Time to shrink the atomic clock

Sounds interesting, but I wonder about the accuracy of the article. HP has made 4U rack mount atomic time sources for years and the GPS satellites all have atomic clocks on board. The clocks that may be built with this new technique may be smaller than current ones, but the current clocks can be smaller than refrigerators.

If this thing isn't even going to be based on cesium, then comparing its size and expense to a standards-agency fountain clock is a bit unfair. Rubidium oscillator modules are common and cheap (cell phone base stations tend to be equipped with them) and pretty small (Symmetricom's smallest module is I think about 2" square).

Anyway, I'm curious how well the frequency stability of this thing can compare to tube-based atomic clocks.