The Quantum Galileo Interferometer compared the relative phase of a freely falling 87Rb wave-packet component with a second component held stationary relative to Earth. The measured phase was consistent with the quantum free-fall phase predicted by the equivalence principle in the low-energy configuration tested. That is a meaningful checkpoint at the boundary between quantum mechanics and gravity—not a demonstration that gravity itself is quantum.
What the quantum equivalence-principle test measured
The experiment began with one coherent quantum state, not two ordinary atoms sent along separate tracks. A microwave pulse created two linked wave-packet components in an atomic system. Magnetic-gradient pulses then gave those components different physical roles: one was launched upward and allowed to fall, while the other was held in place relative to Earth.
The important observable was their relative phase. In quantum mechanics, a wave has a phase that changes as it evolves. When the two components were brought back together, that phase difference appeared in the populations of the atoms’ internal states. The apparatus therefore did more than watch an atom drop: it compared the quantum evolution of a falling component with that of a supported reference component.
The measured behavior followed the predicted free-fall phase for the tested configuration. The result supports applying the equivalence principle to this quantum system in the regime examined.
The classical idea behind the quantum test
The equivalence principle says, in simple terms, that the effects of gravity and acceleration can be indistinguishable in a sufficiently local setting. It is one of the foundations of general relativity.
The QGI asks a narrower question: does the phase predicted for a freely falling quantum wave packet remain consistent when that packet is part of a superposition with a second component held stationary relative to Earth?
That distinction matters. Seeing an atom fall would not by itself be surprising; classical physics already predicts falling objects. The difficult part is preserving a coherent quantum relationship between two trajectories and reading the phase difference after they are recombined.
Inside the Quantum Galileo Interferometer
The atoms came from a Bose–Einstein condensate containing about 2 × 10⁴ 87Rb atoms. The condensate was positioned approximately 113 μm below an atom chip, whose magnetic structures helped control the wave packets.
A microwave π/2 pulse created a superposition of internal atomic states. A magnetic-gradient pulse launched one component upward. That component was transferred into the magnetically insensitive state |F = 1, mF = 0⟩, allowing it to follow a ballistic path under gravity.
The other component remained in the magnetically sensitive state |F = 2, mF = 1⟩. A tuned magnetic gradient supplied an upward force that opposed its weight, holding this reference component stationary relative to Earth. It was not gravity-free; magnetic support simply counteracted the gravitational force during the measurement.
A later magnetic-gradient pulse removed the returning component’s velocity difference. The two wave packets were then overlapped in position and momentum, and a final π/2 pulse converted their relative phase into measurable state populations. Absorption imaging read out that population pattern.
The sequence is easier to summarize as a physical comparison:
| Wave-packet component | Physical treatment | Role in the measurement |
| Ballistic component | Launched upward, made magnetically insensitive, and allowed to fall under gravity | Accumulated the free-fall phase |
| Reference component | Held stationary relative to Earth by a magnetic gradient opposing its weight | Provided the phase reference |
| Recombined state | The two components were brought together and their internal-state populations were measured | Converted the relative phase into an observable signal |
The experiment therefore does not describe two independent classical particles. It tracks two coherent components of one atomic quantum state and compares them through interference.
What the measurements show
The main measured quantities describe a very small and carefully controlled interferometer:
| Quantity | Value | What it describes |
| Atomic species | 87Rb | Rubidium isotope used for the wave packets |
| Atoms in the condensate | About 2 × 10⁴ | Approximate population of the Bose–Einstein condensate |
| Atom-chip distance | About 113 μm | Position of the atoms below the atom chip |
| Maximum spatial splitting | About 7.5 μm | Largest separation between the wave-packet components |
| Phase accumulation | About 80 rad | Accumulated phase across approximately 13 oscillations |
| Experimental cycles | 633 | Each cycle lasted 30 seconds |
| Data-to-simulation residuals | About 2.5% | Difference between the measured data and a numerical simulation |
The 2.5% figure is not a universal phase error or a claim that the apparatus was simply “97.5% accurate.” It refers to residuals between the data and the numerical simulation. The study describes the measured phase as consistent with the predicted cubic free-fall dependence within the tested conditions.
The measurement series reached 2T = 2,000 μs. Separately, the experimental scheme for a 7.5 μm split specifies a 2.4-millisecond sequence. Those are distinct timing descriptions and should not be collapsed into one duration.
Why this is not proof that gravity is quantum
The experiment tested whether a specific quantum phase behaves as predicted when one wave-packet component freely falls and another is held stationary. It found consistency between quantum mechanics and the equivalence-principle prediction in that configuration.
It did not demonstrate that gravity is a quantized field, produce a theory of quantum gravity, or unite quantum mechanics with general relativity. The result is narrower—and more useful—than those headline claims: it places a measured quantum system on a defined point of the map where the two theories meet.
That boundary is important because the QGI operates in a low-energy atomic regime. Agreement there does not automatically answer what happens when much larger objects, stronger gravitational effects, or genuinely superposed spacetime geometries enter the experiment.
The proposed nanodiamond test
A planned follow-up would use nanodiamonds, objects described as roughly 10 orders of magnitude heavier than atoms. The aim is to move into a more ambitious regime in which a massive object could occupy two locations associated with different spacetime curvatures.
That proposal also connects with gravity-related wave-function collapse ideas discussed by Roger Penrose and Lajos Diósi. It is future work, not a completed result: the nanodiamond experiment has not turned the present QGI measurement into proof of quantum gravity or a collapse model.
The practical bottom line is clear. The QGI showed that, in its carefully defined low-energy setup, a quantum system’s free-fall phase behaves consistently with the equivalence principle. The next question is whether increasingly massive superpositions continue to follow quantum mechanics—or reveal where that description begins to break.
