Scientists measure time-cubed quantum phase in freely falling atomic wave packets

Researchers have directly measured a long-predicted quantum phase picked up by a freely falling atomic wave packet, using ultracold rubidium atoms in an interferometer that compared one part of an atom’s wave held still in the lab with another allowed to fall under Earth’s gravity. The measurements matched the expected time-cubed, or (T^3), behavior and were consistent with Einstein’s equivalence principle in the quantum regime tested.

The experiment relied on a basic quantum idea: a single atom can behave like a wave that takes more than one path at once. The team used about 20,000 ultracold rubidium atoms in what it calls a Quantum Galileo Interferometer. Using microwave pulses and magnetic fields from tiny wires on an atom chip, the researchers split each atom’s wavefunction into two parts. One internal atomic state was made sensitive to magnetic fields, letting the magnetic force cancel gravity and hold that part stationary relative to the laboratory. The other state was made nearly insensitive to the field, so it underwent ballistic free fall. When the two parts were brought back together, the scientists measured the phase difference between them.

That phase is the key result. The team found that it grew approximately as the cube of the free-fall time, in line with theory. In press materials and related coverage, the researchers said the observed growth matched the predicted (T^3) dependence to within about 2.5% in the regime they tested. The maximum free-fall time was about 2.4 milliseconds, and the two wave-packet components were separated by about 7.5 micrometers at the top of the arc.

The authors say this is the first direct measurement of this specific free-fall quantum phase. They also say the result is consistent with the equivalence principle applied to quantum matter in this regime. In Einstein’s framework, the equivalence principle says that freely falling motion is locally indistinguishable from inertial motion, meaning motion without gravity’s effects. This experiment probes how that principle appears when the falling object is described by quantum mechanics.

What the work does not do is just as important. It is not a demonstration that gravity itself is quantized. It does not provide a theory of quantum gravity, and it does not resolve the long-standing tension between quantum mechanics and general relativity. Instead, it is a precise test of a predicted quantum effect in Earth’s gravitational field.

There is some historical precedent here. In 1975, the Colella-Overhauser-Werner neutron interferometry experiment showed that gravity can affect quantum interference. What is new in the latest work is the specific arrangement: one part of the quantum wave packet was held fixed in the Earth frame while the other was placed in true free fall, producing the predicted (T^3) phase signature.

“We have no consistent theory telling us why quantum physics should fail,” co-author Vlatko Vedral said in a University of Oxford press release. “This experiment pushes quantum mechanics into one of its most intriguing frontiers, gravity, and shows that, once again, its predictions hold.”

The study, “Observation of the quantum phase of free fall and the consistency with the equivalence principle,” was published Sept. 2 in Science Advances. The work was led by Ben-Gurion University of the Negev, with collaborators from the University of Oxford, the University of Southampton, the German Aerospace Center’s Institute of Quantum Technologies in Ulm, Universität Ulm and Texas A&M University. The lead author is Or Dobkowski, and the senior corresponding author is Ron Folman. The DOI is 10.1126/sciadv.aec8045.