A Johnson noise thermometer with traceability to electrical standards

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 k(B); or, conversely, it can be employed to perform measurements at the triple point of water and obtain a determination of k(B). 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 microK/K of both instruments. A path towards future improvements of JNT accuracy is also sketched.