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 f

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.