An ultra-precise clock that ticks to the beat of hydrogen molecules could probe fundamental constants and maybe spot signs of new physics.

Atomic clocks are the world’s most accurate timekeepers. They measure the frequency of radiation that makes an atom’s electrons jump from one energy level to another. This never changes for a given element, so a number of cycles of this radiation can define the second.

The clocks don’t just keep time. They have a range of applications from simulating quantum systems to measuring gravity. But current clocks are reaching limits when it comes to the precision needed to probe some mysteries.

Changing constants

One experiment uses atomic clocks to study the mass ratio between protons and electrons. The ratio seems constant, but it may be changing very slightly as the universe expands. Spotting even a small shift in the ratio could tell us whether this fundamental constant actually had different values in the past. This in turn might hint at what the cosmos was like in its earliest moments.


Stephan Schiller at the University of Düsseldorf in Germany and his colleagues think clocks using molecules rather than atoms offer a better way to test the idea. According to the team’s calculations, a clock with a molecule of two hydrogen atoms at its centre could combine multiple frequencies at once, providing a more accurate probe of possible changes in the electron-proton mass ratio.

And because hydrogen molecules consist of just a handful of particles, compared with the larger caesium atoms used in atomic clocks, it would be easier to do theoretical calculations and compare them with real experiments, the team says.

Wim Ubachs at VU University Amsterdam in the Netherlands agrees that molecular clocks could be used as high-precision physics instruments. But he says there are still technical hurdles to overcome, such as tuning different lasers to vibrate the molecules at a variety of frequencies at the same time.

Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.113.023004