Einstein is wrong: MIT’s laser-cooled atoms solve the debate

In the feats of atomic accuracy and quantum clarity, MIT physicists have launched the most ideal version of the famous double-slit experiment.

The team used a single photon and laser to cool atoms as gaps in the quantum scale to confirm what Einstein wanted was incorrect: light really cannot manifest as particles and waves at the same time. These findings are published in Physical comment letterrevisiting a century-old debate between Einstein and Nielsbohr and strengthening the core of quantum physics.

Single atoms, single photons and the mystery of a century of history

The double-slit experiment was first designed by Thomas Young, which reveals the interference patterns that light produces like waves. But over time, quantum theory turns tests into something more specific. Light a beam through two gaps and you get a wave pattern. Try to measure the slits the photon passes, the pattern disappears, and light works like particles.

In 1927, Einstein argued that the photons acting as particles should push their gaps, just as a breeze rustled curtains. If this push can be detected, he thinks we can track the path of the photon without breaking the interference mode. Bohr’s response calls the principle of uncertainty: knowing that the path will destroy the wave pattern. Bohr proved right – MIT proved it only through extraordinary control.

Atomic slit: Use matter instead of metal

“What we do can be seen as a new variant of the double-slit experiment,” said Wolfgang Ketterle, a professor of physics at MIT who chaired the study. “These individual atoms are like the smallest slit you can build.”

The researchers used more than 10,000 ultra-low atoms and arranged them in crystal lattices with lasers. Each atom is isolated enough to act as a unique “crack” with only one photon at a time spreading. A highly sensitive detector records the scattered light to determine whether it behaves like a wave or a particle.

Adjust quantum blur

To switch Light’s dual identity, the team adjusted the “fuzziness” (its quantum uncertainty) at each atom’s position. Loosely held atoms are more blurry and are more likely to record the path of photons, pushing the system towards particle-like behavior. The tightly fixed atoms are not so blurry, and there is no idea where the photons go, thus retaining the wavy interference.

By adjusting the number of atoms that limit space, physicists control the point where light creates interference strips or particles on the detector. The results match the quantum prediction exactly.

• More path information (more blur): Reduce interference
• Less path information (tighter limit): More interference
• The waves and particle identity of photons have never been seen at the same time

What about Einstein’s spring?

Einstein once imagined the gap hanging on tiny springs. The transmitted photon may move one gently, thus providing clues to its path. The MIT team also tested the idea and then removed “Spring.” These atoms were briefly released from the laser trap and floated freely in the vacuum under gravity. Even without the trap, the result is the same.

Lead author Vitaly Fedoseev said: “In many descriptions, springs play a major role. But we show that no, springs don’t matter here; what matters is the ambiguity of the atoms.”

A century of coincidence

The timing of the discovery is poetic. 2025 marks the 100th anniversary of the founding of quantum mechanics. The Einstein-bohr pun debate took place in 1927. Now, a century later, Quantum Science has entered a new era of precision.

“Einstein and Bohr never thought of doing such an experiment on a single atom and a single photon,” Keitel said. “What we did was an idealized Gedanken experiment.”

The research was supported by the National Science Foundation, the U.S. Department of Defense, and the Gordon and Betty Moore Foundation.

Journal Reference

title: Coherent and incoherent light scattering of single-atom wave packets
author: Vitaly Fedoseev, Hanzhen Lin, Yu-Kun Lu, Yoo Kyung Lee, Jiahao Lyu, Wolfgang Ketterle
Magazine: Physical comment letter
doi: 10.1103/ZWHD-1K2T
release: July 22, 2025

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