MIT physicists have recreated the most iconic experiment in quantum physics — this time with individual atoms acting as the slits and single photons barely grazing past. Their results have firmly answered a question that has lingered for nearly a century: Can we ever observe light’s particle and wave nature at the same time?
The answer, once again, is no. Not even Einstein’s clever workaround holds up.
“Einstein and Bohr would have never thought that this is possible,” said Wolfgang Ketterle, the Nobel Prize-winning physicist who led the study. “What we have done is an idealized Gedanken experiment.”
The study, published in Physical Review Letters, represents the most stripped-down, precise realization of the double-slit experiment to date. It confirms that even the faintest interaction — like a photon subtly “rustling” an atom as it passes — wipes out the interference pattern that reveals light’s wave nature. This tiny rustle, it turns out, is enough to collapse a photon’s quantum superposition, transforming its behavior from wavelike to particle-like.
A Thought Experiment Made Real
The original double-slit experiment dates back to 1801, when Thomas Young showed that light behaves as a wave by shining it through two slits and producing an interference pattern on a screen. In the 20th century, quantum mechanics complicated the picture: light, like all quantum objects, can behave as both a wave and a particle, but never both at the same time.
Einstein wasn’t satisfied. At a 1927 conference, he suggested a way to catch light in both modes. If you could detect the minute recoil of the slit apparatus — imagine the “rustling of atoms,” as he put it — you could know which path a photon took without disrupting the wave interference pattern on the far side.
Niels Bohr countered, invoking the uncertainty principle. Any such measurement would necessarily disturb the system and erase the wave pattern. And so, the debate became one of the most famous in quantum theory.
Since then, many variations of the double-slit experiment have shown Bohr to be right. But the MIT study goes further, turning what was once a hypothetical scenario into a real-world test.
The Smallest Slits Imaginable
To recreate Einstein’s “recoiling slit” idea, the MIT team used more than 10,000 ultracold atoms — cooled to microkelvin temperatures (very close to absolute zero) and arranged in a tight lattice held in place by laser light. These atoms acted as the slits, and weak beams of light ensured that photons passed through one at a time.
Each atom was far enough apart that it could be treated as an isolated slit. The researchers could then fine-tune how loosely each atom was held — making it more or less “fuzzy” in space. The fuzzier the atom, the more likely it was to be jostled by a passing photon and thus record which-way information (in quantum mechanics, this refers to whether we know which path a quantum particle took in an experiment).
When that rustling occurred, even at the level of a single photon disturbing a single atom, the interference pattern faded.
“We realized we can quantify the degree to which this scattering process is like a particle or a wave,” said first author Vitaly Fedoseev. “And we quickly realized we can apply this new method to realize this famous experiment in a very idealized way.”
In doing so, the team stripped away all confounding factors — no mirrors, no detectors, no spring-loaded screens. Just atoms and light.
“These single atoms are like the smallest slits you could possibly build,” Ketterle explained.
No Need for Springs
Einstein’s proposal involved using a slit mounted on a spring. Detecting the motion of that spring would reveal which slit the photon passed through. In past experiments, such spring-like setups have indeed shown that gaining path information erases the interference pattern.
But Ketterle’s group showed that even without the “spring” — without any recoil-measuring device at all — the same result holds. The only thing that matters is whether the atom becomes entangled with the photon’s path.
“In many descriptions, the springs play a major role,” Fedoseev said. “But we show, no, the springs do not matter here; what matters is only the fuzziness of the atoms.”
In quantum terms, this fuzziness translates to uncertainty in position. And it turns out that’s enough. The more spatially spread out the atom is, the more it becomes entangled with the photon, and the more likely it is to destroy the interference pattern.
Why It Matters
The findings are subtle but significant. They show that the boundary between coherent and incoherent scattering — the difference between a clean interference pattern and a blotchy, particle-like one — can be fully described by quantum entanglement. Even without transferring energy to an atom, a photon can leave behind enough information to destroy its own wave-like behavior.
And crucially, this holds even in free space, after the atoms have been released from their lattice confinement. The researchers demonstrated that light scattering before and immediately after trap release was statistically identical.
That’s because the key quantity isn’t the atoms’ momentum or their confinement. It’s the size of their wavepackets — their spatial uncertainty. The broader the wavepacket, the more the light’s interference gets muddled.
“Our derivation shows that the fractions of coherently and incoherently scattered light are the same regardless of the presence of a trapping potential,” the authors write.
This insight may seem abstract, but it touches on the very heart of quantum mechanics. It reinforces the idea that observation — or any physical interaction that encodes information — collapses quantum superpositions. No clever loophole, not even Einstein’s, can dodge this fundamental limit.
A Timely Conclusion
The United Nations has declared 2025 the International Year of Quantum Science and Technology, marking a century since quantum mechanics took shape. The debate that began in 1927 between Einstein and Bohr is still being tested — now with unprecedented precision.
“It’s a wonderful coincidence that we could help clarify this historic controversy in the same year we celebrate quantum physics,” said co-author Yoo Kyung Lee.
