ABSTRACT
Over the past several decades, the field of atomic physics has seen outstanding developments, which have had an important impact on the development of atomic frequency standards. This is mainly due to improvements in laser technology, which has had a direct effect on the technique of optical pumping and made possible the cooling and trapping of ensembles of atoms to sub-Kelvin (sub-K) temperatures. The laser as such was developed in the 1960s. In early stages of its development, however, the use of the laser in the study of interactions between electromagnetic radiation and atoms was limited by the very restricted number of wavelengths available with the existing lasers and the rather narrow tunability of the instruments developed. The so-called dye laser extended the range of possible studies, but such an instrument is rather cumbersome and its size and complexity restricted its use to limited laboratory applications. The advent of room temperature solid-state diode lasers, which could be fabricated to emit wavelengths in resonance with many transitions in alkali atoms such as Cs and Rb, for example, as well as in ions such as Sr+ and Yb+, among many others, changed the whole picture. The power emitted by such lasers may be several tens of milliwatts. The spectrum of the radiation emitted is relatively narrow and the frequency can be adjusted over a relatively broad range by means of doping the substrate with selected elements at the time of fabrication. Fine frequency tuning of the device can also be done by means of the temperature of the substrate and its driving current. This property allows the exciting of transitions in selected atoms from one single ground-state hyperfine level to an excited state. This makes possible nearly complete inversion of population in ground states through optical pumping. Furthermore, as will be seen below, the exchange of momentum between a photon and an atom makes possible alteration of atomic velocities leading to so-called laser cooling of atoms. These two possibilities by themselves have totally altered the landscape of research in the field of atomic frequency standards leading to new realizations in both the microwave and the optical ranges. We will review selected subjects and recent advances in atomic physics that have had a specific impact on
the development of atomic frequency standards. For a broader overview of such advances in atomic physics, the reader is referred to the excellent review on the subject by Cohen-Tannoudji and Guéry-Odelin (2011).
