Following a recommendation of Comité International des Poids et mesures (CIPM) in 1990, the quantum Hall effect (QHE) is now used worldwide as a representation of the resistance unit, the ohm. The use of the QHE as primary standard for resistance has improved the reproducibility in the field of resistance metrology by a factor of 100 and has reached the level of a few parts in 109 in well equipped laboratories. Despite this success, there is still room for improvement:
Up to now, QHE standards are only available as single values of 6.45 kohm and 12.9 kohm. To continuously cover the whole range of values needed by industry, the technological basis must be developed for a massive series and parallel connection of single quantum devices and for adequate scaling devices.
For quantised Hall resistors a first attempt has been made to extend the range of resistance to lower and higher values by means of a parallel or serial connection of single Hall elements. By means of a sophisticated circuit lay-out, the influence of the connecting leads on the quantised resistance can be made negligible. As a consequence a more complex circuit lay-out is needed which presently limits the number of elements connected in series or parallel. The developments aim at a quantised resistance scale which ranges from about 1 ohm to 1 Mohm and devices which allow higher operation currents.
Up to now, electrical quantum standards have a very limited transportability and can only be operated under a controlled laboratory environment. Therefore, industry can not make direct use of these standards. If transportable quantum standards would be available, the uncertainty of measurement and the down time of the reference standards of industrial calibration laboratories could be lowered, because on-site calibrations could be performed instead of sending the reference standards to a NMI for calibration.
The target is to make compact, transportable quantum standards available. A necessary prerequisite is the development of special cryo-electronic circuits which are suited for an operation in compact cryostats. For QHE devices the operation with magnetic fields as low as possible will be in the foreground which, at the same time, could lead to simplified cryostats. To operate QHE devices at low magnetic fields, new classes of materials must be investigated which show distinct quantum Hall plateaux at low magnetic fields. Graphene, is a new very promising material. Its original and exotic electronic properties make it very interesting for metrology. For instance one can imagine to use it for the realization of a room temperature QHE based resistance standard, or for a quantum resistance standards operating at lower magnetic field (1 Tesla for instance). Such a quantum standard would drastically reduce the experimental implementation cost of the standard. Besides, a graphene based QHE resistance standard could allow to reach better accuracies of calibration.
This project is carried out by a consortium of six National Metrology Institutes across Europe; it is partly funded by the European Community’s Seventh Framework Programme, ERA-NET Plus under Grant Agreement No 217257.
