The quantum standard for resistance is based on the quantum-Hall effect (QHE) in a two-dimensional electron gas. The conventional approach of layered semiconductor structures has recently been challenged by graphene, the two dimensional form of carbon. Graphene has many desirable properties over semiconductors in quantum-Hall effect (QHE) resistance standards. It offers the possibility to greatly simplify a QHE standard as it allows easier implementation and operates at lower magnetic fields and higher temperatures. VTT MIKES metrology has been developing quantum standards based on graphene and was recently involved in measurements showing that quantum standards for resistance as well as for impedance can be implemented by means of graphene.
Precision measurements at the International Bureau of Weights and Measures (BIPM) demonstrated for the very first time that a QHE standard based on graphene gives the same value with very high accuracy for the quantum Hall resistance RH = h/2e2 » 12.906 kW as traditional GaAs multi-layer structures. MIKES developed in tandem with Aalto University the graphene structures used in the measurements. By connecting several QHE graphene elements together, resistance standards for other values than the quantum Hall resistance can be constructed. MIKES manufactured a resistance standard having four graphene elements in series. Preliminary tests were very promising but the actual precision measurements remain to be done.
Inspired by the good results of the direct current (DC) measurement, MIKES together with the German Metrology Institute (PTB) examined the suitability of graphene for alternating current (AC) measurements. The first AC quantum-Hall effect measurements conducted at PTB demonstrated that graphene, due to its smaller losses, is even better suited for the implementation of an impedance standard than traditional GaAs structures.
The results obtained so far are so promising that already in the near future, graphene might be used to realise quantum standards based on the constants of nature for resistance and impedance, which operate at lower magnetic fields and at higher temperatures than the present standards. In addition, they are simpler and less expensive to implement, and, above all, much easier to use.