Nuclear clock could steal atomic clock's crown - New Scientist - New Scientist

Nuclear clock could steal atomic clock's crown: A nuclear clock has not yet been made but the idea would be to use the atomic nucleus like a tuning fork. A nucleus will jump to a higher energy state, then fall back down, and jump up again, only if it is hit with a very specific frequency of light. Tuning a laser so that it prompts these jumps is a way to set its frequency with a phenomenal level of precision. The frequency can then be used like a clock's tick to keep time.

A similar method is used in atomic clocks, except it is the electrons orbiting the nucleus that make the energy jump...

A thorium clock controlled in this way would drift by just 1 second in 200 billion years...