Physicists have performed a new quantum experiment showing that an atomic wave in free fall behaves in a way consistent with Einsteinโs equivalence principle, one of the foundations of modern gravitational physics.
Using ultra-cold rubidium atoms, researchers split the quantum wave associated with an atom into two paths. One part was held stationary while the other was allowed to rise and fall freely under gravity.
When the two parts were brought back together, the scientists measured a difference in their quantum phase.
That phase difference matched the prediction obtained by applying Einsteinโs equivalence principle to the quantum system.
The work, involving researchers from Ben-Gurion University of the Negev, the University of Ulm and the University of Oxford, was published in Science Advances.
Putting Gravity and Quantum Physics in the Same Experiment
Modern physics relies heavily on two extraordinarily successful theories.
Quantum mechanics describes atoms, particles and other phenomena at very small scales.
Einsteinโs theory of relativity describes gravity and the behaviour of objects and spacetime at much larger scales.
Both theories have passed enormous numbers of experimental tests.
Yet physicists still do not have a complete theoretical framework explaining how gravity and quantum mechanics fit together.
The new experiment explores this boundary by testing a fundamental gravitational principle using an object that behaves according to quantum mechanics.
What Einsteinโs Equivalence Principle Says
Einsteinโs equivalence principle states, in simple terms, that the effects of gravity locally disappear for an observer in free fall.
A familiar example is a freely falling elevator.
Someone inside the elevator would appear temporarily weightless because both the person and the elevator are accelerating downward together under gravity.
This principle helped Einstein develop general relativity.
It has been tested extremely accurately using classical objects.
The new study asks what happens when the falling object is not treated as an ordinary particle but as a quantum wave capable of following multiple paths at once.
One Atom, Two Quantum Paths
The researchers worked with clouds of rubidium atoms cooled to temperatures just above absolute zero.
At such low temperatures, atoms can be controlled extremely precisely and their quantum properties become easier to manipulate.
Using microwave pulses, the team placed the atoms into a quantum superposition.
This allowed the atomic wave to effectively travel along two different paths.
Importantly, the atom itself was not physically broken into pieces.
Instead, its quantum state was distributed between two possible trajectories.
Holding One Wave Still While the Other Falls
The researchers manipulated the two parts of the atomic wave differently.
One part was held stationary relative to the laboratory.
Tiny electrical wires on an atom chip generated a carefully controlled magnetic field that produced an upward force.
That magnetic force precisely balanced the downward pull of gravity.
As a result, this part of the atomic wave remained effectively suspended.
The second part was treated differently.
Researchers gave it a carefully controlled upward push before switching it into a state that was almost unaffected by the magnetic field.
It was then allowed to move freely under gravity.
Like a ball thrown upward, it rose and later fell back down.
The Quantum Galileo Interferometer
The experiment relied on an apparatus known as the Quantum Galileo Interferometer.
Its purpose was to separate, control and later reunite the two quantum paths.
At the end of the free-fall trajectory, another magnetic pulse brought the falling atomic wave back together with the stationary one.
When the two waves overlapped, they interfered.
That interference pattern contained information about the relative quantum phase accumulated along the two paths.
Quantum phase describes where a wave is within its oscillation cycle.
A difference in phase therefore provides a highly sensitive way of detecting how the two quantum paths evolved differently.
The Falling Wave Matched Einsteinโs Prediction
The key result was that the measured phase difference agreed with the prediction based on Einsteinโs equivalence principle.
According to the researchers, this represents the first direct measurement of the predicted quantum phase associated with a freely falling object in this type of comparison.
Earlier atom-interferometry experiments have used quantum particles to measure gravity.
The new experiment differs because it directly compares a freely falling atomic wave with another part of the same quantum system that is held stationary.
The agreement indicates that the equivalence principle remains consistent with quantum mechanics under the conditions explored in the experiment.
A Quantum Test of a Classical Idea
The experiment is especially interesting because Einsteinโs equivalence principle emerged from classical gravitational physics.
Quantum mechanics, however, allows behaviours that have no ordinary classical equivalent.
A quantum object can exist in superposition, interfere with itself and behave like a wave.
The experiment therefore asks whether a gravitational principle developed for ordinary falling objects remains valid when the falling object is placed in an explicitly quantum state.
Within the precision and scale of the experiment, the answer was yes.
Why Quantum Phase Matters
The researchers were not simply observing whether the atom physically fell downward.
That would already be expected.
Instead, they measured how gravity affected the phase of the quantum wave.
Quantum phase is central to interference experiments because small changes in phase can alter the final interference pattern.
By comparing the freely falling and stationary components of the same atomic wave, the team could test whether the quantum phase accumulated in the way predicted by the equivalence principle.
This provides a different kind of gravitational test from simply measuring an objectโs trajectory.
A Small Step Toward the GravityโQuantum Puzzle
The experiment does not solve the long-standing problem of unifying gravity and quantum theory.
It also does not prove that gravity itself must be quantum mechanical.
What it does show is that quantum mechanics continues to produce results consistent with Einsteinโs gravitational principle in this new experimental regime.
Study coauthor Professor Vlatko Vedral of the University of Oxford emphasized that there is currently no consistent theory demonstrating exactly where quantum mechanics should fail.
Experiments such as this one therefore help researchers push quantum theory into increasingly unusual physical situations.
Why the Experiment Matters
Tests of gravity using quantum systems could eventually reveal whether established theories continue working when pushed into regimes where both quantum effects and gravitational effects become important.
So far, no confirmed breakdown has been observed.
Each new experiment therefore places additional constraints on theories proposing modifications to quantum mechanics or gravity.
The new result provides another data point showing that quantum mechanics and the equivalence principle remain compatible at the scale tested.
Roger Penrose and a Different Possibility
The study also connects with a long-standing idea proposed by physicist Sir Roger Penrose, who is a coauthor of the paper.
Penrose has argued that quantum superpositions may not remain stable indefinitely when sufficiently massive objects are involved.
In some versions of this idea, gravity itself could contribute to the collapse of a quantum superposition.
The rubidium experiment does not test that regime.
The atoms used were far too light, and the quantum superpositions did not persist long enough to probe the mass and timescales relevant to Penroseโs proposal.
Moving Toward Heavier Quantum Objects
The researchers hope to extend the experimental approach to much heavier systems.
One proposed target is nanodiamonds.
These objects are vastly more massive than individual rubidium atoms while still potentially allowing quantum states to be prepared and studied.
Experiments involving heavier quantum objects could test whether quantum superpositions continue behaving normally as mass increases.
According to the research team, work toward such an experiment is already underway at Ben-Gurion University.
Testing Where Quantum Mechanics Might Break
One of the major unanswered questions in physics is whether quantum mechanics remains universally valid.
Quantum theory works extremely well for atoms and subatomic particles.
But physicists continue investigating whether there might be a scale at which its usual rules begin to change.
Experiments with increasingly massive quantum systems are one way of addressing that question.
If quantum behaviour eventually deviates from standard predictions, gravity could potentially play a role.
If no deviations appear, researchers can place increasingly strict limits on theories proposing such effects.
Gravity Meets Quantum Superposition
The experiment demonstrates how modern technology can bring ideas from gravitational physics and quantum mechanics into the same laboratory setup.
Rather than testing gravity using a conventional falling object, scientists used a quantum wave that followed two paths at once.
One path remained stationary.
The other moved freely under gravity.
When the two were reunited, their interference revealed how their quantum phases had evolved.
The result agreed with Einsteinโs equivalence principle.
Einstein Passes Another Test โ But the Bigger Mystery Remains
More than a century after Einstein transformed scientistsโ understanding of gravity, his ideas continue to survive increasingly unusual experimental tests.
This time, the test involved an atom behaving as a quantum wave and following two different paths simultaneously.
The result strengthens the evidence that the equivalence principle remains valid even when applied to quantum systems under the conditions explored.
But the deeper puzzle remains unresolved.
Physicists still do not know how to construct a complete theory that unifies gravity with quantum mechanics.
By pushing experiments toward heavier objects, longer quantum superpositions and greater precision, researchers hope to discover whether the two theories continue fitting together โ or whether nature eventually reveals a point where something new must take their place.
Journal reference
Or Dobkowski, Barak Trok, Peter Skakunenko, Yonathan Japha, David Groswasser, Maxim Efremov, Chiara Marletto, Ivette Fuentes Guridi, Roger Penrose, Vlatko Vedral, Wolfgang P. Schleich and Ron Folman. โObservation of the quantum phase of free fall and the consistency with the equivalence principle.โ Science Advances, 2 September 2026.