Physicists Observe a Quantum Phase Shift Caused by Earth’s Gravity
Physicists have measured a tiny quantum phase shift that develops when atoms move through Earth’s gravitational field. The effect, predicted nearly 100 years ago, provides a new test of Einstein’s equivalence principle and strengthens the connection between quantum mechanics and general relativity.
In the experiment, researchers placed ultracold rubidium atoms into a quantum superposition—a state in which a single atom follows two paths at the same time. One part of each atom was allowed to fall freely under gravity, while the other part was held stationary. When the two paths were recombined, the researchers detected a subtle difference between them.
The findings, published September 2 in Science Advances, show that Einstein’s equivalence principle remains valid when tested with quantum matter. The result offers a precise demonstration of how gravity influences the phase of a quantum wave.
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“According to the equivalence principle, acceleration and gravity are locally indistinguishable,” Vlatko Vedral, a physicist at the University of Oxford and co-author of the study, told Live Science by email.
Einstein’s “happiest thought”
The equivalence principle can be illustrated with Einstein’s famous elevator thought experiment. A person inside a windowless elevator cannot determine whether the floor is pressing against their feet because the elevator is resting on Earth or because the elevator is accelerating through empty space.
Einstein reportedly called this realization “the happiest thought” of his life. He later used it as a foundation for his theory of general relativity, which describes gravity as a consequence of the curvature of spacetime.
For everyday objects, scientists have tested the equivalence principle to an exceptionally high level of precision. Quantum systems are more difficult to study because particles behave like waves as well as objects.
Every quantum wave has a property called phase, which describes the position of its peaks and troughs. The phase cannot be observed directly, but it becomes measurable when two versions of the same particle are brought back together. Their phases combine to create an interference pattern, causing the waves to reinforce or cancel one another in different locations.
According to theoretical predictions, a quantum wave in free fall should gain phase relative to an identical wave held at rest. Crucially, the phase difference should increase with the cube of the free-fall time. Doubling the fall time should therefore make the effect eight times larger.
Charles Galton Darwin and Earl Kennard first described this prediction in 1927. Until now, however, researchers had not directly measured the predicted cubic relationship.
How the quantum gravity experiment worked
The device, called a 2D MOT, supplies cold atoms to the laboratory.
(Image credit: Or Dobkowski)
To observe the effect, the research team led by physicist Ron Folman of Ben-Gurion University of the Negev cooled approximately 20,000 rubidium atoms into a Bose-Einstein condensate. This exotic state of matter allows atoms to behave collectively like a single quantum wave.
The atoms were released from a magnetic trap inside an instrument called the Quantum Galilean Interferometer. The apparatus created two versions of each atom and separated them by approximately 7.5 micrometers—about seven times the width of the atomic wave.
Radio-frequency and microwave pulses placed the atoms into a superposition of two magnetic states. A magnetic force counteracted gravity for one component, keeping it suspended. The other component became insensitive to the magnetic field and moved freely under Earth’s gravity.
After a few milliseconds, the two paths were brought together. The resulting interference pattern revealed the phase difference between the freely falling and stationary portions of the atom.
“Two different accelerations can be superimposed: zero acceleration and Earth’s gravitational acceleration. The resulting quantum interference was measured,” Vedral said.
A quantum phase predicted in 1927
The researchers varied the free-fall time, extending it to approximately 2.4 milliseconds. Across 633 experimental runs conducted over 5.3 hours, they recorded 13 complete oscillations in the interference signal.
The measured phase shift increased with the cube of the fall time, matching the theoretical prediction to within approximately 2.5%. This is the first direct observation of the long-standing prediction for a quantum wave in free fall.
“The phase between the two elements of the superposition grows as the cube of the experimental period. It was predicted in 1927, but now it has finally been observed for the first time,” Vedral said.
The result can be explained in two equivalent ways. In one description, gravity acts as a force that changes the quantum wave. In another, the experiment is viewed from a freely falling reference frame, where gravity disappears locally. Both approaches produce the same phase shift.
The agreement supports the idea that quantum mechanics and general relativity remain compatible under these experimental conditions.
“At this level of precision, there is no contradiction between quantum physics and gravity,” Vedral said. “In other words, the equivalence principle is fully consistent with quantum mechanics.”
Why recombining the atoms was so difficult
The most challenging part of the experiment was bringing the two portions of each atom back together. After following different paths, the two components were moving at different speeds and had to overlap in both position and motion.
Physicists Marlan Scully, Berthold Georg Englert and Julian Schwinger previously referred to this challenge as the “Humpty Dumpty effect.” Variations in the magnetic field across the atomic cloud also distorted the two quantum paths in different ways.
The interference contrast was approximately 80% during the shortest measurements but fell to about 20% during the longest runs. This limited the maximum duration of the experiment.
The researchers also emphasized that the results do not eliminate every possible theory that violates the equivalence principle. Some alternative theories could produce the same quantum phase shift measured in this study.
What comes next for quantum gravity research?
The team plans to test the equivalence principle under more complex conditions, including experiments performed in a rotating reference frame.
Researchers also want to place two quantum systems into superposition and study how they interact through gravity. Such experiments could help determine whether gravity itself follows quantum laws.
If scientists can scale this method from atoms to heavier objects, such as nanodiamonds, they may be able to test the controversial possibility that gravity destroys quantum superposition. This idea has been associated with physicist and Nobel laureate Roger Penrose, a co-author of the study.
Dobkowski, O., Trok, B., Srakunenko, P., Japha, Y., Groswasser, D., Efremov, M., Marletto, C., Guridi, I. F., Penrose, R., Vedral, V., Schleich, W. P., and Folman, R. (2026). “Observation of quantum phase in free fall and consistency with the equivalence principle.” Science Advances, 12(36).
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Source: www.livescience.com