A Compact Self-Contained Cryogen-Free Instrument for the Realization of the Ohm in the Revised SI

The redefinition of the SI base units ampere and kilogram in 2019 formalized the use of the quantum Hall effect (QHE) to provide resistance traceability (the SI ohm) from the fundamental constants h and e. Traditionally, realization of the ohm via the QHE has required large complex liquid helium cryostats (including a high field superconducting magnet), and been largely confined to National Measurement Institutes. In recent years, graphene has been demonstrated as an ideal material for QHE samples, offering access to the quantum resistance reference (RK=h/e2) at lower magnetic fields and higher temperatures than previously possible. We present a system that builds on this technological advance, combined with liquid helium-free (closed cycle) cryogenic cooling techniques. The system integrates both a graphene QHE reference and a Cryogenic Current Comparator (CCC) instrument into a single compact enclosure. Resistance bridges based around a CCC offer the ultimate accuracy and noise performance for comparisons of conventional room temperature standard resistors to the QHE reference, and for scaling between different decade values, but this technology has not previously been demonstrated without the use of liquid helium. Our CCC system also integrates a second cryogenic SQUID detector to operate as the critical nanovoltmeter in the bridge electronics. We use a latest generation polymer-encapsulated molecular doped epigraphene sample optimized for operation at the 5 T field of our compact magnet, which does not require any user tuning of device properties on repeated cool-down cycles. Combined with the cryogen-free cooling, this gives a truly ‘turn-key’ system, making the quantum resistance reference and CCC accuracy available 24/7 in the metrology laboratory with no regular user intervention. The system is designed for both the realisation of the ohm at 100 Ω and regular calibration of standard resistors in the range 1 Ω to 10 kΩ, with combined relative standard uncertainties down to 0.01 ppm in the best cases.