Quantum Experiment Measures Einstein’s Predicted Phase Shift in Falling Atoms

Physicists have observed a quantum effect predicted nearly a century ago by measuring how an atom’s wave changes when one part of it falls under Earth’s gravity and another part remains suspended. The experiment, reported by Live Science and published Sept. 2 in Science Advances, used roughly 20,000 ultracold rubidium atoms prepared as a Bose-Einstein condensate. The researchers placed each atom into a quantum superposition, allowing it to follow two trajectories at once. One component was made insensitive to magnetic fields and allowed to rise and fall under gravity. The other remained magnetically sensitive, with an opposing force adjusted to cancel gravity and hold it in place. When the two components were brought back together, their interference pattern revealed a difference in their quantum phases. The measured phase change increased with the cube of the free-fall time, as predicted by theoretical work from 1927 by Charles Galton Darwin and Earle Kennard. Researchers extended the free-fall period to about 2.4 milliseconds and recorded 13 complete oscillations during 633 runs conducted over 5.3 hours. The results matched the theoretical model to within about 2.5%, according to the report. The apparatus, developed by a team led by Ron Folman of Ben-Gurion University of the Negev in Israel, was operated about 113 micrometers beneath an atom chip. At the greatest separation, the two parts of each atom were approximately 7.5 micrometers apart. The finding concerns Einstein’s equivalence principle, which states that gravity and acceleration cannot be distinguished locally. The researchers said the observed phase can be derived both by treating gravity as acting on a quantum wave and by using a freely falling reference frame in which gravity disappears. The agreement between those approaches is consistent with quantum mechanics and general relativity applying together at the precision tested. The experiment had practical limits. Recombining the two portions was difficult because they traveled at different speeds, while variations in the magnetic fields distorted the atom cloud. Interference contrast fell from about 80% in shorter trials to 20% in the longest ones. The researchers also cautioned that the result does not rule out every theory involving a breakdown of the equivalence principle, since some such theories predict the same phase. Future tests may examine the setup in a rotating frame or investigate whether two superposed systems can gravitationally influence each other.