National Physical Laboratory

Electrical Quantum Standards

NPL is the UK’s centre for precise electrical measurement. All electrical measurements at frequencies below 10 MHz are traceable to our two primary standards, the Quantum Hall effect resistance, and the Josephson volt.

We offer calibration of DC voltage and resistance standards with uncertainties as low as 20 parts per billion. Our research interests include quantum current standards for closure of the metrological triangle, and AC Josephson standards for providing more accurate traceability of AC voltages. NPL is a world leader in developing instrumentation for precise voltage and resistance measurement.

  • 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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