Quantum Galileo Interferometer Measures Gravity’s Phase on a Falling Atom

A team involving researchers at Ben-Gurion University of the Negev, the University of Ulm and the University of Oxford has directly measured a gravitational phase predicted for a freely falling quantum object. Their Quantum Galileo Interferometer used ultracold rubidium atoms to compare a matter-wave branch held in the laboratory frame with another branch allowed to fall freely. The observed phase agreed with the prediction obtained by applying Einstein’s equivalence principle in this low-energy quantum setting. 12

How the Quantum Galileo Interferometer worked

The experiment began with rubidium atoms cooled to extremely low temperatures, where their wave-like quantum behavior can be coherently controlled. The apparatus split the atoms’ quantum wave function into two spatially distinct wave-packet paths. 12

One path was held approximately stationary relative to the laboratory using magnetic fields. The other was launched and then placed in free fall, meaning the experimenters did not apply forces to that branch during its fall. 2

After a controlled interval, the researchers recombined the two paths. Like overlapping ripples, the waves interfered. That interference encodes their relative phase—the difference in the quantum-wave evolution accumulated along the two paths. Because one branch had fallen freely while the other had been supported, the measurement isolated the phase associated with quantum free fall. 2

What the result says about Einstein’s equivalence principle

Einstein’s equivalence principle holds that, locally, the effects of gravity are absent for an observer in free fall. The new result tests how that principle fits with a matter wave that is coherently spread over different paths.

The measured relative phase matched the predicted free-fall phase. In the regime tested, that is evidence that the equivalence principle can be consistently applied to a quantum object—not merely to a classical particle with a definite trajectory. 12

This matters because the experiment measures a specific feature of quantum evolution rather than simply tracking an atom cloud’s average acceleration. The atom’s wave function was placed in a superposition, and the team read out the phase difference between its two branches. 12

What the experiment does not show

The result should not be described as a discovery of quantum gravity.

It does not unify quantum mechanics with general relativity. In this type of low-energy calculation, the atom is treated quantum mechanically while gravity supplies a classical background field. The experiment confirms that this widely used framework correctly predicts the observed phase in the tested conditions. 2

It also does not establish that gravity itself is a quantum field or that it carries quantum information. No experiment has yet provided conclusive evidence for quantum features of the gravitational interaction. 4855

A common proposed route toward that question is to prepare two masses in quantum states and look for entanglement generated solely through their gravitational interaction. But the interpretation of such experiments depends on theoretical assumptions, and recent work continues to debate whether some classical-gravity frameworks could reproduce entanglement-like effects. 515359

Why this is not a test of Penrose-style gravitational collapse

Roger Penrose has proposed that sufficiently large spatial superpositions could spontaneously lose coherence because of their differing mass distributions. The Quantum Galileo Interferometer instead measured a coherent phase for atomic matter waves; it was not designed to seek anomalous loss of interference visibility in massive, long-lived superpositions.

Testing collapse proposals requires experiments that can vary mass, spatial separation and coherence time while excluding ordinary sources of decoherence. Collisions, electromagnetic noise, vibrations and technical imperfections can all reduce interference visibility without revealing a new gravitational mechanism.

The new measurement therefore supports ordinary quantum coherence in the atomic regime it studied. It does not confirm or decisively rule out collapse models aimed at much larger objects.

Why nanodiamonds are a logical next target

Proposed interferometers with nanodiamonds aim to push matter-wave experiments into a substantially higher mass range. A nitrogen-vacancy center embedded in a nanodiamond can provide a spin degree of freedom that magnetic gradients use to create and control a spatial superposition. 1721

One chip-based proposal targets particles in the approximate mass range of (10^{-19}) to (10^{-15}) kilograms. 20 Such experiments would make it possible to test whether quantum coherence persists as the object becomes more massive and more susceptible to environmental disturbance.

If a sufficiently isolated, massive superposition survives beyond the limit predicted by a particular collapse model, that result could constrain the model. If interference disappears, researchers would first need to demonstrate that conventional decoherence cannot account for it.

How this differs from Tiangong’s in-orbit atom-fall test

The Quantum Galileo Interferometer and the China Space Station’s Tiangong atom-interferometry experiment both concern gravity and ultracold atoms, but they ask different questions.

Tiangong used a dual-species (^{85})Rb/(^{87})Rb atom interferometer to compare the free-fall accelerations of two rubidium isotopes in orbit. From 280 days of data, the reported weak-equivalence-principle test uncertainty was (2.8\times10^{-8}), with a reported result of ((-3.1\pm4.6)\times10^{-7}). 3541

That is a test of the universality of free fall: do different test bodies accelerate alike in the same gravitational field?

The Quantum Galileo Interferometer instead compares two branches of the same coherent matter wave, one supported and one freely falling, to measure the predicted quantum phase of free fall. 2

Both experiments support equivalence-principle physics with quantum atoms. Neither, by itself, demonstrates that gravity is quantized.

The careful conclusion

The Quantum Galileo Interferometer achieved a narrow but important milestone: it directly observed the gravitationally predicted phase of a freely falling atomic matter wave. The agreement with theory extends an equivalence-principle test into a setting where the object is in a controlled quantum superposition. 12

Its significance is strongest when stated precisely. This is compelling evidence that quantum mechanics and Einstein’s equivalence principle remain compatible in the low-energy regime measured—not a completed theory of quantum gravity, and not yet a test of gravity-driven quantum collapse.