Origin of the noise difference between helium-gas and liquid-helium cooling

Determine the origin of the observed difference in noise between the helium-gas-cooled and liquid-helium-cooled cryogenic current comparator during quantum Hall resistance–to–100 Ω resistance-ratio measurements.

Background

The paper reports that, for QHR/100 Ω resistance-ratio measurements, the Allan deviation under liquid-helium cooling was approximately 35% lower than under helium-gas cooling. This difference was resolvable because the quantum Hall device operates below 1 K, reducing thermal noise and making noise originating in the cryogenic current comparator more significant.

The authors note that pulse-tube-related noise may contribute to the discrepancy, but the available measurements do not establish its cause. Resolving the origin is important for understanding and potentially improving the precision of helium-gas-cooled cryogenic current comparator systems. Its inclusion is supported by the authors’ explicit statement that the origin remains unclear.