Cryogenic Current Comparator Research Articles

Ultrastable low-noise current amplifier: A novel device for measuring small electric currents with high accuracy

An ultrastable low-noise current amplifier (ULCA) is presented. The ULCA is a non-cryogenic instrument based on specially designed operational amplifiers and resistor networks. It involves two stages, the first providing a 1000-fold current gain and the second performing a current-to-voltage conversion via an internal 1 MΩ reference resistor or, optionally, an external standard resistor. The ULCA's transfer coefficient is highly stable versus time, temperature, and current amplitude within the full dynamic range of ±5 nA. The low noise level of 2.4 fA/√Hz helps to keep averaging times short at small input currents. A cryogenic current comparator is used to calibrate both input current gain and output transresistance, providing traceability to the quantum Hall effect. Within one week after calibration, the uncertainty contribution from short-term fluctuations and drift of the transresistance is about 0.1 parts per million (ppm). The long-term drift is typically 5 ppm/yr. A high-accuracy variant is available that shows improved stability of the input gain at the expense of a higher noise level of 7.5 fA/√Hz. The ULCA also allows the traceable generation of small electric currents or the calibration of high-ohmic resistors.

Precision high-value resistance scaling with a two-terminal cryogenic current comparator.

We describe a cryogenic two-terminal high-resistance bridge and its application in precision resistance scaling from the quantized Hall resistance (QHR) at RH = RK/2 = 12 906.4035 Ω to decade resistance standards with values between 1 MΩ and 1 GΩ. The design minimizes lead resistance errors with multiterminal connections to the QHR device. A single variable voltage source and resistive ratio windings are utilized to achieve excellent dynamic stability, which is not readily obtained in low-current measurements with conventional cryogenic current comparators (CCCs). Prototypes of this bridge have been verified by a successful international comparison of high-resistance scaling using two-terminal CCCs in the national metrology institutes of Argentina, Mexico, and the United States.

A programmable quantum current standard from the Josephson and the quantum Hall effects

We propose a way to realize a programmable quantum current standard (PQCS) from the Josephson voltage standard and the quantum Hall resistance standard (QHR) exploiting the multiple connection technique provided by the quantum Hall effect (QHE) and the exactness of the cryogenic current comparator. The PQCS could lead to breakthroughs in electrical metrology like the realization of a programmable quantum current source, a quantum ampere-meter, and a simplified closure of the quantum metrological triangle. Moreover, very accurate universality tests of the QHE could be performed by comparing PQCS based on different QHRs.

Final report on RMO comparison SIM.EM-S10: High value resistance comparison with two-terminal cryogenic current comparators

This work presents a supplementary comparison of high value resistance standards performed during 2012 and January 2013, following the guidelines presented in a document about measurement comparisons in the CIPM MRA. The purpose of this task was to compare the high resistance cryogenic current comparator scaling of the participating institutes: National Institute of Standards and Technology, USA (NIST), Centro Nacional de Metrología, Mexico (CENAM) and Instituto Nacional de Tecnología Industrial, Argentina (INTI), all of which are members of the Sistema Interamericano de Metrología (SIM) Regional Metrology Organization.All the measurements of this comparison were performed with two-terminal cryogenic current comparators (CCC). Degrees of equivalence of the participating institutes relative to the comparison reference values are given in the report for the measured resistance values.Main text.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 kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by SIM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Cryogenic Current Comparators for the Realisation of Electrical Quantum Standards

The cryogenic current comparator has played a central role in the realisation of electrical quantum standards over the last 40 years. It is the method of choice for National Measurement Laboratories in realising a quantum Hall effect primary standard of resistance. Single electron transport devices are expected to provide a primary standard of current within the next few years and this paper considers how they will be integrated into electrical metrology.

Precision quantum Hall resistance measurement on epitaxial graphene device in low magnetic field

Precision quantum Hall resistance (QHR) measurements were performed on large-area epitaxial graphene device at temperature T = 1.5 K and at magnetic fields B from 8 T down to 2.5 T, that is much lower than typically used in precision QHR measurement. QHR was measured using cryogenic current comparator resistance bridge with relatively large biasing current Isd = 41 μA to reduce measurement uncertainty. The results showed that at B = 8 T, the relative deviation of Hall resistance from the expected quantized value h/2e2 is within experimental uncertainty of 3.5 parts in 108 and remained below 0.35 parts per million (ppm) down to B = 3 T.

Aspects of Application and Calibration of a Binary Compensation Unit for Cryogenic Current Comparator Setups

We have developed a binary compensation unit for balancing measurement bridges with cryogenic current comparators (CCCs) commonly used in resistance metrology. We present the basic design ideas for the new microcontroller-operated module. It offers balance over a gapless range of resistance ratios of the resistors to be compared. The binary setup allows for convenient calibration when being combined with a binary CCC. The calibration can be performed using the CCC's control electronics and software. Examples of measurements demonstrate the unit's high flexibility and stability of resistance calibration well below one part in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> over a period of 21 hours.

New Design of Quantized Hall Resistance Array Device

We propose a new design for a 10- $\hbox{k}\Omega$ quantized Hall resistance (QHR) array device. This design realizes quantized resistance values that approach decade values with fewer Hall bars. In this design, the 10- $\hbox{k}\Omega$ QHR array device consists of only 16 Hall bars, and this number is about 16 times lower than that in the previous design, although the nominal value is identical to the previous one. The nominal value of this device shows a relative difference of only 0.034 $\mu\Omega/\Omega$ , based on $R_{K - 90} (=25\,812.807 \Omega)$ , from the exact value of 10 $\hbox{k}\Omega$ . This fact might allow us to evaluate each Hall bar in the array device. The parameters of this 10- $\hbox{k}\Omega$ QHR array device were measured by a conventional QHR and cryogenic current comparator using the 100- $\Omega$ standard resistor. According to the measurement, the value of the 10- $\hbox{k}\Omega$ QHR array device agrees with its nominal value within around one part in $10^{8}$ . In the same manner, new combinations of Hall bars were designed for the 100- $\Omega$ –1- $\hbox{M}\Omega$ range.

Resistance Scaling From 10 ${\rm k}\Omega$ Up to 100 ${\rm T}\Omega$ With New Designs of Hamon Transfer Devices

This paper describes the realization of a new system for resistance scaling from 10 ${\rm k}\Omega$ to 100 ${\rm T}\Omega$ . The resistance unit has been transferred from the quantum hall resistance (QHR) primary resistance standard to the resistance standard with a nominal value of 10 ${\rm k}\Omega$ by a cryogenic current comparator. For further transfer of the resistance unit from 10 ${\rm k}\Omega$ to 100 ${\rm T}\Omega$ , Hamon transfer devices (Hamon networks) are used. For its complete realization five devices have been developed: 10–100–1000 ${\rm k}\Omega$ , 1–10–100 ${\rm M}\Omega$ , 0.1–1–10 ${\rm G}\Omega$ , 10–100–1000 ${\rm G}\Omega$ , and 1–10–100 ${\rm T}\Omega$ . The first two devices have a single insulation, and the next three have a double isolation. In the transfer devices with the double insulation, triax-type connectors are used. All transfer devices have individual temperature stabilization within ${\pm}{\rm 0.01}^{\circ}{\rm C}$ . In the paper, the factors influencing device accuracy have been described. The insulation leakage is recognized, and the error caused by it is calculated. The guarding network for the new design of Hamon transfer devices is presented. Voltage coefficients of resistance and settling times of resistors used in these devices have been determined and discussed. Uncertainties for the Hamon transfer devices ratios have been calculated. Calibration results have been presented. The developed system is currently tested in the Central Office of Measures in Poland.

Robust Control of a Two-Terminal Cryogenic Current Comparator

A digital $H\infty$ controller for a two-terminal cryogenic current comparator is designed. To this end, a set of mathematical models covering the actual system is proposed. Simulation results compare the open- and closed-loop systems based on the proposed controller and the traditional integral control. According to these results, the new controller can significantly reduce the noise in the superconducting quantum interference device sensor.

Quantum metrological triangle experiment at LNE: measurements on a three-junction R-pump using a 20 000:1 winding ratio cryogenic current comparator

The direct closure of the quantum metrological triangle consists in achieving Ohm's law with the three effects used and investigated in quantum electrical metrology: the Josephson effect (JE), the quantum Hall effect (QHE) and the single-electron tunnelling effect (SET). The aim is to check the consistency of the phenomenological constants KJ, RK and QX associated with these effects and theoretically expressed with the fundamental constants e and h (elementary charge and Planck constant, respectively). Such an experiment is a contribution to a future redefinition of the International System of units (SI). In this paper, the experimental setup developed at LNE is described and the main obtained results are given. From a set of four measurements, agreement at a level of 1.3 parts in 105 was found between the quantum charge involved in the SET, i.e. QX, and the CODATA value of the elementary charge. The best measurement has shown a relative type-A uncertainty of 1.9 parts in 106, while the amplitude of the current generated by a metallic electron pump was as low as 3.6 pA.

Low temperature permeability and current noise of ferromagnetic pickup coils

For a non-destructive measurement of intensities of charged particle beams a Cryogenic Current Comparator is used which captures the azimuthal magnetic field of the beam by a superconducting pickup coil at 4.2K and transforms it into a current which is detected by a SQUID based current sensor. The current noise of the pickup coil and the bandwidth of this transformer depend on the frequency response curve of the complex permeability of the ferromagnetic core material embedded in the pickup coil. A measurement of the series inductance LS and series resistance RS of such a coil allows an indirect evaluation of the current noise contribution of the core using the Fluctuation–Dissipation-Theorem. These measurements were done with a commercial LCR-Meter in a frequency range from 20Hz to 2MHz. The current noise density was also directly measured using a SQUID-sensor. A comparison with between the direct and indirect measurement showed a good coincidence. Due to the critical temperature of the LTS-SQUID, noise measurements above 4.2K are not possible apart from using an anti-cryostat. The measurement of the series inductance LS and series resistance RS with an LCR-Meter works in the whole temperature range and provides a comfortable access to the magnetic properties of core materials. Compared to direct measurements, the indirect measurement thus allows a technologically simpler and broader determination of the core noise.

Quantum Hall effect in exfoliated graphene affected by charged impurities: Metrological measurements

Metrological investigations of the quantum Hall effect (QHE) completed by transport measurements at low magnetic field are carried out in a-few-$\mu\mathrm{m}$-wide Hall bars made of monolayer (ML) or bilayer (BL) exfoliated graphene transferred on $\textrm{Si/SiO}_{2}$ substrate. From the charge carrier density dependence of the conductivity and from the measurement of the quantum corrections at low magnetic field, we deduce that transport properties in these devices are mainly governed by the Coulomb interaction of carriers with a large concentration of charged impurities. In the QHE regime, at high magnetic field and low temperature ($T<1.3 \textrm{K}$), the Hall resistance is measured by comparison with a GaAs based quantum resistance standard using a cryogenic current comparator. In the low dissipation limit, it is found quantized within 5 parts in $10^{7}$ (one standard deviation, $1 \sigma$) at the expected rational fractions of the von Klitzing constant, respectively $R_{\mathrm{K}}/2$ and $R_{\mathrm{K}}/4$ in the ML and BL devices. These results constitute the most accurate QHE quantization tests to date in monolayer and bilayer exfoliated graphene. It turns out that a main limitation to the quantization accuracy, which is found well above the $10^{-9}$ accuracy usually achieved in GaAs, is the low value of the QHE breakdown current being no more than $1 \mu\mathrm{A}$. The current dependence of the longitudinal conductivity investigated in the BL Hall bar shows that dissipation occurs through quasi-elastic inter-Landau level scattering, assisted by large local electric fields. We propose that charged impurities are responsible for an enhancement of such inter-Landau level transition rate and cause small breakdown currents.

Precision comparison of the quantum Hall effect in graphene and gallium arsenide

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 quantized resistance values within the relative standard uncertainty of our measurement of 8.7 × 10−11. The result places new tighter limits on any possible correction terms to the simple relation RK = h/e2, and also demonstrates that epitaxial graphene samples are suitable for application as electrical resistance standards of the highest metrological quality. We discuss the characterization of the graphene sample used in this experiment and present the details of the cryogenic current comparator bridge and associated uncertainty budget.

Design Considerations for a CCC Bridge With Complete Digital Control

The design considerations for a completely digitally controlled cryogenic current comparator resistance ratio bridge, including the feedback loops for voltage and current balance, are described. The numerical algorithms have been modeled and optimized, and the overall system performance is illustrated with example measurement data. It is shown that the final performance of the bridge is limited only by the accuracy of the current comparator, the noise sources associated with the current and voltage null detectors, and the resistors under test. An explanation of how the effect of leakage resistance in the system is minimized is also given.

Cryogenic current comparators and their application to electrical metrology

The cryogenic current comparator (CCC), first demonstrated nearly 40 years ago, has become a key component of electrical metrology. It utilises a superconducting screen to achieve very high ratio accuracy and hence has found many applications where the electrical units need to be scaled over decade values. It has been deployed over a wide range of currents from 100 A to 1 pA and has been used to verify the accuracy of electrical quantum effects such as the quantised Hall effect. This study is a review of the theory, design principles and most common applications of the CCC. In addition to the summary of its use in top-level electrical metrology, some recent developments such as the use of high temperature superconducting materials and applications outside the realisation of electrical units are described.

Settling Behavior of the Bridge Voltage in Resistance Ratio Measurements With Cryogenic Current Comparators

A long settling time of the bridge voltage in resistance comparison setups with cryogenic current comparators might become crucial with respect to the type-A uncertainty, because it effectively limits the frequency of current reversals. Using a new bridge voltage detector with considerably increased bandwidth, we were able to trace the transient distortions caused by current reversal. Significant influence of the cabling is observed. Comparing a 12.9-kΩ quantum Hall resistor and a 100-Ω standard resistor, we achieved an expanded type-A uncertainty of about three parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> after 24 min of measurement for current reversal frequencies down to 100 mHz.

Design and Fabrication of Integrated Cryogenic Current Comparators

A cryogenic current comparator (CCC) is an instrument of great importance in electrical metrology, which provides a ratio of two currents with ultimate accuracy based on superconductivity. It is used for dc resistance calibration in many national metrology institutes. Conventional CCCs consist of multi-turn coils of wire windings, a multi-layered shield of superconductor foils and a separate SQUID sensor. This implementation results in a bulky device too massive to cool with a mechanical cryocooler, making a system inconvenient. A new implementation of CCC, an integrated CCC (ICCC) consisting of thin-film spiral coils, a thin-film superconducting shield and a SQUID sensor integrated on a single chip, enables a user-friendly system operated with a compact cryocooler. Prototype ICCC chips were designed and fabricated by using a superconducting Nb integrated circuit technology with chemical-mechanical polishing. The basic operation of the prototype ICCCs was confirmed by monitoring periodic flux-voltage characteristics of the SQUIDs.

A Non-Destructive Beam Monitoring System Based on an LTS-SQUID

Monitoring of beam currents in particle accelerators without affecting the beam guiding elements, interrupting the beam or influencing its profile is a major challenge in accelerator technology. A solution to this problem is the detection of the magnetic field generated by the moving charged particles. We present a non-destructive beam monitoring system for particle beams in accelerators based on the Cryogenic Current Comparator (CCC) principle. The CCC consists of a high-performance low-temperature DC superconducting quantum interference device (LTS DC-SQUID) system, a toroidal pick-up coil, and a meander-shaped superconducting niobium shield. This device allows the measurement of continuous as well as pulsed beam currents in the nA-range. The resolution and the frequency response of the detector strongly depend on the toroidal pick-up coil and its embedded ferromagnetic core. Investigations of both the temperature and frequency dependence of the relative permeability and the noise contribution of several nanocrystalline ferromagnetic core materials are crucial to optimize the CCC with respect to an improved signal-to-noise ratio and extended transfer bandwidth.

Precise Measurement of the Quantized Hall Resistance on the Basis of a Cryogenic Current Comparator

Precise Measurement of the Quantized Hall Resistance on the Basis of a Cryogenic Current Comparator