[fmajid]

I'm not sure I would call a rubidium oscillator an "atomic clock".

This, on the other hand, certainly qualifies:

https://coverclock.blogspot.com/2017/05/my-stratum-0-atomic-...

The CSAC module is somewhat more expensive, but still cheaper than a Swiss watch :-)

That said, just because you have an atomic frequency reference doesn't mean you have a clock. You still need to compensate for gravitational time dilation (because of General Relativity, time flows faster as your altitude increases and it's actually measurable using atomic clocks) to get TAI atomic time, and then factor in leap seconds for UTC.

[adrian_b]

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.