Fifty years of advance in frequency standards

Fifty years ago next October, the V^e Congres International de Chronometrie was held in Paris. In its proceedings there is a paper by L. Essen which shows a drawing of a caesium-beam frequency standard then under construction. Measurements in the paper were referred to quartz ring oscillators. The caesium apparatus was still for the future. Here I draw comparisons with that time. I also collate new data and present a list of precise frequency measurements, primarily at laser frequencies, following previous presentations of the list in 1997 and 2001. Recent advances have been accentuated by the introduction of femtosecond lasers for measuring optical frequencies against the caesium standard, and by a laser-cooled fountain version of that standard. Measurements noted include narrow transitions in atoms or ions susceptible to cooling by optical techniques, and so capable of observation with reduced second-order Doppler effect. Applications include searches for change with time of the fundamental constants.