Realization of the metre by frequency-stabilized lasers

After a short review of the history of the definition of the SI base unit of length—the metre—precise methods for its realization are discussed. The paper gives an overview of current realizations of the metre by frequency-stabilized lasers. Furthermore, a detailed description of a precise optical frequency standard based on cold neutral calcium atoms is presented together with an estimation of the relative uncertainty (2 × 10−14) of its frequency.