A quantum particle does not choose a single path - it seems to traverse all possible ones at once, and only the sum of these trajectories yields the observable result. It is this idea, proposed by Richard Feynman in 1948, that Chinese scientists have now verified in a direct experiment for the first time.
A team led by Shi-Liang Zhu from South China Normal University and the Hefei National Laboratory conducted measurements on single photons. The researchers built a complex optical setup where a photon passed through a sequence of "mazes" of phase shifts and polarizers. They precisely measured five propagators - mathematical objects describing the evolution of a quantum state between two points - and multiplied them together, ultimately reconstructing 1 419 857 individual trajectories.
The results matched the predictions of Feynman's formula with high precision: the probabilities emerged from the coherent superposition of all paths, and the amplitudes of all trajectories had the same magnitude, while the phase was determined by the classical action in units of Planck's constant. This is direct confirmation of two fundamental postulates that were previously taken on faith.
Imagine throwing a stone into a pond: classical physics sees a single trajectory, but quantum mechanics requires summing the contributions from billions of "ghost" paths, each adding its tiny correction. The experiment showed that without accounting for all these corrections, the prediction fails - and that is exactly how a real photon behaves.
The work, published on 26 August 2026 in the journal Science Advances, opens the way to new tests in quantum optics and to more precise control of quantum states. Now it will be possible to directly verify how the environment or measurement "cuts off" extra paths and to create more robust quantum devices.
The experiment did not merely confirm an old theory - it provided a tool to look deeper into the very nature of quantum reality.
