Testing general relativity principles at the quantum level

The universality of free fall states that two objects will follow the same acceleration regardless of their mass, internal composition, or structure.

So far, this assumption has not failed experimentally. However, Einstein’s equivalence principle is expected to be violated when gravity is included in quantum mechanics. This led Barrett et al. to examine the free fall of quantum objects, specifically isolated atoms.

Few research groups have tested the universality of free fall for quantum objects. Of those groups, many examine two isotopes of rubidium. In this study, the authors chose to test two different atoms, potassium and rubidium, which is advantageous due to their difference in mass but more challenging experimentally.

The atom gases are cooled to very low temperatures using low noise ultra-stable fiber-based lasers. This lowers the velocity dispersion of the atom cloud, allowing enough time for measurement. The vacuum system brings the experiment to ultra-low pressures, and magnetic fields associated with the laser beams locally trap the atoms at the center of the experiment.

“At low temperature, the wave behavior of the atoms allows us to do atom interferometry. We split the matter waves and recombine them coherently to create an atom interferometer,” said author Baptiste Battelier. “By splitting them spatially, each atom is composed of two wave packets, each following a different path. We measure the accumulated phase shift along the two paths, which is directly proportional to the acceleration.”

The researchers characterized the limitations and errors of the experiment in depth, a key step in increasing future accuracy. They plan to improve the experiment by orders of magnitude with lower temperatures of the atomic sample and increased measurement times reachable in weightlessness.

Source: “Testing the universality of free fall using correlated 39K - 87Rb atom interferometers,” by Brynle Barrett, Gabriel Condon, Laure Chichet, Laura Antoni-Micollier, Romain Arguel, Martin Rabault, Celia Pelluet, Vincent Jarlaud, Arnaud Landragin, Philippe Bouyer, and Baptiste Battelier, AVS Quantum Science (2022). The article can be accessed at https://doi.org/10.1116/5.0076502 .

This paper is part of the Celebrating Sir Roger Penrose’s Nobel Prize Collection, learn more here .