In a stunning demonstration of experimental precision, physicists at the Massachusetts Institute of Technology (MIT) have performed the most idealized version of the famous double-slit experiment, settling a century-old debate between Albert Einstein and Niels Bohr. By using ultracold atoms and single photons, the team has definitively confirmed Bohr’s view on quantum mechanics: it is impossible to observe the dual wave and particle nature of light at the same time.
The experiment revisits a historic scientific rivalry that began in 1927. At its heart was the double-slit experiment, which reveals that light behaves as a wave when unobserved but as a particle when measured. Einstein argued that one could, in theory, devise a way to detect which of the two slits a single photon passes through (its particle nature) by measuring the slit’s recoil, while simultaneously observing the wave-like interference pattern it creates. Bohr countered, invoking the quantum uncertainty principle to argue that the very act of measuring the photon’s path would inevitably destroy the interference pattern.
Now, a team led by MIT Professor Wolfgang Ketterle has brought this thought experiment—or Gedanken experiment—to life. “Einstein and Bohr would have never thought that this is possible, to perform such an experiment with single atoms and single photons,” says Ketterle. The researchers replaced the traditional slits with more than 10,000 atoms, super-cooled to just above absolute zero and held in a crystal-like lattice by lasers. This setup allowed them to scatter single photons off two adjacent atoms, treating the atoms as the smallest possible “slits.”
The key to the experiment was the ability to tune the quantum state of the atoms. By adjusting the lasers holding an atom in place, the researchers could control its “fuzziness,” or the uncertainty of its location. A “fuzzier,” more loosely held atom was more easily “rustled” by a passing photon, which in turn recorded information about the photon’s path. The results, published in Physical Review Letters, confirmed the predictions of quantum theory: the more information they obtained about the photon’s particle-like path, the weaker the visibility of the wave-like interference pattern became.
Most significantly, the team directly tested Einstein’s idea of detecting a slit’s recoil, which he conceptualized as the slit being mounted on a spring. In previous experiments, such a “spring-like” component was a factor. The MIT team performed their experiment without the proverbial springs. By momentarily turning off the lasers holding the atoms in place, they measured the outcome while the atoms were effectively floating in free space. They observed the exact same phenomenon, proving that the loss of wave interference is not due to a mechanical force, but to a more profound “quantum correlation between photons and atoms,” as first author Vitaly Fedoseev explains.
This work provides the clearest demonstration to date of Bohr’s principle of complementarity and clarifies a foundational concept of quantum physics. Its timing is fitting, as the United Nations has declared 2025 the International Year of Quantum Science and Technology, celebrating the 100th anniversary of the formulation of quantum mechanics. “It’s a wonderful coincidence that we could help clarify this historic controversy in the same year we celebrate quantum physics,” says co-author Yoo Kyung Lee.