Lab experiment measures gravity’s quantum phase in rubidium atoms

On a specially designed atom chip at Ben-Gurion University of the Negev, clouds of ultracold rubidium atoms were split into two quantum paths. One part was held stationary relative to the laboratory and Earth; the other was put into free fall. Their interference revealed the first direct measurement of the quantum phase predicted for a freely falling object, according to the researchers from Ben-Gurion, the University of Ulm and the University of Oxford.

The test targets Einstein’s equivalence principle. For an observer in free fall, the principle says, gravity must disappear locally—a lift or airplane falling freely can leave its occupants weightless. Quantum objects complicate the picture because they can behave like waves and travel along more than one path. The experiment asked whether that same gravitational rule still applies when an atomic wave occupies both paths.

Microwave pulses first placed the ultracold rubidium atoms into quantum superposition. Tiny electrical wires on the chip then created magnetic fields: an upward force held one wave against gravity, while a magnetic pulse pushed the other upward before the magnetic field was removed to simulate free fall. A final pulse recombined the two parts, allowing the interferometer to measure their tiny difference in quantum phase.

That phase difference matched Einstein’s prediction. Similar experiments had already used quantum objects to measure gravity, but the researchers say this is the first direct measurement of the predicted phase of a freely falling object. The findings were published in Science Advances. Oxford physicist Vlatko Vedral said the result takes quantum mechanics into gravity without showing that the two major frameworks have been unified.

Concretely, the advance gives researchers a demonstrated laboratory route for testing gravity with quantum matter and points toward experiments on more massive objects. For now, the boundary has moved in a precise but limited way: Einstein’s equivalence principle holds in this quantum test, while Penrose’s question about long-lived superpositions of massive objects remains open.