelling standards belonging to the BIPM were used. The comparison was carried out with an 'A-B-A' pattern of measurements; the standards were measured first at the BIPM for a period of a...

This report gives the result of a bilateral comparison of resistance between the CMI (Czech Republic) and the BIPM carried out in 2015. Two 1 Ω and two 10 kΩ travelling standards belonging to the BIPM were used. The comparison was carried out with an 'A-B-A' pattern of measurements; the standards were measured first at the BIPM for a period of abou...

This report gives the result of a bilateral comparison of resistance between the SMD (Belgium) and the BIPM carried out in 2017. Two 1 Ω and two 10 kΩ travelling standards belonging to the BIPM were used. The comparison was carried out with an 'A-B-A' pattern of measurements; the standards were measured first at the BIPM for a period of about one m...

Within the framework of the supplementary comparison EURAMET.EM-S31, 'Comparison of capacitance and capacitance ratio', five participants (the BIPM, METAS, LNE, PTB, and VSL) inter-compared their capacitance realisations traced to the quantum Hall resistance measured at either ac or dc. The measurands were the capacitance values of three 10 pF stan...

Improved magnetic materials open the perspective of operating room-temperature current comparators at lower frequency without compromising metrological accuracy. Two prototypes of low-frequency current comparator (LFCCs) have been successfully tested in precision resistance ratio measurements at drive frequencies of 0.5 Hz and 1 Hz using the BIPM's...

Adjustment of the charge carrier density in quantized Hall resistance (QHR) devices based on epitaxial SiC graphene films can be performed using photochemical and chemical gating. Our experiments show that the carrier concentration in fabricated graphene devices can be reversibly varied by either treatment with UV light or using molecules from aqua...

The CCEM-K4 key comparison of capacitance at 10 pF was last carried out between 1996 and 1999. The Consultative Committee for Electricity and Magnetism of the International Committee for Weights and Measures decided that it should be repeated in 2016. We present here the main technical choices adopted for the next comparison.

A project aiming at measuring the von Klitzing constant with an uncertainty of the order of one part in 10^8 is currently ongoing at the BIPM. This measurement will be performed by linking a calculable cross-capacitor to a quantum Hall resistor through a quadrature transfer. The main parts of the overall measuring setup as well as the current statu...

Recent proposals to re-define some of the base units of the SI make use of definitions that refer to fixed numerical values of certain constants. We review these proposals in the context of the latest results of the least-squares adjustment of the fundamental constants and against the background of the difficulty experienced with communicating the...

This paper presents a systematic study of the effects of reversals of current polarity on 1 Ω standard resistors. A selection of 1 Ω standards were measured on a dc cryogenic current comparator bridge and also on an ac current comparator bridge operating at 1 Hz. We find that the apparent 4 terminal resistance depends strongly on the dc reversal ra...

This report gives the result of a bilateral comparison of capacitance between the BIM (Bulgaria) and the BIPM carried out in 2012. Two 10 pF travelling standards belonging to the BIPM were used. The comparison was carried out with an 'A–B–A' pattern of measurements; the standards were measured first at the BIPM for a period of about one month, then...

This report gives the result of a bilateral comparison of capacitance between the NPLI (India) and the BIPM carried out in 2011. Two 10 pF travelling standards belonging to the BIPM were used. The comparison was carried out with an 'A–B–A' pattern of measurements; the standards were measured first at the BIPM for a period of about one month, then f...

This report gives the result of a bilateral comparison of capacitance between the NPLI (India) and the BIPM carried out in 2011. One 100 pF travelling standard belonging to the BIPM was used. The comparison was carried out with an 'A—B—A' pattern of measurements; the standard was measured first at the BIPM for a period of about one month, then for...

The proposed redefinition of several International System (SI) base units is a topic that has been on the metrology agenda for the last decade. Recent progress on several determinations of the fundamental constants means that we now have a good idea of the defined numerical values that will be given in the new system to the Planck constant, h, and...

Two prototypes of a current comparator for operation at low frequencies in the 0.1 Hz to 1 Hz range and at room temperature have been realized. Experimental tests include the determination of the frequency-dependent sensitivity and a comparative measurement of the ratio of two standard resistors with nominal values of 1 ko and 100 o. The realized c...

This paper considers the impact of the planned redefinition of the SI on electrical traceability, with a focus on resistance and voltage calibration. The new SI will replace the 1990 values for the critical constants RK and KJ. With the present state of experimental data, we can say with good confidence that the required relative changes will be of...

This paper considers the problem of defining measurement conditions that approximate 'DC' for resistance standards. A recent on-site comparison between the BIPM and the PTB highlighted this difficulty in dealing with Peltier heating in 1 o standards, but it is a general problem with a variety of causes. We aim to draw attention to this potential so...

The BIPM has an ongoing project to establish a Thompson Lampard calculable capacitor for use in the determination of the von Klitzing constant, RK. This paper describes key parts of the measurement chain used to provide the link between the dc quantum Hall effect and the calculable capacitor. The bridge used to calibrate a conventional 1 pF standar...

In 2011, the General Conference on Weights and Measures (CGPM) confirmed its intention to adopt new definitions for four of the base units of the SI based on fixed numerical values of selected constants. These will be the kilogram, the ampere, the kelvin and the mole. The CGPM was not able to adopt the new definitions at that time because certain e...

An on-site comparison of the quantum Hall effect (QHE) resistance standards of the Physikalisch-Technische Bundesanstalt (PTB) and of the Bureau International des Poids et Mesures (BIPM) was made in November 2013. Measurements of a 100?? standard in terms of the conventional value of the von Klitzing constant, RK-90, agreed to 5 parts in 1010 with...

The quantum Hall effect allows the standard for resistance to be defined in terms of the elementary charge and Planck's constant alone. The effect comprises the quantization of the Hall resistance in two-dimensional electron systems in rational fractions of RK=h/e2=25812.8074434(84) Ω (Mohr P. J. et al., Rev. Mod. Phys., 84 (2012) 1527), the resist...

This paper describes the development of a thermally regulated enclosure for standard resistor elements. The design, using a Peltier element, can work in a wide range of possible laboratory temperatures, with mK stability. Combined with a high quality resistance element, a robust working standard with 10-9 level relative stability is produced.

This paper describes the progress on the implementation of a calculable cross capacitor for the measurement of RK at the BIPM. The main electrodes are installed and the alignment process has been demonstrated. The optical interferometer required to measure the guard displacement is operational. The chain of bridges for the transfer from RK to 1 pF...

The half-integer quantum Hall effect in epitaxial graphene is compared with high precision to the well known integer effect in a GaAs/AlGaAs heterostructure. We find no difference between the quantised resistance values within the relative standard uncertainty of our measurement of $8.7\times 10^{-11}$. The result places new tighter limits on any p...

This report describes the results obtained from a KRISS–BIPM bilateral comparison of 10 kΩ resistance standards in 2011. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.b. Two BIPM 10 kΩ travelling standards of SR104 type were calibrated first at the BIPM, then at the KRISS and again at the BIPM after...

This report describes the results obtained from a NML(Ireland)–BIPM bilateral comparison of 1 Ω resistance standards in 2010. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.a. Two BIPM 1 Ω travelling standards of CSIRO type were calibrated first at the BIPM, then at the NMLI and again at the BIPM afte...

This report describes the results obtained from a NML(Ireland)–BIPM bilateral comparison of 10 kΩ resistance standards in 2010. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.b. Two BIPM 10 kΩ travelling standards of SR104 type were calibrated first at the BIPM, then at the NMLI and again at the BIPM...

The Syst\`eme Internationale d'unit\'es (SI system) is about to undergo its biggest change in half a century by redefining the units for mass and current in terms of the fundamental constants h and e, respectively. This change crucially relies on the exactness of the relationships which link these constants to measurable quantities. Here we directl...

This report describes the results obtained from a GUM(Poland)–BIPM bilateral comparison of 10 kΩ resistance standards in 2009–2010. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.b. Two BIPM 10 kΩ travelling standards of SR104 type were calibrated first at the BIPM, then at the GUM and again at the BI...

This report describes the results obtained from a GUM(Poland)–BIPM bilateral comparison of 1 Ω resistance standards in 2009–2010. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.a. Two BIPM 1 Ω travelling standards of CSIRO type were calibrated first at the BIPM, then at the GUM and again at the BIPM a...

We investigate some statistical properties of ac voltages from a white noise source measured with a digital lock-in amplifier equipped with finite impulse response output filters which introduce correlations between successive voltage values. The main goal of this work is to propose simple solutions to account for correlations when calculating the...

This report gives the results of a bilateral comparison of capacitance between the CMI (Czech Republic) and the BIPM carried out in 2009. One 100 pF travelling standard belonging to the CMI was used. The comparison was carried out with an 'A–B–A' pattern of measurements; the standard was measured first at the CMI for a period of about one month, th...

This report gives the results of a bilateral comparison of capacitance between the CMI (Czech Republic) and the BIPM carried out in 2009. One 10 pF travelling standard belonging to the CMI was used. The comparison was carried out with an 'A–B–A' pattern of measurements; the standard was measured first at the CMI for a period of about one month, the...

This report describes the results of a bilateral comparison of capacitance between the NIST (United States) and the BIPM carried out in 2007–2008. Four 10 pF travelling standards belonging to the BIPM were used. The comparison was carried out with an 'A–B–A' pattern of measurements; the standards were measured first at the BIPM for a period of one...

We measure the frequency dependence of a sample of commercial air- and gas-dielectric capacitors between 1 kHz and ≈0.01 Hz. We find that the capacitance of sealed-gas capacitors can change by more than 100 μF/F over this range. This has important implications for the use of gas capacitors in reference sub-nA current generators.

This paper presents a simple resistance bridge dedicated to measuring a pair of 51.6 kΩ resistors (which form part of a quadrature bridge) directly against a quantum Hall effect (QHE) reference. It uses a room temperature current comparator of ratio 4, operating with low frequency AC in the range 1 to 10 Hz, and is energized from a single supply in...

This report describes the results obtained from a NIMT (Thailand)–BIPM bilateral comparison of 10 kΩ resistance standards in 2009. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.b. Two BIPM 10 kΩ travelling standards of SR104 type were calibrated first at the BIPM, then at the NMIT and again at the BI...

This report describes the results obtained from a NML(Ireland)–BIPM bilateral comparison of 1 Ω resistance standards in 2008. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.a. Two BIPM 1 Ω travelling standards of CSIRO type were calibrated first at the BIPM, then at the NMLI and again at the BIPM afte...

This report describes the results obtained from a NML(Ireland)–BIPM bilateral comparison of 10 kΩ resistance standards in 2008. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.b. Two BIPM 10 kΩ travelling standards of SR104 type were calibrated first at the BIPM, then at the NMLI and again at the BIPM...

A measurement system for ionisation chamber currents below 1 nA is presented. The system is based on a standard capacitor charging technique, and uses a commercially available electrometer and air-gap capacitor. We find that the uncertainties in the current range 100 fA–1 nA are up to an order of magnitude better than those available from a state-o...

This report describes the results obtained from a NMIA(Australia)–BIPM bilateral comparison of 1 Ω resistance standards in the period 2008–2009. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.a. Three BIPM 1 Ω travelling standards of CSIRO type were calibrated first at the BIPM, then at the NMIA and a...

Results of applying time series analysis techniques to measurements on a coaxial AC bridge are presented. A simple formula is used to account for correlations introduced by a low pass filter, and the Allan variance is applied to a study of sources of noise and instability.

A new set of ac-dc resistance transfer standards are being developed for use in the BIPM quadrature bridge. These resistors use commercial thick film resistance elements, whose small ac-dc difference is calibrated against a calculable coaxial reference. The improved stability of the new resistors should reduce the overall uncertainty of the quadrat...

A comparison of 100 pF capacitance standards at a measuring frequency of 1 kHz was carried out between the BIPM and the NML (Ireland). Two travelling standards belonging to the BIPM were used. Measurements were made over the period June to October 2007. One of the travelling standards had to be eliminated from the comparison due to a failure of its...

This report describes the results obtained from a NIST–BIPM bilateral comparison of 1 Ω resistance standards in 2007. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.a. Three BIPM 1 Ω travelling standards of CSIRO type were calibrated first at the BIPM, then at the NIST and again at the BIPM after thei...

This report describes the results obtained from a CMI?BIPM bilateral comparison of 1?? resistance standards in 2007?2008. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.a. Two BIPM 1?? travelling standards of CSIRO type were calibrated first at the BIPM, then at the CMI and again at the BIPM after the...

This report describes the results obtained from a CMI–BIPM bilateral comparison of 10 kΩ resistance standards in 2008. The comparison was carried out in the framework of the BIPM ongoing key comparison BIPM.EM-K13.b. Two BIPM 10 kΩ travelling standards of SR104 type were calibrated first at the BIPM, then at the CMI and again at the BIPM after thei...

This paper describes two systems recently developed at the National Physical Laboratory (NPL), Teddington, U.K., for the generation and measurement of dc currents in the range 100 fA to 1 nA. Both systems are based on the capacitor charging technique. One system generates known currents, and constitutes a new reference at NPL. The other was designe...

We present acoustoelectric current measurements through a double quantum dot. Due to background impurity potential fluctuations an unintentional quantum dot is situated next to an intentionally induced single dot. By changing the top gate voltages, each of the two dots alone can be addressed separately but also a situation can be realized where bot...

The piezoelectric potential of a surface acoustic wave (SAW) can drive carriers in a semiconductor device over macroscopic distances. An application where optically generated electron-hole pairs are separated, transported and deliberately recombined was demonstrated a few years ago [Phys. Rev. Lett. 78 (1997) 4099]. Also around that time it was sho...

We studied the influence of a quantum dot in a 1D-channel on the quantised acoustic charge transport. Current plateaux at record-low RF-powers have been observed. More importantly, our findings show a possible way to a systematic engineering of quantum dots for an optimised high accuracy device

We present a new realization of quantized charge pumping operating at 3 GHz. Surface acoustic waves on a GaAs/AlGaAs heterostructure are used to pump electrons through a quantum dot defined by metallic split gates. The current quantization is determined by the electronic states of the dot

We present a detailed study of the surface acoustic wave mediated quantized transport of electrons through a split-gate device containing an impurity potential defined quantum dot within the split-gate channel. A regime of quantized transport is observed at low rf powers where the surface acoustic wave amplitude is comparable to the quantum dot cha...

We present a realization of quantized charge pumping. A lateral quantum dot is defined by metallic split gates in a GaAs/AlGaAs heterostructure. A surface acoustic wave whose wavelength is twice the dot length is used to pump single electrons through the dot at a frequency f=3GHz. The pumped current shows a regular pattern of quantization at values...

We present a detailed study of the surface acoustic wave mediated quantized transport of electrons through a split-gate device containing an impurity potential defined quantum dot within the split-gate channel. A regime of quantized transport is observed at low rf powers where the surface acoustic wave amplitude is comparable to the quantum dot cha...

We present a detailed study of surface acoustic wave devices produced at Physikalisch-Technische Bundesanstalt and Cambridge University. From the temperature dependence of the acoustic current, information on a number of critical device parameters such as the intrinsic electron temperature and Coulomb charging energy can be deduced. The results of...

A cryogenic current comparator (CCC) bridge for quantum Hall resistance ratio measurement has been constructed. Compared to previous CCC systems at the National Physical Laboratory, UK, the new system differs in two aspects. Firstly, it uses a type-II CCC, i.e. the ratio windings are on the outside and the detection winding is on the inside. This h...

Measurements of the slopes (ΔI/ΔV_g)_n of the acoustoelectric current plateaux I_n=nef for n=1 and 2 have been made, as a function of the bath temperature T in the range 0.3 to 4.2 K. Electrons, constrained in one-dimensional channels, are transported by a surface acoustic wave of frequency f≈2.8 GHz, generated by tran...

We present a quantitative comparison between surface acoustic wave devices produced at PTB and Cambridge University. From the temperature dependence of the acoustic current, information on a number of device parameters such as the electron temperature and self-heating can be deduced. The results of this study will be discussed with respect to the l...

A cryogenic current comparator with a potential current noise of 1 fA//spl radic/Hz has been built. The comparator will provide accurate current ratios for a new high resistance bridge, using a novel method of balancing two currents from a single source.