Summary

Background

The units of electricity play a key role in industrial, scientific and technological applications since the measurement of nearly all other quantities relies on them at some point. Electrical units are traced to quantum standards of voltage and resistance, using the quantum Hall Effect (QHE) for the latter. In this project a QHE system will be created which exploits unique properties of the material graphene, namely its high robustness against increased drive current and operating temperature at still moderate magnetic fields. Simplified QHE systems will become available which can broaden the QHE user base considerably. In a first step, the main part of the project will progress beyond state of the art by fabricating stable graphene devices tailored to operate under the above mentioned relaxed conditions of temperature and magnetic field, and by establishing their exact operation margins. Two material fabrication routes will be explored, both based on scalable material growth techniques. In another part of the project a simplified measurement infrastructure, tailored to make best use of graphene’s advantages, will be developed.

 

Need for the project

The need for a better quantum resistance standard is triggered by the fact that for resistance no user of metrological services, and often not even National Metrology Institutes (NMIs), can take direct advantage from the perfect quantum standards: Equipment needed for exploiting the QHE is very specialised and expensive and requires highly experienced staff. While quantum standards of voltage are widespread and easily transported, the same level of simplicity is not possible for quantum standards of resistance. Hence long chains of hierarchical calibrations are required, with the primary quantum reference only available in large NMIs. This leads to loss in precision at each chain link, and to loss of time and money due to periodic recalibrations of secondary standards.

A QHE system which is simpler, transportable, and provides the primary reference closer to the end user will reduce the cost and inconveniences of traditional calibration chains, constituting a real breakthrough for European and worldwide metrology. For the quantum Hall effect there is now a first-time opportunity to create such a system: graphene exhibits the QHE at lower magnetic fields and higher temperatures than any other material, and systems become possible that use closed cycle cryo-coolers or simple liquid helium dewars. Combined with compact superconducting magnets such systems fulfil the main requirements of simplicity and transportability – eventually becoming a ‘bench-top QHE’. Their reduced cost and complexity will allow deploying them more widely - into smaller NMIs, into industry, or allowing a dedicated QHE reference at the point of use.

 

Scientific and technical objectives

The project will employ a wide scope of methods, from fundamental and material science studies, precision measurements, to fabrication and instrument development, in order to supply a new generation of robust quantum Hall devices as well as novel instrumentation, enabling a largely simplified dissemination of the unit of resistance. A simpler to use, yet ultimately precise quantum standard of resistance based on graphene will be created by:

  • Advancing the device fabrication methods to fulfil requirements of metrology in terms of homogeneity, contact resistance, variety of sizes, controlled disorder, edge structure, etc., and by developing the procedures for precise, quantitative, non-destructive characterisation of graphene and graphene devices combining structural, chemical and physical methods. (Work package 1)

  • Performing precision QHE measurements on graphene to test the limits of achievable uncertainty under relaxed experimental requirements of temperature and magnetic field. (Work package 2)

  • Investigating ac-losses in graphene devices and demonstrating the ac-QHE in graphene, thereby assessing the potential of a graphene-based standard of impedance. (Work package 3)

  • Developing customized instrumentation which allows using graphene as an intrinsically referenced resistance standard, including simplified instrumentation required to scale the singular QHE value of 12.9 kΩ to other values. (Work package 4)

 

Expected results and potential impact

The key outcome of this JRP will be new, simpler to operate quantum resistance standards and adapted instrumentation to exploit them. This will have a groundbreaking effect on the metrology landscape where it will directly impact the electricity community formed by all the NMIs and calibration laboratories by enabling them to disseminate the units ohm and farad in a simpler and quicker way.

The availability of the best imaginable standard of resistance, the fundamental constant based QHE reference, will significantly improve calibrations by shortening the calibration chains, making them more efficient and economic. A standard that is more distributed, with the primary reference available wherever and whenever it is needed, will reduce the costs and inconveniences of traditional approaches. Such improved dissemination will constitute a real breakthrough not only for calibration laboratories, but also for those NMIs which up to now cannot afford to operate the complex quantum standards.

The impact of the project will result from two developments:

  • Graphene-based quantum Hall effect devices operating under strongly relaxed conditions of temperature and magnetic field, along with a technological method of fabricating them, will largely increase the user base of ultimately precise intrinsically referenced standards.

  • The specific instrumentation to utilize the graphene QHE devices, differing from instrumentation exploiting traditional devices in terms of simplicity and added capability for device conditioning, will be available as well as simplified accessory instrumentation which is vital for scaling the QHE resistance from its singular quantized value to all values relevant in practice.