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A molecular clock for testing fundamental forces

22 August 2019

The vibrational frequencies of trapped ultracold molecules can serve as a check on what we think we know about the universe.

Gravity over long distances is well understood. The simple inverse-square law that Isaac Newton proposed centuries ago still accurately describes the motions of planets, stars, and galaxies in the nonrelativistic regime. Short distances—microns or less—are another matter. Extra dimensions or undiscovered massive particles might alter the functional form of gravity at short range. But because electromagnetic forces are so overwhelmingly dominant in microscopic experiments, those theoretical possibilities are extremely difficult to test. According to the best experimental constraint so far, the force of gravity at the nanometer scale is no more than 1021 times what Newton’s law says it is. That’s not a typo.

Columbia University’s Tanya Zelevinsky and colleagues hope to improve on that upper bound with their ultraprecise measurements of the vibrational frequencies of a diatomic molecule. Their experimental setup is similar to an atomic optical-lattice clock (see Physics Today, March 2014, page 12), so they call it a molecular lattice clock, even though precision timekeeping isn’t one of their immediate goals.

Energy difference diagram

To probe the energy difference between two vibrational states, shown in blue and orange in the figure, the researchers use Raman spectroscopy, a two-photon process that connects the two states by way of a higher-energy virtual state. The difference between the Raman laser frequencies (gold and bright red dashed lines) can be stabilized to within 0.1 Hz or better. Accordingly, the measurement precision is limited by molecular, not optical, effects.

To limit Doppler broadening of the resonance, Zelevinsky and colleagues immobilize their molecules in an array of optical traps. But optical trapping creates its own problem: Through the AC Stark effect, the trapping laser separately shifts each of the vibrational states and thus alters the resonance energy. The researchers solve that problem by choosing a so-called magic trapping wavelength, which imposes identical Stark shifts on both vibrational states. In their setup, the trapping laser frequency (dark red dashed line) gets its magic character from its near-coupling to yet another molecular excited state (green solid line).

All told, the researchers measure the 25 THz resonance with a linewidth of just 32 Hz, for a quality factor of nearly 1012. And they anticipate that by overcoming technical challenges, they’ll be able to improve that number by several orders of magnitude.

Their molecule of choice—the strontium dimer, chosen for its compatibility with ultracold techniques—has several isotopic variants, which will allow the researchers to test the effect of nuclear mass on the interatomic force. In addition to constraints on non-Newtonian gravity, Zelevinsky and colleagues also want to study the stability of the electron-to-proton mass ratio. If the proton mass changes even slightly over time (as it would, for example, if the strong nuclear force were not constant), the clearest signature of the change would be in molecular vibrational frequencies. (S. S. Kondov et al., Nat. Phys., 2019, doi:10.1038/s41567-019-0632-3.)

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