Current status of the quantum metrology triangle

The quantum metrology triangle is a test of the consistency of three quantum electrical standards: the single-electron tunnelling current standard, the Josephson voltage standard and the quantum Hall resistance standard. This paper considers what is known about each of these effects separately in terms of (1) theory, (2) empirical tests of universality and (3) 'direct' tests involving fundamental constants. The current status of each of the three 'legs' of the triangle is quite different, with the single-electron leg being the weakest by far. This leads to the conclusion that a recent experimental result for the triangle should be interpreted primarily in terms of corrections to the quantum of charge transferred by single-electron devices.