The state of the art is the chip-scale atomic clock https://en.wikipedia.org/wiki/Chip-scale_atomic_clock https://en.wikipedia.org/wiki/Chip-scale_atomic_clock

The state of the art is the chip-scale atomic clock https://en.wikipedia.org/wiki/Chip-scale_atomic_clock https://en.wikipedia.org/wiki/Chip-scale_atomic_clock which came out of a research project at NIST. It is useful for devices that need good accuracy for long holdover times without radio reception. A Microsemi SA.45 CASC retails for around $1500.