Old-school timekeeping (Image: Dan Sparham/Rex Features)

ATOMIC clocks are the basis of GPS devices, they define the official length of the second and recently played a role in tracking subatomic particles that seemingly travelled faster than the speed of light. Now this “ultimate” timekeeper has a rival: a new method for making nuclear clocks suggests such devices could be 60 times as accurate as their atomic rivals.

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. The most accurate atomic clocks drift by an amount equivalent to just 4 seconds since the big bang. In principle, a nuclear clock could smash that. Ambient electric and magnetic fields affect electrons in atomic clocks, causing errors, but they would influence the tightly bound particles in the nucleus much less.


While nuclear clocks made from thorium atoms, which can be excited with relatively low-energy ultraviolet light, were first proposed in 2003, whether they would actually be more accurate than their atomic rivals was unclear. The exact frequency needed to excite a nucleus depends on the configuration of its orbiting electrons, which can vary, introducing uncertainty.

Now Corey Campbell at the Georgia Institute of Technology in Atlanta and colleagues have devised a scheme that uses lasers to carefully control the spatial orientation of the electron orbits in atoms. A thorium clock controlled in this way would drift by just 1 second in 200 billion years, the team claims – that is more than 14 times the age of the universe (arxiv.org/abs/1110.2490).

“They show that indeed these [drifts] can be very small or even negligible,” says Ekkehard Peik of the National Metrology Institute of Germany in Braunschweig, who is not a member of the team.

Before nuclear clocks become a reality, researchers must identify the precise frequency of light needed to excite thorium nuclei.

Such clocks could shed light on string theory. The frequency of the jumps in a nuclear clock will depend on the strong nuclear force, while the jumps by electrons in atomic clocks depend on a different fundamental force. So together they could reveal if the relative strength of the forces changes, as string theory has it.