The new mise en pratique for the metre—a review of approaches for the practical realization of traceable length metrology from 10−11 m to 1013 m

The revised International System of Units (SI) came into force on May 20, 2019. Simultaneously, updated versions of supporting documents for the practical realization of the SI base units (mises en pratique) were published. This review paper provides an overview of the updated mise en pratique for the SI base unit of length, the metre, that now gives practical guidance on realisation of traceable length metrology spanning 24 orders of magnitude. The review begins by showing how the metre may be primarily realized through time of flight and interferometric techniques using a variety of types of interferometer. Examples of techniques for measuring the interferometric phase and coping when the integer interference order is unknown are then described, together with examples of typical uncertainty contributions that may be encountered. The requirements for traceable nanoscale metrology and the need for an alternative secondary metre as identified by the Consultative Committee for Length’s Working Group on Nanometrology are outlined. These led to the inclusion in the mise en pratique of secondary realisations of the length unit at the nanometre and sub nanometre scale, based on the lattice spacing of silicon. Three measurement techniques using this secondary realisation are then described in detail. The paper concludes by emphasising that measurements made today over 24 order of magnitude are still compatible with measurements made using the metre as adopted over 200 years ago.