Universality of the quantized Hall resistance

The discovery of the quantum Hall effect (QHE) in 1980 by Klaus von Klitzing has revolutionized the resistance metrology. Today, the quantized Hall resistance (QHR), observed in a two-dimensional electron gas exposed to a high magnetic flux at low temperatures, is used worldwide as an invariant reference for electrical calibrations. It has improved the reproducibility of calibration results by up to two orders of magnitude during the last two decades. The best resistance bridges such as the cryogenic current comparator or the Josephson potentiometer allow the comparison of the QHR with conventional resistance standards at a level of a few parts in 10¹⁰.