A century-old thought experiment proposed by Albert Einstein to challenge Niels Bohr's ideas on quantum mechanics has been realized in a real-world test. Researchers led by Chao-Yang Lu at the University of Science and Technology of China used modern tools to confirm Bohr's principle of complementarity. The findings show that light exhibits both wave and particle properties, but not simultaneously in a clear way.
The rivalry between Albert Einstein and Niels Bohr over quantum mechanics dates back to 1927, centered on the double-slit experiment first demonstrated by Thomas Young in 1801. Young proved light behaves as a wave by showing an interference pattern of light and dark stripes on a screen after passing through two slits. Einstein argued light is a particle, while Bohr introduced complementarity, suggesting quantum objects can act as waves or particles but not both at once.
Einstein proposed modifying the experiment with a recoiling slit to track a photon's path, which he believed would reveal both behaviors simultaneously, contradicting Bohr. Nearly a century later, Lu and his team at the University of Science and Technology of China brought this to life. They fired a single photon at an ultracold atom, acting as the slit, cooled by lasers and electromagnetic forces for precise control.
The atom recoiled upon interaction, putting the photon into a state mimicking passage through two paths and producing an interference pattern. By tuning the atom's momentum uncertainty, as predicted by Heisenberg's uncertainty principle, the team could erase the pattern when measuring which-way information precisely. In intermediate regimes, a blurry interference appeared alongside partial recoil data, showing both properties partially. "Seeing quantum mechanics ‘in action’ at this fundamental level is simply breathtaking," Lu said.
Bohr's counterargument held: precise position knowledge fuzzes momentum, destroying the pattern. A related experiment this year by Wolfgang Ketterle at the Massachusetts Institute of Technology used two ultracold atoms and lasers, confirming similar results even without recoiling mechanisms. "In atomic physics, with cold atoms and lasers, we have real opportunities to showcase quantum mechanics with clarity which was not possible before," Ketterle noted.
Philipp Treutlein at the University of Basel praised the work for matching historical predictions and its educational impact. Lu hopes it inspires wonder in quantum mechanics' beauty. The results appear in Physical Review Letters (DOI: 10.1103/93zb-lws3).