Although gravity is well understood at large scales, it is more difficult to measure in small regions where it’s overpowered by other forces. Researchers have used interferometers to measure the acceleration of free-falling atoms interacting with a small source mass, but the measurement times are limited to a couple of seconds, and the atoms have less gravitational attraction to the source mass as they fall. As a result, it has been difficult to garner precise results for the small changes in acceleration.
In 2019, Holger Müller’s group at the University of California, Berkeley, used a new interferometry method in which atomic wavepackets were held in place in an optical lattice (see Physics Today, January 2020, page 14). Like free-fall atomic interferometers, the apparatus used laser pulses to split atomic wavepackets in two; the researchers could then measure the gravity-induced phase difference between the wavepackets after recombination. Because the wavepackets were held at a constant height, the atoms felt the gravity from the source mass consistently. The researchers showed that the novel method increased the measurement time 10-fold. But the signal from Earth’s gravity, some 200 million times as large as the precision of the measurement, still made small-scale gravity measurements difficult.
Now Müller and his group have refined their interferometry technique so that it isolates the underlying gravitational forces from the small masses with a precision not previously achieved in atomic interferometry. The researchers swap the locations of the atomic superposition above and below a tungsten source mass, and separately they move the source mass closer to or farther from the atoms. The technique enables the researchers to determine how much acceleration is from the source mass and to isolate Earth’s gravity and other systemic effects. They also were able increase the measurement time from 20 seconds to 70 seconds by limiting small shakes in the laser beams and the extra thermal velocity of the cold cesium atoms.
Using a nanometers-per-square-second measurement that is more than four times as accurate as measurements from free-fall interferometers, the researchers were able to increase the constraints on some dark-energy models. So-called fifth-force models assume that the forces responsible for dark energy are detectable only in regions with less matter density. Measuring gravitational attraction at high precision for small regions allows for the investigation of those screened models. The study improves constraints on two fifth-force models and excludes the available parameter space for one of them.
Müller and colleagues plan to construct a new optical lattice interferometer that addresses some of the limiting factors revealed in the latest experiments. The new apparatus should bring researchers a step closer to testing the limits of Newtonian mechanics and general relativity at small scales. (C. D. Panda et al., Nature 631, 515, 2024.)
