Israeli scientists’ falling-atom experiment points to new tests at the quantum-gravity frontier

For more than a century, quantum mechanics and Albert Einstein’s theory of general relativity have each passed countless tests, but physicists have struggled to reconcile the two. Now, scientists in Israel and abroad have taken a new step toward understanding how they interact by directly measuring how gravity changes a quantum object as it falls.

The findings could eventually contribute to more sensitive quantum sensors for measuring gravity and acceleration, with potential applications in surveying, geological mapping and navigation without relying on GPS.

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The advance is not simply that scientists measured gravity using quantum atoms; such experiments have been done before. Instead, the researchers directly measured a specific change in the “phase” of a quantum wave as an atom freely fell under gravity—an effect predicted by Einstein’s equivalence principle.

The experiment was led by Ben-Gurion University of the Negev, the University of Ulm in Germany and the University of Oxford, with Nobel Prize-winning physicist Sir Roger Penrose among the researchers involved. It was published in the peer-reviewed journal Science Advances.

“Quantum mechanics tells us that a particle is also a wave,” Professor Ron Folman of Ben-Gurion University’s Ilse Katz Institute for Nanoscale Science and Technology told The Press Service of Israel. “Waves, whether sea waves, or sound waves, or light waves, or quantum waves, have a feature called phase.”

“While people have measured the acceleration of free-falling objects ever since Galileo, no one has ever measured the quantum features of a free-falling quantum object, and specifically, how its phase changes due to free-fall,” he said.

The equivalence principle, a cornerstone of general relativity, states that the effects of gravity can locally disappear for an observer in free fall—a person falling freely in an elevator, for example, would experience weightlessness. The principle has been tested with ordinary matter to extraordinary precision. But quantum objects can behave like waves and can effectively follow two paths at once.

“In our experiment we used this rule to put an atom in two places and two trajectories at the same time. In one trajectory it was free-falling and in the other it was static relative to Earth, where the latter was used as a reference,” Folman explained.

The researchers cooled clouds of rubidium atoms to temperatures just above absolute zero and manipulated them near a specially designed atom chip. Magnetic fields held one part of the atomic wave stationary while the other was allowed to fall freely. The two parts were then brought back together, and their waves interfered, allowing the researchers to measure the tiny difference in quantum phase that had accumulated between the falling and stationary portions. The measured phase shift matched the value predicted by Einstein’s equivalence principle for a quantum object.

Folman said the experiment is significant because scientists still lack a theory explaining how gravity and quantum mechanics work together. “Both have been proven to work, but scientists could never formulate a theory which describes how both work together in the same universe,” he said.

The experiment does not show that gravity itself is quantum, nor does it establish a unified theory of gravity and quantum mechanics. Instead, it demonstrates that, under the conditions tested, Einstein’s equivalence principle is compatible with quantum superposition. It also does not test Penrose’s proposal that quantum mechanics could break down for sufficiently massive objects held in quantum superpositions for long enough.

However, the researchers hope to use the technique with much heavier objects. “Our experiment opened the door for future more elaborate experiments to test the interface between the two theories,” Folman told TPS.

The team is already developing an experiment using a nanodiamond instead of an atom. A sufficiently massive object placed in a quantum superposition could let researchers explore whether quantum mechanics and gravity behave differently at a scale where their interaction becomes more pronounced. “Once it is put in a superposition, it will create a superposition of space-time itself, and here we might be able to see completely new physics,” Folman said.

If successful, such experiments could offer new ways to investigate the unresolved relationship between quantum mechanics and gravity. The researchers also see longer-term potential for the underlying techniques in quantum sensors capable of extremely precise measurements of gravity and acceleration.

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