Ben-Gurion University team measures the quantum phase of a falling atom for the first time

Physicists led by Prof. Ron Folman of Ben-Gurion University of the Negev reported on 4 September 2026 in Science Advances that they had directly measured how the quantum phase of an atom changes as it falls freely under gravity. The paper, "Observation of the quantum phase of free fall and the consistency with the equivalence principle", was a collaboration between Ben-Gurion University's Ilse Katz Institute for Nanoscale Science and Technology, the University of Ulm in Germany and the University of Oxford, and its authors include the Nobel laureate Sir Roger Penrose and physicists Vlatko Vedral, Chiara Marletto and Wolfgang Schleich.

Quantum mechanics and Einstein's general relativity are each confirmed by countless experiments, but no one has found a theory that combines them. A cornerstone of relativity is the equivalence principle, which says that an observer in free fall locally feels no gravity. It has been tested with great precision on ordinary objects, but theorists predicted that for a quantum particle, which behaves as a wave and can follow two paths at once, the principle should show up as a specific shift in the wave's phase. As Folman put it, people have measured the acceleration of falling objects since Galileo, but nobody had measured how the phase of a falling quantum object changes.

To do so, the team cooled clouds of rubidium atoms to just above absolute zero next to a specially designed atom chip. Each atom's wave was split in two: magnetic fields held one part still relative to the laboratory while the other part was allowed to fall. When the two parts were recombined, their interference revealed the tiny phase difference that had built up between them. The measured value matched the prediction of the equivalence principle for a quantum object, showing that, at least at low energies, Einstein's principle is compatible with quantum superposition.

The researchers stress that the result does not show gravity itself is quantum, and does not test Penrose's own proposal that superpositions of heavy objects should collapse. Instead it provides a new kind of interferometer for probing the boundary between the two theories. The group is now developing a version using a nanodiamond, far heavier than an atom, which Folman says could put space-time itself into superposition and possibly reveal new physics. The techniques may also lead to more sensitive quantum sensors of gravity and acceleration for navigation without GPS and for geological surveying.