The route to atomic and quantum standards

Over the past half-century, there has been a shift away from standards based on particular artifacts toward those based on physical effects, the most stable being based on quantum properties of systems. This change was proposed at the end of the 19th century but is still not complete at the start of the 21st. We discuss how this vision has been implemented through recent advances in science and metrology and how these may soon lead to an SI system finally free from artifact standards, with a consistency based on fundamental constants.
Quantities, Units, and Standards To investigate any physical phenomena, we must make measurements, communicate them to others, and record them in a way that will be understandable in the future. [...]