Ordinary atomic clocks are like a tuning fork that must be kept in perfect silence and stillness. The slightest movement, and the note goes flat. The same goes for atoms: their thermal motion shifts the frequency of radiation, so ultra-precise clocks require almost complete immobilization of the particles. That is difficult, expensive, and bulky.
The authors of the study figured out how to "hear" a clean note even in a noisy orchestra of three million strontium atoms heated to room temperature. The key is two clever excitation protocols. The first hits the atom with three laser pulses at once, with frequencies tuned so that the Doppler effect cancels out. The second protocol works in two steps: first one photon, then a pair. Both methods cut manipulation time to a microsecond — thousands of times faster than before.
Now that same tuning fork can be used not in a silent lab, but right out on a noisy street. The result is not just speed, but the removal of stringent vacuum and cooling requirements. This means ultra-precise atomic clocks could become smaller, cheaper, and more accessible — for instance, on satellites or in mobile quantum devices.
as written by the authors arXiv:2607.06789
We demonstrate two Doppler-free (DF) excitation schemes for coherent manipulation of the $${}^1\!S_0 - {}^3\!P_0$$ clock transition in $$^{88}$$Sr that achieve microsecond-scale excitation times. The first approach uses three-photon excitation with distinct phase-coherent spectral components to couple the ground and clock states while canceling the first-order Doppler shift. The second approach is a sequential protocol that combines a single-photon excitation with a two-photon Raman excitation, reducing coherent clock transition manipulation times to below a microsecond. With both methods, we perform high-contrast Ramsey spectroscopy on thermal ensembles of $$3 \times 10^{6}$$ atoms in free space. We observe three orders of magnitude suppression of Doppler dephasing compared to single-photon excitation, relaxing the need for tight confinement or ultra-low temperatures. These broadly applicable techniques enable fast, coherent manipulation of narrow-line transitions, with implications for optical atomic clocks, matter-wave interferometers, quantum-enhanced metrology, and quantum information processing.
Three million atoms in the experiment is roughly the population of a city like Madrid, yet they all fit in a volume smaller than a thimble.
In Greg Egan's novel "Permutation City," quantum clocks that don't require cryogenic temperatures become the basis for interstellar navigation.