Feynman’s 80-year-old quantum ‘path integral’ verified by experiment

A foundational postulate of quantum physics, formulated by Richard Feynman nearly 80 years ago, has been directly verified by experiment for the first time. The research, published August 26 in Science Advances, confirms that the "path integral" — which predicts a quantum particle's behavior by summing all possible paths it could take — works exactly as Feynman proposed in 1948.science+1

The Experiment

Shi-Liang Zhu and colleagues at South China Normal University in Guangzhou, China, tested Feynman's path integral by sending single photons through a maze of tiny mirrors, lenses, and crystals. The team measured a quantum object known as a propagator, which predicts how a photon's quantum state changes as it travels between points in the experiment.newscientist

By measuring five successive propagators for a given photon and multiplying them together, the researchers reconstructed the path integral from experimental data. The process yielded 1,419,857 distinct paths, all of which were plugged into Feynman's formula. The prediction matched what the photon actually did.newscientist

"The familiar phrase 'sum over all paths' was no longer just a symbolic instruction in a textbook, we could see its consequences emerge directly from experimental data," Zhu said.newscientist

Why It Matters

The Feynman path integral has been a cornerstone of theoretical physics since its introduction, underpinning calculations in quantum electrodynamics, particle physics, and condensed matter theory. Yet until now, it had never been directly tested — scientists simply assumed it worked because the theories built on top of it produced accurate predictions.newscientist

Jörg Götte at the University of Glasgow called the precision of the experiment a real scientific advance. Given that a photon's path cannot be traced directly as it moves, the work showcases sophisticated quantum measurement and control methods, he said. Had the measurement turned up something else, "the whole framework of using the path integral for quantum calculations would have crumbled."newscientist

Technical Challenges

Zhu noted that the experiment required improving "almost every aspect" simultaneously. The sheer number of paths created opportunities for errors and experimental noise to accumulate across the five propagator multiplications. Without those improvements, the final data would have appeared nearly completely random.newscientist

The team hopes researchers from other fields can adapt their experimental approach to different quantum systems, potentially enabling more complex tests — such as directly measuring how paths add up when photons travel through materials rather than air.newscientist