Frequency standards and frequency measurement

Recent years have brought major breakthroughs in the development, operation and mutual comparison of frequency standards in the microwave as well as in the optical frequency domain. Several cold-atom fountains have been developed, using caesium as well as rubidium atoms. Some of them have been compared among each other. Mutual agreement of the order of one part in 10(15) was demonstrated for two caesium fountains operated side by side but also for two operated simultaneously in the US and Germany. Ultra-narrow resonances of optical transition lines were observed using single trapped ions, and a maximum line quality factor in excess of 10(14) was reported. Measurements of frequencies in the optical range of atomic transitions in neutral hydrogen and calcium, as well as in ions of indium, mercury, strontium and ytterbium, were performed. The last-named achievements were to a large extent enabled by the use of Kerr-effect mode-locked Ti:sapphire femtosecond lasers and techniques of broadening the output spectrum of these lasers to an octave-wide comb. This paper reviews the current status in these fields, with similar emphasis on the standards and on the feasibility to transfer the properties of the standards from a local laboratory to a broader community.