Cryogenic Current Comparator Research Articles

Application of the quantum Hall effect to resistance metrology

Abstract The quantum Hall effect (QHE) discovery has revolutionized metrology by providing with a representation of the unit of resistance, R K , that can be reproduced within a relative uncertainty of one part in 109 and is theoretically only linked to Planckʼs constant h and the electron charge e. This breakthrough also results from the development of resistance comparison bridges using cryogenic current comparator (CCC). The QHE experimental know-how now allows the realization of perfectly quantized Quantum Hall Array Resistance Standards (QHARS) by combining a large number of single Hall bars. In the context of an evolution of the Systeme International (SI) of units by fixing some fundamental constants of physics, the determination of the von Klitzing constant R K through the use of the so-called Thompson–Lampard calculable capacitor and the realization of refined universality tests of the QHE are of prime importance. Finally, the fascinating graphene material might be a new turning point in resistance metrology.

Dark current measurements on a superconducting cavity using a cryogenic current comparator

This paper presents nondestructive dark current measurements of tera electron volt energy superconducting linear accelerator cavities. The measurements were carried out in an extremely noisy accelerator environment using a low temperature dc superconducting quantum interference device based cryogenic current comparator. The overall current sensitivity under these rough conditions was measured to be 0.2 nA/Hz(1/2), which enables the detection of dark currents of 5 nA.

An automated cryogenic current comparator resistance ratio bridge for routine resistance measurements

A design of an automated cryogenic current comparator (CCC) resistance ratio bridge for routine measurements in a national metrology institute or standards laboratory is described. It employs a type II CCC for use in a low loss liquid helium storage vessel and can be continuously operated from the mains power via specialized isolated supplies. All parameters including the servo control loops are digitally controlled. Noise sources in the system are analysed using the Allan deviation and it is demonstrated that non-white noise sources can be eliminated by choosing an appropriate current reversal rate.

Improving the stability of cryogenic current comparator setups

We have realized an improved resistance calibration setup based on a cryogeniccurrent comparator (CCC). The comparator, with 18 windings and 4647turns in total, is well-suited for all the necessary comparisons with a 100 Ω standard resistor with the quantum Hall effect and inside the range of standard resistance values from1 Ω to 1 MΩ. The new state-of-the-art setup is equipped with a low-noise dc SQUID (superconductingquantum interference device) and a digital double current source. Proper damping of theCCC resonance, careful electronics design (‘box-in-box’) and application of internalwideband feedback to the SQUID sensor improve the dynamic stability considerably andlower the overall measurement time as compared against our previous setup.

Resistance metrology based on the quantum Hall effect

The Quantum Hall effect (QHE), a macroscopic effect of solid state physics, provides a universal representation of the unit of resistance which depends on the elementary charge e and the Planck constant h only. If implemented according to specific technical guidelines, the quantum resistance standard can be reproduced with a relative uncertainty below one part in 109. Calibrations of wire resistors in terms of the QHE can be carried out with similarly low uncertainties by using resistance bridges equipped with cryogenic current comparators, the performance of which relies on the magnetic flux sensitivity of superconducting quantum interference devices (SQUID). Using a special connection technique, the fundamental properties of the QHE allow the fabrication of arrays combining a large number of single Hall bars connected in series or in parallel and which demonstrate quantum accuracy. Similar to the case of voltage metrology with Josephson array voltage standards, an improvement of resistance metrology is expected from the availability of quantum Hall array resistance standards (QHARS). The QHE Wheatstone bridge, which is another application of the same connection technique, opens the way to new universality tests of the QHE with a relative uncertainty below one part in 1011. At frequencies in the kilohertz range, the recent progress in the application of coaxial bridges to the QHE allows metrologists to operate a quantum resistance standard with alternating current reaching an accuracy of some parts in 108. Finally, the discovery of the QHE in graphene opens new horizons for the resistance metrology.

Application of LTS-SQUIDs in Nuclear Measurement Techniques

Low temperature superconducting quantum interference devices (LTS SQUIDs) are used to make precision measurements of electromagnetic fields in applications ranging from biomedicine to high energy physics. We have previously described an LTS SQUID-based device for nuclear physics which employs the Cryogenic Current Comparator principle (CCC). The CCC consists of a high-performance LTS DC SQUID system, a toroidal pick-up coil, and a meander-shaped superconducting niobium shield. Theoretical investigations show that as external noise decreases, improvements in performance depend on the properties of the ferromagnetic core material embedded in the pick-up coil. Here we present the temperature- and frequency-dependence of several candidate ferromagnetic and nanocrystalline materials. We discuss these results in light of the optimization of the CCC sensor performance.

QUANTUM HALL EFFECT AND OHM METROLOGY

The quantum Hall effect (QHE) discovery has revolutionized the ohm metrology: the representation of the unit of resistance is now universal and the ohm can be maintained in each national metrology institute with a relative uncertainty of one part in 109. This breakthrough also results from the development of resistance comparison bridges using cryogenic current comparator (CCC). The fundamental properties of the QHE allow the realization of Quantum Hall Array Resistance Standards (QHARS) by combining a large number of single Hall bars connected in series and/or in parallel. These standards can be as accurate as a single Hall bar. More generally, the multiple connection technique allows metrologists to design useful circuits based on quantum Hall resistors like voltage dividers or Wheatstone bridges. The QHE Wheatstone bridge is particularly suitable for comparing quantum standards. By detecting the unbalance current of this bridge with a CCC, new universality tests of the QHE with a target uncertainty less than 1011 can be realized.

Internally Damped CCC for Accurate Measurements of Small Electrical Currents

We have developed and tested an internally damped cryogenic current comparator (CCC) with a main 1:20 000 winding ratio for the accurate measurement of small electrical currents. The key features of the CCC are the use of resistive CuNi wire for the windings and brass internal shields for additional damping of the internal resonance of the CCC. Inductance measurements of the CCC overlapping tube at 300 and 77 K, as well as the superconducting quantum interference device (SQUID) noise spectra of the complete CCC setup at 4 K, confirm the effectiveness of the damping of the CCC self-resonance with respect to other designs. At 4 K, the CCC is remarkably stable in measurements with external feedback mode; even mains-operated current sources can be used without unlocking the SQUID null detector of the measurement bridge. First measurements of currents in the range of 100-300 pA using this CCC are performed, resulting in deviations from the expected value in the range of about 20-50 muA/A.

CCC Bridge With Digitally Controlled Current Sources

We have built a cryogenic current comparator (CCC) resistance bridge for the comparison of standard resistors in the range of 1 Omega-100 kOmega. The use of fully programmable current sources allows for smooth polarity reversals, regardless of the time constant of each side of the bridge. We measured a CCC transfer function of 2.25 muA middotturn/phi0 and a noise level of 54 pA middot turn/radic(Hz) at 1 Hz, with a 1/f corner of about 0.6 Hz. Two different readout schemes were implemented and compared with good agreement. We performed a few measurements to assess the accuracy of the bridge and evaluated all the significant sources of error. The total uncertainty of the bridge is estimated to be 2.6 parts in 109 between 1 Omega and 10 kOmega.

Uncertainty Evaluation in a Two-Terminal Cryogenic Current Comparator

We present an uncertainty evaluation of a new cryogenic current comparator (CCC) bridge that is designed to compare two-terminal 1 and 10 MOmega standard resistors with the quantized Hall resistance (QHR) and scale from these decade values to other decade values between 10 kOmega and 1 GOmega. Use of the bridge over a wide range of resistance values results in a spectrum of dominant contributions to the uncertainty budget, and this analysis helps determine the optimum parameters of the measurements. Our theoretical analysis is compared with experimental data.

Improved Cryogenic Current Comparator Setup With Digital Current Sources

We have realized an improved resistance calibration setup based on a cryogenic current comparator (CCC). The new comparator with 18 windings and a total of 4647 turns suits well for all the necessary comparisons of a 100-Omega standard resistor with the quantum Hall effect (QHE) and within the range of standard resistance values from 1 Omega to 1 MOmega. The new state-of-the-art setup is equipped with a dc superconducting quantum interference device (SQUID) and a digital double current source, allowing a reduction in the ramping time by at least one order of magnitude and significantly improving the dynamic stability of the system. The latter also benefits from the internal wideband feedback that we present in this paper.

Experimental realization of the quantum metrological triangle experiment

The quantum metrological triangle experiment (QMTE) consists in realizing Ohm's law with Josephson (JE), quantum Hall (QHE) and single electron tunneling (SET) effects. The aim is to check the consistency of the link among the phenomenological constants K J, RK and Q X involved in these effects and theoretically expressed with the fundamental constants e and h. Such an experiment could be a contribution for a new definition of the système international d'unités (SI) base units.In the QMTE, a current generated by a SET device flows through a resistor calibrated against QHE standard and the voltage induced at its terminals is compared to the metrological voltage generated by a Josephson junctions array. At LNE, the studied SET devices are 3 junctions single electron pumps with on chip resistors. The quantized current generated by this pump is theoretically equal to ef (f is the frequency of the driving signals applied on the gates) and is measured through a cryogenic current comparator (CCC), which allows to amplify the low pumping current with a metrological accuracy.We will present and discuss the experimental set-up developed at LNE and the first results. In addition to the main aim of QMTE described above, these preliminary results are also a first step towards a determination of e.

A new measurement tool for characterization of superconducting rf accelerator cavitiesusing high-performance LTS SQUIDs

This paper presents a new system to measure very low currents in an acceleratorenvironment, using a cryogenic current comparator (CCC). In principle a CCC is aconventional current transformer using the high-performance SQUID technology to sensethe magnetic fields caused by the beam current. Since the system is sensitive on a pA level,it is an optimum device to detect dark currents of superconducting cavities. The systempresented here is designed for the test facilities of the superconducting accelerator modulesfor the European XFEL at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg.Measurements in a quiet environment showed that an intrinsic noise level of the CCC of40 pA Hz−1/2 could be achieved.

Testing universality of the quantum Hall effect by means of the Wheatstone bridge

We report on a study concerning an on-chip quantum Hall effect (QHE) circuit made of two series arrays of two Hall bars set parallel to each other. This array, the design of which is based on the quadruple connection technique, can be studied either as an RK∕i resistance standard (RK is the von Klitzing constant, i is the Hall plateau index) or as a Wheatstone bridge. Metrological characterizations were carried out by varying the temperature, the magnetic field and the bias current. In particular, by measuring the unbalance current with a cryogenic current comparator, it has been possible to compare the four quantum resistances constituting the Wheatstone bridge with a relative uncertainty, never achieved so far, of 8 parts in 1011. This work shows not only the efficiency of the multiple connection technique but also the interest of a QHE Wheatstone bridge made of Hall bars of different natures to realize QHE universality tests with attainable accuracies as low as some parts in 1012. The verification of the QHE universality property with a relative uncertainty one order of magnitude lower than that of the previous universality tests should support, as recommended by the Comité International des Poids et Mesures, the redefinition of the Système International of units based on the fundamental constants of physics.

Improvement of Resistance Ratio Bridge Using Cryogenic Current Comparator by Operating with Low-Noise Voltage Amplifier Using Superconducting Quantum Interference Device for Low Resistance Measurements

We improved a resistance ratio bridge using a cryogenic current comparator (CCC bridge) for low-resistance measurement by operated with a laboratory-developed low-noise voltage amplifier using a superconducting quantum interference device (SQUID voltage amplifier). The combined uncertainty of the resistance ratio measurement of 10 mΩ/1 Ω was evaluated to be about 0.03 ppm.

An LTS-SQUID Based Measurement Tool for Characterization of Superconductive RF Cavities

This paper presents a new system to measure very low currents in an accelerator environment, using a cryogenic current comparator (CCC). In principle a CCC is a conventional current transformer using the high performance SQUID technology to sense the magnetic fields caused by the beam current. Since the system is sensitive on a pA level, it is an optimum device to detect dark currents of superconducting cavities. The system presented here is designed for the test facilities of the superconducting accelerator modules for the European XFEL at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg. Measurements in a quiet environment showed that an intrinsic noise level of the CCC of 40/radicHz could be achieved.

Magnetic-field calculation for a high-temperature superconducting cryogenic current comparator

The problem considered in this paper arises in the design of a high-temperature superconducting cryogenic current comparator (CCC). The CCC consists of two currents flowing in opposite directions inside a toroidal superconducting shield. The shield has a radial cut, necessary for the measurement of the current ratio, but causing an error in the obtained ratio. The problem of interest is the dependence of the error on the geometric parameters of the device: the major and minor radii of the shield, the cut width, the material thickness, and the location of the currents. In the first part of the paper, a toroidal shield with an infinitesimal cut is considered and analytic expressions are derived for the magnetic field and the surface-current distribution. In the second part, a cut of finite width is introduced. Since all the perturbing currents are present in the narrow region around the cut, a shield of cylindrical shape is assumed. Expressions are derived for the flux through the cut and the magnetic field around the cut. Analytical results are in good agreement with the numerical results obtained by a finite-element method. In the final part, the expression for the ratio error is derived, which shows that in order to minimize the error, currents should be concentrated around the shield axis, the major radius of the shield should be maximized and the bore radius minimized. The error depends logarithmically on the cut width.

Precise Determination of Hydrostatic Pressure Correction Coefficient of Triple Point Cell of Water using Cryogenic Current Comparator Bridge

Applying a cryogenic current comparator bridge to resistance thermometry, the temperature distributions along the thermometer wells of two triple point cells of water are precisely measured using a standard platinum resistance thermometer with low measuring currents of about 0.1 mA. The average hydrostatic pressure coefficient of the two cells calculated from the temperature distributions is -0.854(38) mK/m. This value agrees well with the reported value of -0.856 mK/m, but is not consistent with the recommended value of the International Temperature Scale of 1990, -0.73 mK/m.

Direct resistance comparisons from the QHR to100 M/spl Omega/ using a cryogenic current comparator

Measurements of room-temperature 100 M/spl Omega/ standard resistors and cryogenic thin-film resistors based directly on a quantized Hall resistance standard have been made with a cryogenic current comparator (CCC) bridge. This 15 496:2 ratio CCC attains a current sensitivity of 10.7 fA/Hz/sup 1/2/ in measurements of cryogenic thin-film resistors, without extensive shielding or filtering. A resistive primary winding helps the CCC maintain stability in the presence of external noise. The resistive-winding technique may be useful for the absolute measurement of small currents delivered by single-electron tunneling devices.

The IEN CCC Bridge to Scale the Quantized Hall Resistance to 1-<tex>$Omega$</tex>Standards

A resistance bridge based on a cryogenic current comparator has been built at the Istituto Elettrotecnico Nazionale "G. Ferraris" (IEN). The use of this bridge simplifies the scaling down from the quantized Hall resistance to decadic standard resistors, at the same time reducing the measurement uncertainty. The bridge is described and the result of a comparison at the level of 100 /spl Omega/, with the traditional scaling method used at IEN is reported. The two methods agree well within their uncertainties, for which detailed uncertainty budgets are given.