arXiv subjects

Stephen Giblin

Publications and source records attributed to Stephen Giblin.

5 recordsLinked to original sources

arXiv2025

Discovery and rectification of an error in high resistance traceability at NPL: a case study in how metrology works

We broach a seldom-discussed topic in precision metrology; how subtle errors in calibration processes are discovered and remedied. We examine a case study at the National Physical Laboratory (NPL), UK, involving the calibration of DC standard resistors of value 100 MOhm and 1 GOhm. Results from the period 2001 to 2015 were in error by approximately 0.7 parts per million (ppm), with quoted uncertainties (k=2) of 0.4 ppm and 1.6 ppm respectively. Inter-comparisons did not detect the error, mainly because the uncertainty due to the transportation drift of the comparison standards was too large to resolve it. Likewise, research into single-electron current standards did not detect the error because at this resistance value it was on the borderline of statistical significance. The key event was a comparison between PTB (Germany) and NPL (UK) of a new small-current measuring instrument, the ultrastable low-noise current amplifier (ULCA). At that time, the transport stability of the ULCA was not well established. Nevertheless, calibrations of the ULCA at NPL using a 100 MOhm resistor were sufficiently discrepant with the PTB calibrations to motivate a thorough investigation into the NPL traceability chain, which uncovered the error. This instructive episode illustrates a positive interplay between calibration and research activities and shows that cutting-edge calibration uncertainties must be supported by a vigorous research programme. It is also important for NMIs to maintain a comfortable buffer (at least a factor of 10) between their claimed uncertainty and the uncertainty that their customers require, so that small errors can be resolved without significant impact on measurement stakeholders.

physics.ins-det↗

arXiv2023

Precision measurement of an electron pump at 2 GHz

A well-characterised sample of silicon tunable-barrier electron pump has been operated at a frequency of 2 GHz using a custom drive waveform, generating a pump current of 320 pA. Precision measurements of the current were made as a function of pump control parameters, using a blind protocol, over a 7-week campaign. The combined standard uncertainty for each 10 hour measurement was 0.1 parts per million. The pump current exhibits a plateau along the exit gate voltage flat to approximately 0.1 parts per million, but offset from ef by 0.2 parts per million. This offset may be a sign of errors in the current traceability chain, indicating a limit to the accuracy of small current scaling using existing methods based on cryogenic current comparators.

cond-mat.mes-hall↗

arXiv2020

From Counting Electrons to Calibrating Ammeters: Improved Methodologies for Traceable Measurements of Small Electric Currents

New technology, the ultrastable low-noise current amplifier and the electron pump, provide new methods for making traceable measurements of small DC electric currents. We review four traceability routes for small current measurements and discuss the merits of each one. We present three case studies of small current calibrations, highlighting the role of noise and drifting instrument offsets. We show how the Allan deviation is used as a statistical tool for designing a calibration cycle to correctly eliminate drifting instrument offsets from calibration data. We also present a simplified noise model for a low-current ammeter which predicts a lower limit to the achievable statistical uncertainty in a calibration.

physics.ins-det↗

arXiv2020

Realisation of a quantum current standard at liquid helium temperature with sub-ppm reproducibility

A silicon electron pump operating at the temperature of liquid helium has demonstrated repeatable operation with sub-ppm accuracy. The pump current, approximately 168 pA, is measured by three laboratories, and the measurements agree with the expected current ef within the uncertainties which range from 0.2 ppm to 1.3 ppm. All the measurements are carried out in zero applied magnetic field, and the pump drive signal is a sine wave. The combination of simple operating conditions with high accuracy demonstrates the possibility that an electron pump can operate as a current standard in a National Measurement Institute. We also discuss other practical aspects of using the electron pump as a current standard, such as testing its robustness to changes in the control parameters, and using a rapid tuning procedure to locate the optimal operation point..

physics.ins-det↗

arXiv2018

Inter-laboratory nanoamp current comparison with sub-part-per-million uncertainty

An ultrastable low-noise current amplifier (ULCA) has been transferred between two laboratories, NPL and PTB, four times, with three of the transfers yielding a relative change in the trans-resistance gain of less than $2 \times 10^{-7}$. This is a new bench-mark for inter-laboratory transfer of small current. We describe in detail the use of a cryogenic current comparator to calibrate the ULCA at NPL, and the use of the ULCA to measure $1$ G$Ω$ resistors with relative uncertainties at the $10^{-7}$ level.

physics.ins-det↗