Atoms fall under gravity, but their quantum wave accumulates a phase that cannot be predicted without combining the two great theories. An international group of physicists has directly measured this phase for the first time in an experiment on an atom chip.
The experiment was conducted in the laboratory of Ben-Gurion University of the Negev (Israel) with the participation of scientists from Oxford, Ulm, and other centers. The team included Nobel laureate Roger Penrose. The researchers used rubidium atoms cooled to nearly absolute zero and manipulated them on a special atom chip using microwave pulses and magnetic fields. They created a quantum Galilean interferometer: they split the atom's wave function into two parts, held one stationary, allowed the other to fall freely, then recombined the paths and registered interference.
The result matched the prediction that follows from Einstein's equivalence principle applied to the quantum wave. The phase accumulated by the falling part exactly corresponded to the expected magnitude. This is the first direct measurement of the quantum phase of free fall.
Imagine two identical boats starting simultaneously from the same point: one stays still, the second drifts with the current. When they meet again, the difference in path is visible in the shift of waves on the water. Here, gravity plays the role of the current, and the quantum phase of the atom plays the role of the waves.
The experiment does not unify quantum mechanics and gravity, nor does it prove that gravity itself is quantum. It shows that the equivalence principle remains valid in the quantum regime. The work was published on 2 September 2026 in the journal Science Advances.
Now physicists have a new tool to test the limits of applicability of quantum theory in gravitational fields and for future tests with heavier objects.
