Atomic clocks are the most precise timekeepers in the world, which use lasers to measure the vibrations of atoms. They keep time with such precision that, if they had been running since the beginning of the universe, they would only be off by about half a second today. Still, they could be even more precise. If atomic clocks could more accurately measure atomic vibrations, they would be sensitive enough to detect phenomena such as dark matter and gravitational waves, which would give answers to questions like what effect gravity might have on the passage of time and whether time itself changes as the universe ages.

The researchers Pedrozo-Peñafiel et al. report in the journal Nature that they have built an atomic clock that measures not a cloud of randomly oscillating atoms, as state-of-the-art designs measure now, but instead atoms that have been quantumly entangled. Their findings would lead to clocks being less than 100 milliseconds off. To keep perfect time, clocks would ideally track the oscillations of a single atom, but they are subject to the Standard Quantum Limit that introduces an uncertainty when measuring at such small scales.

The solution would be quantum entanglement which describes a nonclassical physical state, in which atoms in a group show correlated measurement results, even though each individual atom behaves like the random toss of a coin. The team reasoned that if atoms are entangled, their individual oscillations would tighten up around a common frequency, with less deviation than if they were not entangled. In this way, the researchers quantumly entangle the atoms, and then use another laser, similar to existing atomic clocks, to measure their average frequency. When the team ran a similar experiment without entangling atoms, they found that the atomic clock with entangled atoms reached a desired precision four times faster.  

This version was adapted and abridged from the original MIT article (New type of atomic clock keeps time even more precisely, MIT, 16.12.2020)

Original paper: Pedrozo-Peñafiel, E., Colombo, S., Shu, C., Adiyatullin, A.F., Li, Z., Mendez, E., Braverman, B., Kawasaki, A., Akamatsu, D., Xiao, Y. and Vuletić, V., 2020. Entanglement on an optical atomic-clock transition. Nature, 588(7838), pp.414-418. Online