Nearly a century ago, a debate began in Brussels between a pair of renowned physicists that has remained at the heart of our modern understanding of quantum mechanics.
Now, the famous dispute may finally be resolved, and a clear winner seems to have emerged, according to new findings that appeared in Physical Review Letters.
The controversy began in 1927 amid the proceedings of the fifth annual Solvay Conference in Brussels. In attendance were Albert Einstein and Niels Bohr—a pair of scientists whose names remain synonymous with physics and the quantum realm more than a century after their most formative contributions to the field.
A Historic Debate
It was at the 1927 event that Bohr and Einstein initially described a now-famous double-slit interferometer involving a “movable slit” capable of spotting the otherwise indiscernible momentum recoil associated with the movement of a single photon. This famous thought experiment was significant not only for its cultural impact on science but also for offering a remarkable perspective on concepts that would become a focal point in the modern study of quantum mechanics.
Yet, while there have been many attempts to reproduce this conceptual experiment, doing so in practice with tangible optical equipment comprising a tiny slit that meets the necessary quantum limit, paired with a tunable momentum width, has remained elusive.
That is, until now. According to new research by Chinese physicists, the successful engineering of a long-sought “Einstein-Bohr interferometer” has finally come to fruition.
To achieve this, the team employed a specialized scientific instrument known as an optical tweezer, which uses a highly focused laser beam to manipulate objects, including atoms and nanoparticles.
Optical tweezers manipulate objects that are so small—and under unique enough conditions—that when objects are held in the absence of any additional support (such as within a vacuum), researchers often refer to this as optical “levitation.” By introducing a repulsive or attractive force, lasers in these unique devices allow researchers to control such tiny objects with remarkable precision.
An Atom Reaches Its Ground State
In their recent research, the Chinese team manipulated a single atom with their optical tweezer, which they cooled to such a degree that it reached its ground state—the point at which the atom reaches its lowest energy—meaning that its motion was reduced in three dimensions “such that its momentum uncertainty is comparable to that of a single photon,” according to the researchers.
From here, the team employed a novel configuration for their interferometer, in which the atom, cooled to its ground state, serves as “an ultralight, quantum-limit beam splitter that is momentum entangled with the input photon.”
By manipulating the optical tweezer to vary its trap depth, the researchers report that they were able to “tune” the atom’s momentum uncertainty. Fundamentally, this allowed them to observe a practical manifestation of Bohr and Einstein’s famous debate, in that they were able to watch a very minute, but discernible alteration in the visibility of single-photon interference produced under such conditions.
The team’s real-world double-slit interferometer “also allows us to distinguish the classical noise caused by atom heating from the quantum-limited noise due to the momentum transfer,” the study’s authors write, arguing that their observations detail a “quantum-to-classical transition.”
Settling a Century-Old Score
At the heart of the recent experiment is the longstanding question over differences in opinion between Bohr and Einstein that began during the event in Brussels in 1927.
Unfortunately for Einstein, the results of the new double-slit experiment favor Bohr’s position in this case. Famously, Bohr’s principle argued that paired properties of particles, such as their position and momentum, can not be measured at the same time.
Thus, the recent experiment appears to support Bohr’s ideas, and specifically the concept known as complementarity, which essentially argues that two contrasting theories may together be able to explain various phenomena—most notably, the seemingly contradictory wave and particle properties exhibited by light—but each by itself only explains part of the phenomena in question.
While Einstein may have lost out in this longstanding debate, it was both men’s visionary ideas—and their differing opinions in this case—that gave rise to one of the most tantalizing questions in quantum physics, and one that may now finally be resolved.
The team’s recent paper, “Tunable Einstein-Bohr Recoiling-Slit Gedankenexperiment at the Quantum Limit,” appeared in Physical Review Letters on December 2, 2025.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.