The CSACs that I have in a couple devices are 'atomic', and use Rubidium, but they're a bit lower accuracy than Cesium clocks [1] or Hydrogen Masers [2].

There are a few folks on the time-nuts mailing list who own such exotic pieces of hardware, but those are pretty far out of reach for most!

[1] https://www.microchip.com/en-us/products/clock-and-timing/co...

[2] https://www.microchip.com/en-us/products/clock-and-timing/co...