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Cesium-based atomic clocks ("fountain clocks") like these use the natural resonance frequency of electron orbital energy states under a microwave laser which can be counted to measure time. Since there is a natural background noise of microwaves and many frequencies can interact with orbitals it's important to isolate the atoms from outside sources of electromagnetic radiation and heat in order to maintain accuracy.

Earlier this year there was a big leap in so-called "nuclear clocks" which uses the resonant frequency of energy states of a nucleus itself as opposed to electron orbitals around it. Besides the "more frequency = more better" factor that has always driven clock accuracy -- thorium-229 nuclei excites in ultraviolet wavelengths -- nuclear clocks are better isolated than electron orbital-based clocks because the frequency band where they interact is impossibly narrow. In fact, the reason why it was only recently demonstrated is due to the difficulty of producing the required frequency at a high enough precision to interact reliably. This could lead to more accurate and more compact and cheaper clocks.

Discussion 4 months ago: https://news.ycombinator.com/item?id=42362215 | Major Leap for Nuclear Clock Paves Way for Ultraprecise Timekeeping (nist.gov)

Some years back there was talk of an atomic clock that came in a very small package (matchbox / cigarette pack size?). Iirc, saw a price indication of $1500 somewhere.

Something like that still around, and/or available? Any updated designs?

Personally I have no need for ultra-accurate timekeeping. But hey... an atomic clock is way cooler than a Nixie clock or oven-controlled Xtal oscillator. And no... huge 2nd hand atomic clock found on eBay etc doesn't cut it. Too big /heavy / power-hungry.

I recall reading that our ability to measure time accurately exceeds that of any other quantity. According to the NIST, the newest Optical Lattice clocks would drift by less than 1 second if they were started 13 billion years ago at the big bang. What else can we measure down past 1 part per 10e18?

How do you sync atomic clocks? When the error rate is 1/1e16, your error in propagating the time from one clock to another is going to be off by many orders of magnitude vs the timekeeper itself.

Can't mention a fountain clock without calling to mind one the Lion Clock in Alhambra Palace, of my favorite things ever:

https://www.iflscience.com/this-incredible-islamic-fountain-...

NIST explainer: Cesium Fountain Clock [1999]

https://www.nist.gov/news-events/news/1999/12/nist-f1-cesium...

What are 'limits' on how accurate enough clocks 'should' be?

Presumably there's diminishing returns, but as the article says we're at one part in 2.2e-16, are there practical application of going further?

New goal: upgrading from a Rubidium CSAC to NIST-F4.

I've also been reading about nuclear clocks[1]... skipping over the uncertainty of the entire atom's chaotic oscillations entirely!

[1] https://en.wikipedia.org/wiki/Nuclear_clock

How many atomic clocks are in operation in Colorado now? It would be nice if they could be spread around a bit. I suppose there are logistical issues that keep them centralized?