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Physicists say they have for the first time observed a minute quantum phase shift produced when matter falls in Earth’s gravity — a subtle effect predicted nearly a century ago that tests whether Einstein’s equivalence principle survives at quantum scales. The result, published Sept. 2 in Science Advances, links general relativity’s central idea to quantum behavior measured in a laboratory.
Bringing Einstein’s “elevator” idea into the lab
The experiment puts a long-standing thought experiment into practice: locally, acceleration and gravity are indistinguishable. That insight underpins the equivalence principle, the conceptual foundation of general relativity. Until now, tests of that principle focused on macroscopic objects; this team probed it using individual quantum particles behaving as waves.
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Splitting atoms between fall and suspension
Researchers cooled roughly 20,000 rubidium atoms into a Bose-Einstein condensate, released them from a magnetic trap and used pulses of radiofrequency and microwaves to place each atom into a superposition — effectively sending the same particle along two different trajectories at once. One branch rose and then fell under gravity alone, while the other was kept stationary by a magnetic force tuned to cancel Earth’s pull. The apparatus sat about 113 micrometers beneath an “atom chip” patterned with gold wires just 2 micrometers wide.
At the apex of the flying arc, the two components of a single atom were separated by about 7.5 micrometers — several times wider than the atomic wavepacket — before the sequence was reversed and the two halves recombined in an interferometer the team calls the quantum Galileo interferometer.
Measuring a century-old prediction
As the two parts of each atom overlapped again, they produced interference fringes that revealed a tiny phase difference. Theory from 1927 by Charles Galton Darwin and Earle Kennard predicted that this quantum phase should grow with the cube of the fall time. The team extended the free-fall interval to roughly 2.4 milliseconds and, over 633 experimental runs spanning 5.3 hours, observed 13 full oscillations of the interference signal. The measured phase followed the predicted cubic dependence to within about 2.5 percent.
“The phase between the two elements of the superposition, which grows as the cube of the duration of the experiment and which was predicted a long time ago, in 1927, has now finally been observed for the first time,” co-author Vlatko Vedral said.
Why the finding matters
The significance lies in how the same phase arises from two very different pictures. One view treats gravity as a force acting on a quantum wave; the other adopts a falling reference frame where gravity disappears and invokes the equivalence principle. Both approaches yield the same phase shift, the authors argue, demonstrating compatibility between quantum mechanics and general relativity at the measured level.
Vedral said the agreement implies, within this experiment’s precision, “there is no conflict between quantum physics and gravity.” That concordance narrows how and where new physics might appear, while leaving open some alternative theories that predict identical phases.
Technical hurdles and limits
Recombining the two atomic components proved the most exacting task. Because the halves travel with very different velocities and experience slightly different magnetic field curvatures, returning them to the same position and momentum is difficult — a challenge known in the field as the Humpty-Dumpty effect. The interference contrast began near 80 percent in short runs and fell to about 20 percent for the longest experiments, limiting how far the team could stretch the observation.
The authors caution that their measurement does not exclude every theoretical scenario in which the equivalence principle might break down, since some such theories predict the same phase behavior observed here.
Next steps for quantum gravity tests
Looking ahead, the researchers plan to probe the principle under different conditions, including experiments in rotating frames and setups where two quantum systems are placed in superposition and can gravitationally influence each other. Such experiments would push the interface between gravity and quantum mechanics further.
Scaling the technique to far heavier objects — for example, nanodiamonds — could open new routes to test proposals that gravity plays a role in destroying quantum superpositions, a conjecture associated with co-author Roger Penrose. Those experiments would address one of the central open questions: does gravity itself obey quantum rules?
Study citation
Observation of the quantum phase of free fall and the consistency with the equivalence principle, Dobkowski et al., Science Advances, 12(36). https://doi.org/10.1126/sciadv.aec8045
