Laser Frequency Measurements and the Redefinition of the Meter

The use of an atomic wavelength to define the meter assumes the permanence and universality of the atomic energy levels, Planck\´s constant, and the speed of light. The new definition proposed for the meter is based upon the path length traveled by light in a specified duration. It would involve only the last postulate, whereas the definition of the second involves the first two. The implementation of the new definition will have its accuracy limited, as usual, by poor realization rather than by fundamentals. On the one hand, one never realizes clean plane waves, even with visible light. On the other hand, one will have to measure the frequency of visible sine waves. This would look easier if one had a set of convenient reference frequencies measured "once and for all." The reliability of such frequencies can only be based upon repeated comparisons between a number of independent laboratories. At present, very few determinations of visible frequencies have been performed that may claim an accuracy exceeding 109. The reliability and convenience of these measurements must be improved. Progress in this respect will open the way to an easier implementation of the new definition of the meter. It is also a key to extend to the IR and visible range the electromagnetic high-frequency techniques. Frequency standards in this range would then have a definite appeal.