Fundamental metrology in the future: Measuring the single quantum

Great progress has been made over the past 50 years in replacing artefact definitions of the SI base units by those based on fundamental constants. However it is important to recognise that the development of the SI is not ‘complete’ once the base units are all defined in this way. New physics and technologies can always be expected to emerge and these will require developments and modifications to the SI. At the present time a dominant trend seems to be that measurements are changing from dealing with average properties of ensembles to metrology on single entities. This is already apparent for single atoms, ions or photon detection and measurement but can be expected to extend into the technologically important areas of single spins, single THz photons and even phonons in the future. Another significant development may be expected towards measurement of quite different physical quantities from those currently required, such as spin current, decoherence time etc. The paper addresses some of these issues, as well as the influence of quantum mechanics on precise measurement, which will also increase as metrology addresses both single entities and the nano scale.