Primary resistance metrology simplified by using graphene
The Table Top Quantum Hall (TTQH) device is used to realise the ohm, the SI unit of electrical resistance. The primary standard for resistance is based on the quantum Hall effect (QHE), which requires low temperatures, high magnetic fields and skilled operators.
NPL has developed a compact, cryogen-free and fully automated system for high‑precision resistance calibration or quick device characterisation. It uses graphene to realise the QHE in a compact and simple device.
Graphene, a single layer of carbon atoms with many unique properties, has a very strong quantisation of the Hall resistance in moderate magnetic fields and at easily achievable temperatures. Using specially-designed graphene devices, we have developed a turn-key table-top quantum Hall system for high-accuracy resistance metrology which combines simple, reliable operation with low running costs.
Larger National Measurement Institutes (NMIs) will find that this simplifies existing QHE equipment in resistance calibration services, giving cost and time savings, and reducing uncertainties by making more direct use of quantum reference. Smaller NMIs and manufacturers will find that the acquisition of a QHE reference more accessible, providing independent SI traceability to underpin calibration and measurement capabilities for customers, avoiding the need send artefacts to external laboratories for calibration.
This service will be of interest to NMIs, accredited calibration laboratories, metrology instrument manufacturers and academic laboratories who want a 24/7 reference for resistance.
Cryogen-free: The price of liquid helium continues to increase and in some places cryogenic liquids are difficult to source. A pulse-tube cooler only requires electrical power and is capable of achieving near-continuous operation (service interval is approximately 20,000 hours). The base temperature of a small table-top system is approximately 2.5 K.
Turn-key: Operation of the pulse-tube cooler is completely automatic. No prior experience of cryogenic techniques is required to operate this system.
High precision: Exploiting the unique properties of graphene, this system is capable of achieving an accuracy of 2 parts per billion for a measurement current of approximately 25 μA through the quantum Hall device.
Dual system: NPL has developed a cryogenic current comparator and null detector which can be included in the same pulse-tube cooler which houses the quantum Hall device. In addition a fast sample exchange mechanism is being considered, which would allowthe system to be used as a tool for quick device characterisation. This can be useful for non-metrological applications such as graphene characterisation in production facilities.
The National Graphene Metrology Centre (NGMC) brings together researchers and facilities from across NPL. It builds capability in metrology for graphene (and 2D materials) in a coordinated and focused way to address the growing demands for this emerging industry area. NPL is part of the EU Flagship Graphene project.