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by mikewarot 898 days ago
Yes they count, but they use a completely different principle of operation.

The Cesium beam clock actually has a stream of atoms flowing through it, which is finite, leading to tube lifetimes of a decade or less.

The rubidium clock has a number of cells of rubidium gas, one of which is a light source, the other of which gets hit with RF, and if the frequency is just right, it absorbes about 1% of the light before it hits the detector.

A much smaller physics package, to be sure. They do drift, though, a tiny bit.