Introduction to optical atomic clocks
Optical atomic clocks use optical transitions in laser cooled neutral atoms or ions as quantum frequency reference (QFR) (see explanation in Figure 1). The invention of the femtosecond frequency comb has made it possible to precisely count frequencies in the optical domain, and to transform optical waves into radiofrequency waves or signals, where they can be used and processed by more traditional techniques.
The scientific challenges for optical atomic clocks are the establishment of techniques for reliable and simple preparation of suitable QFRs, the control of systematic effects to a high degree of accuracy, and the development of the required components, in particular ultrastable laser sources at the frequencies corresponding to the clock transitions. From an application point of view, the technological challenge lies in developing a system that is robust and whose electronic control is sufficiently sophisticated that unattended, automatic operation is possible.
Two approaches towards optical clocks are pursued in the field of time metrology at present. The first is based on a single ion trapped in an electrodynamic trap, where the storage time can exceed many weeks. The second is based on using ensembles of tens of thousand neutral atoms trapped for a relatively short time (seconds) in a trap formed by standing optical waves (an optical lattice) delivered by a laser. Here, a new ensemble of atoms is periodically reloaded into the trap and interrogated.
Figure 1 Prinicple of an optical atomic clock.
A laser, the local oscillator, interrogates an ensemble of ultracold atoms, the QFR. In the case of a lattice optical clock (shown) the atoms are at micro-Kelvin temperature and trapped by laser waves The interrogation of the QFR by the local oscillator results in a signal proportional to the partial absorption of its radiation (frequency n), which is maximum when its freqeuncy n corresponding to the center frequency n0 of the atomic resonance.Using a feedback control system, the laser frequency n is continuously kept tuned on the atomic resonance frequency, so that n = n0 . The resulting ultra-stable optical frequency n0 can be converted to an equally stable radio-frequency by means of a femtosecond laser frequency comb.