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.
High-bandwidth, variable-resistance differential noise thermometry