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 Josephsonjunction waveform synthesizer. The measured value of k = 1.380651(18)×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 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  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)10 -23 J/K, for which the relative standard uncertainty is 3.510 -6 and the relative offset from the CODATA 2010 value is +1.910 -6 .

A value for the Boltzmann constant was measured electronically using an improved version of the Johnson Noise Thermometry (JNT) system at the National Institute of Standards and Technology (NIST), USA. This system is different from prior ones, including those from the 2011 determination at NIST and both 2015 and 2017 determinations at the National Institute of Metrology (NIM), China. As in all three previous determinations, the main contribution to the combined uncertainty is the statistical uncertainty in the noise measurement, which is mitigated by accumulating and integrating many weeks of cross-correlated measured data. The second major uncertainty contribution also still results from variations in the frequency response of the ratio of the measured spectral noise of the two noise sources, the sense resistor at the triple-point of water and the superconducting quantum voltage noise source. In this paper, we briefly describe the major differences between our JNT system and previous systems, in particular the input circuit and approach we used to match the frequency responses of the two noise sources. After analyzing and integrating 49 days of accumulated data, we determined a value: k = 1.380 642 9(69)×10−23 J/K with a relative standard uncertainty of 5.0×10−6 and relative offset −4.05×10−6 from the CODATA 2014 recommended value.

The paper presents an absolute Johnson noise thermometer (JNT), an instrument to measure the thermodynamic temperature of a sensing resistor, with traceability to voltage, resistance and frequency quantities. The temperature is measured in energy units, and can be converted to SI units (kelvin) with the accepted value of the Boltzmann constant kB; or, conversely, it can be employed to perform measurements at the triple point of water and obtain a determination of kB. The thermometer is composed of a correlation spectrum analyzer and a calibrator. The calibrator generates a pseudorandom noise (at a level suitable for traceability to an ac voltage standard) by digital synthesis, scaled in amplitude by a chain of electromagnetic voltage dividers and cyclically injected in series with the Johnson noise. First JNT measurements at room temperature are compatible with those of a standard platinum resistance thermometer within the estimated combined uncertainty of 60 µK K−1 of both instruments. A path towards future improvements of JNT accuracy is also sketched.