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

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