For the first time, a quantum‑mechanical phase generated by Earth’s gravity has been measured directly. In an experiment reported on 2 September in Science Advances, a team led by Ron Folman at Ben‑Gurion University sent about 20,000 ultracold rubidium atoms into a superposition of falling and stationary trajectories and detected the minute phase difference predicted by Einstein’s equivalence principle.
Experiment design and quantum interferometer
The researchers cooled the rubidium atoms to form a Bose‑Einstein condensate, a state of matter where atoms behave as a single quantum wave. Using an “atom chip” patterned with gold wires only a few micrometres thick, they released the atoms from a magnetic trap and applied radio‑frequency and microwave pulses to split each atom into two magnetic states.
One component received a magnetic kick that launched it upward; a subsequent pulse rendered it insensitive to magnetic fields, allowing it to rise and fall solely under gravity. The other component remained magnetically sensitive and experienced a precisely tuned force that cancelled gravity, keeping it suspended. At the apex of the trajectory the two halves of a single atom were separated by roughly 7.5 µm, about seven times the width of the atomic wave itself.
The apparatus, dubbed the quantum Galileo interferometer, recombined the two paths after a free‑fall interval of up to 2.4 ms. Over 633 experimental runs lasting 5.3 hours, the team recorded 13 full interference oscillations, each representing a complete cycle of phase accumulation between the falling and stationary halves.
Results confirm Einstein’s equivalence principle
The observed phase grew with the cube of the fall time, exactly as theoretical models predict. The measured values matched the 1927 prediction by Charles Galton Darwin and Earle Kennard within about 2.5 %. Vlatko Vedral of the University of Oxford, a co‑author, noted that the phase “grows as the cube of the duration of the experiment and was predicted a long time ago… now finally observed.”
Because the same phase can be derived by treating gravity as a force on a quantum wave or by moving to a freely falling frame where gravity disappears, the agreement demonstrates that, at the tested precision, quantum mechanics and general relativity do not conflict. Vedral emphasized that the equivalence principle remains “perfectly compliant with quantum mechanics.”
Recombining the two atom halves proved technically demanding. The differing velocities of the two paths required precise alignment of both position and momentum, a challenge known as the “Humpty‑Dumpty effect.” Interference contrast started at about 80 % for short fall times but fell to roughly 20 % for the longest runs, setting a practical limit on experiment duration.
While the result does not exclude every possible theory that could break the equivalence principle, it establishes a robust experimental link between relativity and quantum physics. The authors suggest future work could involve rotating frames or superposing multiple massive systems, potentially extending the technique to heavier objects such as nanodiamonds. Such advances might address the longstanding question of whether gravity itself obeys quantum rules, a topic championed by co‑author and Nobel laureate Roger Penrose.
Helene Elliott is the Lead Science & Space Reporter at News Raise. She reports on aerospace missions, astrophysics discoveries, quantum research, and environmental technology.
