An automated temperature-controlled electrical DC voltage and DC resistance multiple reference standard (MRS) has been developed by Measurements International (MI) with the scientific support from the Istituto Nazionale di Ricerca Metrologica (INRIM). The MRS includes a 10 V, a 1 Ω, and a 10 kΩ standards selectable via a switch unit. This setup allows the artifact calibration of high-end calibrators and multimeters used in low-frequency electrical measurements. The two resistors are high-stability standards from MI, while the 10 V standard is based on a low-noise circuit developed by INRIM in collaboration with MI. A key innovation is the internal real-time clock calendar, which displays the calibration values of the MRS standards and their updated values internally calculated. This ensures reliable use of the MRS standards over extended periods between calibrations, effectively minimizing uncertainties due to their drift. The standards are housed in a thermal box, minimizing temperature variations. The MRS standards meet the uncertainty requirements defined by calibrators and multimeters manufacturers for artifact calibration and can also serve as laboratory references or travelling standards for interlaboratory comparisons (ILCs). MI is currently commercializing the MRS.

We describe the preliminary measurement conducted to establish a baseline dataset toward a cryogen-free system for quantized Hall resistance (QHR) standard application. This paper illustrates the current state-of-the-art cryogenic system, the quantization verification and ratio measurement between an established GaAs Hall bar and a graphene-base quantum Hall array resistance standard (GQHARS). The technology comparison is carried out with a room temperature ratio bridge.

Johnson noise thermometry (JNT) is a purely electronic method of thermodynamic thermometry. In primary JNT, the temperature is inferred from a comparison of the Johnson noise voltage of a resistor at the unknown temperature with a pseudo-random noise synthesized by a quantum-based voltage-noise source (QVNS). The advantages of the method are that it relies entirely on electronic measurements, and it can be used over a wide range of temperatures due to the ability of the QVNS to generate programmable, scalable, and accurate reference signals. The disadvantages are the requirement of cryogenic operation of the QVNS, the need to match the frequency responses of the leads of the sense resistor and the QVNS, and long measurement times. This review collates advice on current best practice for a primary JNT based on the switched correlator and QVNS. The method achieves an uncertainty of about 1 mK near 300 K and is suited to operation between 4 K and 1000 K.

A graphene quantized Hall resistance (QHR) device fabricated at the National Institute of Standards and Technology (NIST) was measured alongside a GaAs QHR device fabricated by the National Research Council of Canada (NRC) by comparing them to a 1 kΩ standard resistor using a cryogenic current comparator. The two devices were mounted in a custom developed dual probe that was then assessed for its viability as a suitable apparatus for precision measurements. The charge carrier density of the graphene device exhibited controllable tunability when annealed after Cr(CO)3 functionalization. These initial measurement results suggest that making resistance comparisons is possible with a single probe wired for two types of quantum standards - GaAs, the established material, and graphene, the newer material that may promote the development of more user-friendly equipment.

At the National Institute of Metrological Research (INRIM) a measuring setup able to vary, accurately and in a selective way, the temperature, humidity and pressure to detect eventual hysteretic phenomena and to evaluate their effects on electrical standards as high precision standard resistors and Zener-diode based DC Voltage standards. Main element of the setup is a pressure-tight case housing the standards and acting the stresses due to the desired parameters. By means of pressure-tight-connectors, the standards under test are connected to the measuring instruments, while the climatic conditions are acquired through inner and external sensors. Humidity is controlled by means of two conditioning containers filled respectively with water and silica-gel, which regulate the air-moisture percentage the case in a range of 10-90%. A pumping system establishes the pressure in the case from 600 hPa to 1400 hPa. The characterization vs. humidity and pressure is made at constant temperature by means of a commercial air-bath acting from 15 and 45 ° C. Preliminary results have been obtained for pressure dependence of commercial 100 Ω resistors and of an INRIM-developed 10 V Zener-diode based DC Voltage standard.

We describe measurements using a quantum Hall system equipped with a liquefier and a room temperature direct current comparator bridge to transfer the R k-90 /2 value of 12906.4035 Ω to a 1 kΩ. Subsequently we scale from the 1 kΩ down to the 1 Ω to an uncertainty of 0.02 ppm. Estimates of the relative uncertainties of the quantum Hall effective are compared to the estimates of the older wet quantum Hall effective along with scaling methods will be provided in this report and a discussion of the advantages of the scaling paths.

A portable temperature controlled DC Voltage and DC Resistance Reference with Switching Unit (RSU) has been developed at National Institute of Metrological Research (INRIM) to calibrate multifunction electrical instruments involving 10 V, a 1 Ω and a 10 kΩ Standards. The resistors are made with two resistors nets while the 10 V is a low noise-drift INRIM-projected circuit. Preliminary measurement over one-month period shows standards uncertainties span from 3.0×10 -7 to 6.4×10 -7 , suitable for artifact calibration. With a real clock calendar, the RSU can show both the RSU Standards calibration values and updated ones between two calibrations according to an internal algorithm.

In preparation for the redefinition of the International System of Units (SI), five different electronic measurements of the Boltzmann constant have been performed using different Johnson noise thermometry (JNT) systems over the past seven years. In this paper, we describe in detail the JNT system and uncertainty components associated with the most recent National Institute of Standards and Technology (NIST) determination of the Boltzmann constant: k = 1.380642 9(69) × 10−23 J/K, with a relative standard uncertainty of 5.0 × 10−6 and relative offset of −4.05 × 10−6 from the Committee on Data for Science and Technology (CODATA) 2014 recommended value. We discuss the input circuits and the approach we used to match the frequency response of two noise sources. We present new measurements of the correlated noise of the 4 K on-chip resistors in the quantum-accurate, pseudorandom, voltage-noise source, which we used to estimate the correlated, frequency-dependent, nonthermal noise in our system. Finally, we contrast our system with those used in other measurements and speculate on future improvements.

Spectral aberration has been the main source of uncertainty in Johnson Noise Thermometry approach to measuring the Boltzmann constant. Recently, with newly developed hardware and the introduction of a novel fitting algorithm for analyzing the data, we have achieved a frequency independent spectral aberration for measurements with the NIST JNT system. Consequently, we performed a data drift analysis and an electromagnetic interference investigation to explain a residual offset that affects the Boltzmann constant.

Johnson Noise Thermometry is an electronic approach to determine the Boltzmann constant k through measurement of the ratio of the noise power measured across a sense resistor at the triple point of water and the measured noise-power of a synthesized reference waveform. The reference waveform is synthesized by an array of Josephson junctions. In this paper, we show how incorrect averaging to determine the ratio can produce a systematic error in the measured value of k. We describe the appropriate algorithm to avoid this error.

In 2010, NIST measured the Boltzmann constant k with an electronic technique that measured the Johnson noise of a 100 Ω resistor at the triple point of water (TPW) and used a voltage waveform synthesized with a quantized voltage noise source (QVNS) as a reference. In this paper, we present measurements of k using a 200 Ω sense resistor and appropriately modified QVNS circuit and waveform. Preliminary results show agreement with the previous value within the statistical uncertainty.

A quantum-voltage-calibrated Johnson-noise thermometer was developed at NIM, which measures the Boltzmann constant k by comparing the thermal noise across a 100 Omega sense resistor at the temperature of the triple point of water with the pseudo-random frequency-comb voltage waveform synthesized with a bipolar-pulse-driven quantum-voltage-noise source. A measurement with integration period of 10 hours and bandwidth of 640 kHz resulted in a relative offset of 0.5 x 10(-6) from the current CODATA value of k, and a type A relative standard uncertainty of 23 x 10(-6). Benefiting from closely matched noise powers and transmission-line impedances and small nonlinearities in the cross-correlation electronics, the derived k shows self-consistent values and standard uncertainties for different measurement bandwidths.

A new quantum voltage calibrated Johnson noise thermometer (JNT) was developed at NIM to demonstrate the electrical approach that determines the Boltzmann constant k by comparing electrical and thermal noise power. A measurement with an integration period of 10 hours and bandwidth of 640 kHz results in relative offset of 0.5×10 -6 from the current CODATA value of k, and type A relative standard uncertainty of 23×10 -6 . The quadratic fitting parameters of the ratio spectrum show a flat response with respect to the measurement bandwidth. This flat response is a dramatic improvement compared to the response produced by the NIST JNT system that dominated the relative combined uncertainty of previous measurements of k.

Johnson Noise Thermometry is an electronic approach to measuring temperature. For several years, NIST has been developing a switching-correlator-type Johnson-noise thermometer that uses a quantized voltage noise source as an accurate voltage reference. When this method is used to measure resistors at the triple-point of water, the system creates a direct electronic method for determining the ratio of the Boltzmann constant k to the Planck constant h. In 2010, NIST optimized the JNT system for use with 100 Omega sense resistors and produced a determination for k with a relative standard uncertainty of 12 x 10(-6). In order to further validate and improve the measurement method, we modified the system to operate with a 200 Omega resistor source instead of the 100 Omega source. In this paper, we summarize the technical challenges and achievements to date and project what is achievable in the near future.

We report new results for Johnson noise thermometry in the range 693 K to 800 K. The results are based on operation of a quantized voltage noise source (QVNS) which serves as a reference for the relative noise power spectral density. This paper describes the modifications to the system in order to overcome the limitations in our previous work and furthermore enable measurements of T-T-90 at higher temperatures.

The Boltzmann constant was measured by comparing the Johnson noise of a resistor at the triple point of water with a quantum-based voltage reference signal generated with a superconducting Josephson-junction waveform synthesizer. The measured value of k = 1.380651(18) \times 10^-23 J/K is consistent with the current CODATA value and the combined uncertainties. This is our first measurement of k with this electronic technique, and the first noise thermometry measurement to achieve a relative combined uncertainty of 13 parts in 10^6. We describe the most recent improvements to our Johnson Noise Thermometer that enabled the statistical uncertainty contribution to be reduced to seven parts in 10^6, as well as the further reduction of spurious systematic errors and EMI effects. The uncertainty budget for this measurement is discussed in detail.

The National Institute of Standards and Technology has developed a quantum-voltage-noise-source-calibrated Johnson noise thermometer (JNT) to provide a new electronic measurement technique for determining the Boltzmann constant. Improvements in electronics and synthesized noise waveforms have led to reduced uncertainty in the measurement. Recent investigations have shown that some of the distortion in the present electronics arises in the differential stage of both the preamplifier and the analog-to-digital converter (ADC). The distortion can be reduced by compensating the direct current offset of the signal at the inputs to the differential stage. A four-channel cross correlation JNT with optimized preamplifiers and new ADCs is being assembled. The improvements are on track to reach the goal of an electronic measurement of the Boltzmann constant at a relative uncertainty of 6 x 10(-6).

Improved electronics and synthesized noise waveforms for the NIST quantum-voltage-standard-calibrated Johnson noise thermometer (JNT) have lead to reduced uncertainty in the temperature measurement. Recent measurements show that some of the distortion in the present electronics arises in the differential stage of both the preamplifier and the analog-digital converter (ADC). Distortion from the preamplifier can be reduced by compensating the DC offset of the signal at the inputs to the differential stage. A four-channel system with a new ADC is under construction to achieve the goal of re-determining the Boltzmann constant at a relative uncertainty of 6×10-6.

Long integration time is necessary to reach low uncertainty when measuring temperature through Johnson Noise Thermometry (JNT). The main goal of the NIST JNT experiment is to achieve a 6×10-6 relative uncertainty in the measurement of the water triple point, which could contribute to the re-determination of Boltzmann's constant. A four-channel JNT system, which will reduce the measurement time two-fold, is being developed with new components, including a switchboard, an analog to digital converter (ADC) and a programmable, recharging power supply system. While implementing the new ADC, a source of systematic error was revealed, as well as a means to increase the measurement bandwidth.