Physicists in China Put Feynman's 80-Year-Old Quantum Idea to the Test

For nearly 80 years, physicists have relied on a thought experiment devised by Richard Feynman to predict how quantum particles behave. Researchers in China have now tested the idea directly in the laboratory for the first time, using single particles of light. The team was led by Shi-Liang Zhu at South China Normal University in Guangzhou, and the results were published in Science Advances.

Thought experiments have long shaped quantum mechanics — famous examples such as Schrodinger's cat and the double-slit experiment showed that particles can exist in multiple states at once, and that their behavior changes when they are directly measured. In 1948, Feynman proposed his "path integral" idea: a quantum particle moving between two points does not take a single route. Instead, every conceivable path contributes, and all of them add together to produce the outcome that is observed. Feynman also predicted that each of these paths carries the same likelihood, differing only in the phase of their quantum wave functions — assumptions that had never been confirmed experimentally.

Zhu's team tested the idea using single photons. Rather than tracking a photon's path directly, which is impossible without disturbing it, the researchers measured its probability amplitude, a value combining how likely the photon is to take a given route with the timing of that route. By sending photons through a setup of mirrors, lenses and crystals and carefully measuring how their properties shifted, the team reconstructed amplitudes for about 1,419,857 possible paths. Because so many paths were involved, the researchers refined nearly every part of their measurement process to keep small errors from snowballing into meaningless results.

The results closely matched Feynman's predictions: probabilities emerged from combining all the paths, the paths carried equal strength, and phases were set by the particle's classical trajectory. Zhu's team says other researchers could adapt the technique to test how paths combine in different physical systems, such as photons traveling through materials rather than empty space.