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Recent measurements using acoustic gas thermometry have determined the value of the Boltzmann constant, k, with a relative uncertainty less than 1 × 10−6. These results have been supported by a measurement with a relative uncertainty of 1.9 × 10−6 made with dielectric-constant gas thermometry. Together, the measurements meet the requirements of the International Committee for Weights and Measures and enable them to proceed with the redefinition of the kelvin in 2018. In further support, we provide a new determination of k using a purely electronic approach, Johnson noise thermometry, in which the thermal noise power generated by a sensing resistor immersed in a triple-point-of-water cell is compared to the noise power of a quantum-accurate pseudo-random noise waveform of nominally equal noise power. The experimental setup differs from that of the 2015 determination in several respects: a 100 Ω resistor is used as the thermal noise source, identical thin coaxial cables made of solid beryllium–copper conductors and foam dielectrics are used to connect the thermal and quantum-accurate noise sources to the correlator so as to minimize the temperature and frequency sensitivity of the impedances in the connecting leads, and no trimming capacitors or inductors are inserted into the connecting leads. The combination of reduced uncertainty due to spectral mismatches in the connecting leads and reduced statistical uncertainty due to a longer integration period of 100 d results in an improved determination of k = 1.380 649 7(37) × 10−23 J K−1 with a relative standard uncertainty of 2.7 × 10−6 and a relative offset of 0.89 × 10−6 from the CODATA 2014 recommended value. The most significant terms in the uncertainty budget, the statistical uncertainty and the spectral-mismatch uncertainty, are uncorrelated with the corresponding uncertainties in the 2015 measurements.

The neon triple point is one of the defining fixed points of the International Temperature Scale of 1990 (ITS-90). Although recognizing that natural neon is a mixture of isotopes, the ITS-90 definition only states that the neon should be of ?natural isotopic composition?, without any further requirements. A preliminary study in 2005 indicated that most of the observed variability in the realized neon triple point temperatures within a range of about 0.5?mK can be attributed to the variability in isotopic composition among different samples of ?natural? neon. Based on the results of an International Project (EUROMET Project No. 770), the Consultative Committee for Thermometry decided to improve the realization of the neon fixed point by assigning the ITS-90 temperature value 24.5561?K to neon with the isotopic composition recommended by IUPAC, accompanied by a quadratic equation?to take the deviations from the reference composition into account. In this paper, the uncertainties of the equation?are discussed and an uncertainty budget is presented. The resulting standard uncertainty due to the isotopic effect (k = 1) after correction of the calibration data is reduced to (4 to 40)??K when using neon of ?natural? isotopic composition or to 30??K when using 20Ne. For comparison, an uncertainty component of 0.15?mK should be included in the uncertainty budget for the neon triple point if the isotopic composition is unknown, i.e. whenever the correction cannot be applied.

We report on an improved systematic evaluation of the JILA SrI optical lattice clock, achieving a nearly identical systematic uncertainty compared to the previous strontium accuracy record set by the JILA SrII optical lattice clock (OLC) at $2.1 \times 10^{-18}$. This improves upon the previous evaluation of the JILA SrI optical lattice clock in 2013, and we achieve a more than twenty-fold reduction in systematic uncertainty to $2.0 \times 10^{-18}$. A seven-fold improvement in clock stability, reaching $4.8 \times 10^{-17}/\sqrt{\tau}$ for an averaging time $\tau$ in seconds, allows the clock to average to its systematic uncertainty in under 10 minutes. We improve the systematic uncertainty budget in several important ways. This includes a novel scheme for taming blackbody radiation-induced frequency shifts through active stabilization and characterization of the thermal environment, inclusion of higher-order terms in the lattice light shift, and updated atomic coefficients. Along with careful control of other systematic effects, we achieve low temporal drift of systematic offsets and high uptime of the clock. We additionally present an improved evaluation of the second order Zeeman coefficient that is applicable to all Sr optical lattice clocks. These improvements in performance have enabled several important studies including frequency ratio measurements through the Boulder Area Clock Optical Network (BACON), a high precision comparison with the JILA 3D lattice clock, a demonstration of a new all-optical time scale combining SrI and a cryogenic silicon cavity, and a high sensitivity search for ultralight scalar dark matter., Comment: 24 pages, 9 figures

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.

The last 25 years have seen tremendous progress in thermometry in the moderate temperature range (1 K to 1235 K). Various primary thermometers - based on different physics -have uncovered errors in the International Temperature Scale of 1990 and set the stage for the planned redefinition of the kelvin.

In 2006, the CIPM clarified the definition of the kelvin by specifying the isotopic composition of the water to be used in the realization of the triple point. At the same time, the Consultative Committee for Thermometry gave recommended values for the isotopic correction constants to be used for water departing from the specified composition. However, the uncertainties in the values for the correction constants were undesirably large due to unresolved differences between the data sets from which the values were determined. This paper derives improved values of the constants by considering additional data from isotopic fractionation measurements and the heats of fusion and freezing points of the relevant water isotopologues. Values of the corrections determined from the expanded data are AD = 671(10) μK, A18O = 603(3) μK, and A17O = 60(1) μK. A typical correction made with these values lies just within the expanded uncertainty (k = 2) of the corrections made with the older values, but has about half the uncertainty.

Currently, the CODATA value of the Boltzmann constant is dominated by a single gas-based thermometry measurement with a relative standard uncertainty of 1.8×10−6 [P.J. Mohr, B.N. Taylor, D.B. Newell, CODATA recommended values of the fundamental physical constants: 2006, Rev. Mod. Phys. 80 (2008) 633–730]. This article describes an electronic approach to measuring the Boltzmann constant that compares Johnson noise from a resistor at the water triple point with a pseudo-random noise generated using quantized ac-voltage synthesis. Measurement of the ratio of the two power spectral densities links Boltzmann's constant to Planck's constant. Recent experiments and detailed uncertainty analysis indicate that Boltzmann's constant can presently be determined using Johnson noise with a relative standard uncertainty below 10×10−6, which would support both historic and new determinations.

We have developed a Johnson noise thermometry (JNT) system that is calibrated by precision waveforms synthesized with a quantized voltage noise source (QVNS). Improvements to the QVNS and the cross-correlation measurement electronics have significantly reduced systematic errors and measurement uncertainty. We describe in detail a number of these improvements, such as amplifier performance and frequency response. We also show details of recent measurements that characterize the nonlinearities and distortion properties of the measurement electronics. Finally, we present an uncertainty analysis and discuss the potential for the QVNS-JNT technique to contribute to the redefinition of Boltzmann's constant.

A conventional technique for scaling temperatures in Johnson noise thermometry is via resistance ratios. We describe measurements from 1 kHz to 1 MHz using this approach via correlation methods in the frequency domain. We show that the effects of the mismatch in time constants of the input networks may be empirically treated to extract the noise power and temperature ratios in the low-frequency limit with uncertainties < 60 muK/K

We describe a new approach to Johnson noise thermometry (JNT) that exploits recent advances in Josephson voltage standards and digital signal processing techniques. Currently, high-precision thermometry using Johnson noise is limited by the nonideal performance of electronic measurement systems. By using the perfectly quantized voltage pulses from a series array of Josephson junctions, any arbitrary broadband waveform can be synthesized and used as a calculable noise source for calibrating the cross-correlation electronics used in JNT systems. With our prototype JNT system, we have found agreement to two parts in 10/sup 3/ with a 1/spl sigma/ uncertainty of 1/spl times/10/sup -3/ between the voltage noise of a 100-/spl Omega/ resistor in a triple-point Ga cell (T/sub 90/= 302.916 K) and a pseudo-noise waveform with the same average power that is synthesized by a quantized voltage noise source. We estimate the temperature of the resistor to be 302.5 K/spl plusmn/0.3 K (1/spl sigma/ uncertainty based on the uncertainty from the cross-correlation). With better characterization of our JNT system, we expect to achieve relative accuracies of parts in 10/sup 5/ for arbitrary temperatures in the range between 270 and 1000 K.

The unit of thermodynamic temperature, the kelvin, will be redefined in 2018 by fixing the value of the Boltzmann constant, k. The present CODATA recommended value of k is determined predominantly by acoustic gas-thermometry results. To provide a value of k based on different physical principles, purely electronic measurements of k were performed by using a Johnson noise thermometer to compare the thermal noise power of a 200 Ohm sensing resistor immersed in a triple-point-of-water cell to the noise power of a quantum-accurate pseudo-random noise waveform of nominally equal noise power. Measurements integrated over a bandwidth of 550 kHz and a total integration time of 33 days gave a measured value of k = 1.3806514(48)x10^-23 J/K, for which the relative standard uncertainty is 3.5x10^-6 and the relative offset from the CODATA 2010 value is +1.9x10^-6., Comment: 20 pages, 9 figures

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 Ω sense resistors and produced a determination for k with a relative standard uncertainty of 12×10−6. In order to further validate and improve the measurement method, we modified the system to operate with a 200 Ω resistor source instead of the 100 Ω source. In this paper, we summarize the technical challenges and achievements to date and project what is achievable in the near future.

A solenoid geometry for producing magnetic fields is introduced. A 1-A current has been stabilized using nuclear magnetic resonance techniques. A pair of tandem solenoids produces two uniform magnetic fields in opposite directions, and these fields are not affected by external magnetic shielding. The current and background field are controlled to within 0.1 ppm over an 8-h period. >

We report comparisons between the Shot Noise Thermometer (SNT), a primary thermometer based on the electronic noise from a tunnel junction, and the ITS-90 in the temperature range from 4 K to 30 K. The SNT indicates a temperature that is systematically high compared to T90, and we explain the probable origin of this effect. We characterize other systematic effects in the SNT and provide a roadmap for future extensions in the accuracy of the SNT which could make it metrologically useful in the lower-most ranges of the ITS-90 and especially in the range of the PLTS-2000.

We describe our progress towards a high-precision measurement of temperature using Johnson noise. Using a quantized voltage noise source (QVNS) based on the Josephson effect as a calibrated noise source, we have been able to measure the gallium and water triple point temperatures with an accuracy approaching 100 ppm

We describe a new approach to Johnson noise thermometry (JNT) that takes advantage of the recent advances in Josephson voltage standards and digital signal processing techniques. Previous attempts at high-precision thermometry using Johnson noise have been limited by the non-ideal performance of the electronic systems. By using the perfect quantization of voltages from the Josephson effect, any arbitrary broadband waveform can be synthesized and used as a calculable noise source for calibrating the cross-correlation electronics. With better calibration, we should be able to achieve relative accuracies of parts in 10/sup 5/ for an arbitrary temperature in the range between 84 K and 430 K. We present the latest measurements of our recently constructed JNT system.