| That CSAC module has of course a much better long-term accuracy than the rubidium oscillator from this article. Nevertheless, the rubidium clock qualifies as an atomic clock as the distinction has nothing to do with the precision or accuracy of the clock. If the average value of the frequency of the clock is determined by the dimensions of some resonating device, which can never be reproduced perfectly identical, then it is a non-atomic clock. If the average value of the frequency of the clock is determined by a resonance frequency of some atoms, ions or molecules, then it is an atomic clock, because, at least in principle, it should be possible to make clocks with identical average frequencies. (In the cheaper atomic clocks the reproducibility is not ideal, because the output frequency is shifted by various constructive details, e.g. the buffer gas in rubidium oscillators). So a rubidium clock qualifies. A less obvious example is a helium-neon laser with the frequency tuned to a resonance of the methane molecule. If you use that laser with the frequency determined by methane together with a frequency divider (made with a mode-locked fiber laser) providing an output frequency in the MHz range, you have an atomic clock, and one much better than most rubidium clocks and than many of the cheaper cesium clocks. |