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Optical transitions of electrons between atomic energy states provide a means of accurately defining time. Now, Martin Boyd and co-workers at the National Institute of Standards and Technology in Colorado have reached a new level of precision using an ensemble of neutral atoms1.

Experiments using single trapped ions have led to great advances in metrology, but ensembles of atoms promise much larger signals, and they can also provide higher accuracy in the resulting optical clock. This accuracy is determined by the spectral width of the absorption spectrum, as characterized by the Q factor — the ratio of the light frequency to the resonance linewidth. A fundamental limit is imposed by decoherence — interaction of the atoms with their surroundings. Decoherence disrupts the exact timing of the transition, reducing the Q factor, and is caused by either the intrinsic properties of the atom or external influences, for example, the way the atoms are confined.

In the set up used by Boyd et al., the atoms are trapped in an optical lattice specially engineered to reduce decoherence, trapping approximately 10,000 cold strontium atoms in 100 or so lattice sites. The ensemble exhibits a peak in its absorption spectrum at a frequency of 4.3 ⨉ 1014 Hz (a wavelength of 698 nm) and a linewidth of 1.8 Hz. This represents a Q factor of 2.4 ⨉ 1014, the highest value ever recorded in coherent spectroscopy. The impressive spectral resolution of this system is not only a boon for frequency metrology, but could also be used in the direct optical manipulation of nuclear spins.