What did the international team led by Ron Folman, including collaborators from Ben Gurion University, Oxford, Ulm, and Nobel laureate Roger Penrose, discover in its September 2 Science Advances study using the Quantum Galileo Interferometer, ultracold rubidium atoms, an atom chi

The team directly measured the gravitationally induced quantum phase difference between two coherent paths of ultracold rubidium atoms: one magnetically supported against gravity and one launched upward and then freely falling. The interference shift agreed, within the experiment’s precision, with the phase predicted by Einstein’s equivalence principle for those paths. 24

• What they did: Microwave pulses placed the atoms into a superposition of two internal-state/path histories near an atom chip. Magnetic fields held one branch essentially stationary against gravity, while the other followed a ballistic, upward-and-downward free-fall trajectory; recombining them converted their relative phase into a measurable interference signal. 24

• What was established: In this atomic, weak-Earth-gravity regime, ordinary quantum evolution and the equivalence-principle prediction give the observed result. It is a direct quantum-interference test of gravitational free-fall phase, not merely a classical trajectory test. 24

• What was not established: It does not unite quantum mechanics with general relativity, prove that gravity itself is quantized, detect gravitons, or decide between quantum and classical gravity. A test of gravity’s quantum nature would require, for example, evidence that gravity can create entanglement between suitably isolated masses. 23

• Penrose’s relevant proposal: Penrose has proposed an objective, gravity-related collapse mechanism: a massive object put into a spatial superposition would correspond to two differently curved spacetime geometries, making the state unstable. Its coherence would then decay on a timescale related to the gravitational self-energy difference of the two mass distributions. 9

• Why heavier objects matter: A single rubidium atom is far too light for a strong test of that proposed mass-dependent breakdown. Interferometers that create large, long-lived spatial superpositions of heavier systems—potentially levitated nanodiamonds or other mesoscopic particles—could look for an otherwise unexplained loss of interference at the rate Penrose-type models predict, after rigorously excluding ordinary environmental decoherence. Observing coherence beyond that predicted decay would constrain the model; observing a reproducible excess decay with the expected mass, separation, and time scaling would support it, though alternative noise explanations would still need to be eliminated. 19