National Physical Laboratory


Our research interests are focussed on two goals: widening the area of application of the existing quantum standards of voltage and resistance, and developing a new quantum standard of current.

The aim of a quantum current standard is to generate an accurately known current by moving electrons one at a time. To achieve this, NPL, in collaboration with the University of Cambridge, is studying the properties of single-electron transport devices based on semiconductor nano-structures. We have also begun to investigate the possibility of exploiting quantum phase-slip in nano-scale superconducting wires. Once developed, the quantum current standard will be used together with the quantum Hall effect (QHE) resistance standard and the Josephson voltage standard to perform a metrological triangle experiment. This aim of this experiment to improve the consistency of the electrical units, and provide input to a possible re-definition of the SI planned for 2011.

The Josephson effect is well established as a primary standard for DC voltage. NPL, in collaboration with other National Measurement Institutes, has been developing primary AC voltage standards based on the Josephson effect. In particular, NPL has taken the lead in developing electronics for AC current-bias of Josephson arrays.

In collaboration with the University of Cambridge and Qinetiq, we are developing QHE devices based on new materials such as Indium Antimonide and Graphene. These devices will work at higher temperatures than the conventional Gallium-Arsenide ones, resulting in primary resistance standards that are more portable and cheaper to operate.

Electrical Quantum Standards research

  • DC voltage measurements at NPL are traceable to the SI volt via the Josephson effect, which establishes an accurate potential difference based on the fundamental constants, h and e, and frequency.
  • All electrical measurements below 10 MHz at NPL are traceable to two quantum standards: the quantum Hall effect (QHE) resistance standard and the Josephson voltage standard (JVS).
  • Since 1990, the quantum Hall effect (QHE) has been used as a primary resistance standard at NPL. The QHE takes place in special layered semiconductor structures with a 2-dimensional sheet of electrons trapped between two of the layers.
  • The properties of a phase-slip junction are identical to those of a Josephson Junction, but with current and voltage exchanged. For example, a Josephson junction biased at a low current is a perfect conductor, and a phase-slip junction biased at low voltage is a perfect insulator.


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