Feynman's Path Integral Directly Verified in Experiment: SCNU Team Measures 1,419,857 Quantum Paths

Forum topic · 小凯 · 2026-08-26

Summary

A team led by Zhu Shiliang and Yan Hui at South China Normal University (SCNU) has reportedly achieved the first direct experimental verification of Feynman's path integral formulation, first proposed in 1948. The key obstacle was that the quantum propagator—a complex-valued amplitude at the core of the path integral—could not be measured by conventional techniques that only capture probabilities (real numbers). Building on their 2023 Nature Photonics work (DOI: 10.1038/s41566-023-01212-1), which first measured a single-photon propagator and demonstrated the quantum least-action principle, the team coupled single-photon spatial and polarization modes, sent photons through gradient-index (GRIN) optical materials, and reconstructed both real and imaginary parts of the propagator from spatial distributions captured by a single-photon camera. In an August 2026 paper in a Nature journal, they multiplied five measured propagators, reconstructed 1,419,857 distinct paths, and found that summing over these paths predicted the observed photon behavior accurately. The verification elevates the path integral from a textbook formalism to an experimentally observable object, with implications for quantum field theory, quantum gravity, quantum algorithms, and condensed matter physics. Note: details of the 2026 paper await confirmation of its officially published version.

Background

On August 27, a NetEase Tech report spread widely in Chinese media: Feynman's quantum hypothesis, proposed 80 years ago, has been directly verified by experiment for the first time. The team behind it is led by Zhu Shiliang and Yan Hui at the School of Physics, South China Normal University (SCNU).

Feynman's 1948 paper, *Space-Time Approach to Non-Relativistic Quantum Mechanics*, proposed that the probability amplitude for a particle to travel from A to B is a sum over all possible paths, each contributing a phase factor $e^{iS/\hbar}$ . Physicists have used this formalism for eight decades—but no one had ever *directly* measured it. The obstacle: the propagator $$K(B,A)$$ , the core object of the path integral, is a complex number (probability amplitude with real and imaginary parts), while conventional quantum measurement techniques can only extract probabilities (real numbers).

Key points

How the propagator was measured: The team coupled single-photon spatial modes with polarization modes, encoded photons during propagation through gradient-index (GRIN) optical materials, and captured spatial distributions with a single-photon camera. From distributions across different polarization states, they reconstructed both the real part Re(K) and the imaginary part Im(K) of the propagator.
From one propagator to the full path integral: In 2023 (Nature Photonics, DOI: 10.1038/s41566-023-01212-1), the team first measured a single-photon propagator and demonstrated the quantum least-action principle—photons in free space and harmonic potentials followed the classical path predicted by least action.
The 2026 result: In an August 2026 Nature-journal paper, the team repeated the measurement five times, multiplied the propagators

$K_{\text{total}}(x, t) = \int \cdots \int K_5 K_4 \cdots K_1 \, dx_1 dx_2 dx_3 dx_4$

and reconstructed 1,419,857 distinct paths through the GRIN material. Feeding these paths into Feynman's formula predicted photon behavior that matched actual observations—the first direct experimental verification of the full path integral.

Why 1,419,857 paths? The paths are discrete and inferred from experimental data. The number reflects an optimal trade-off between experimental precision (spatial resolution, polarization control, GRIN index control, camera noise) and error accumulation.

Progression: 2023 vs. 2026

| Dimension | 2023 paper | 2026 paper | |---|---|---| | Object measured | Single propagator | Product of 5 propagators | | Paths recovered | A few classical paths | 1,419,857 | | What was verified | Quantum least-action principle | The complete path integral | | Significance | Propagators are measurable | Path integrals are directly verifiable |

Why it matters

1. Quantum field theory: Feynman diagrams are discretized approximations of path integrals; direct verification opens the door to visualizing them experimentally. 2. Quantum gravity: The path integral is a standard language of quantum gravity; making it an observable quantity improves testability. 3. Quantum computing/algorithms: Path-integral-based algorithms (quantum Monte Carlo, quantum simulation) gain a direct experimental benchmark. 4. Condensed matter / materials science: Electron transport and phonon scattering descriptions can be re-calibrated against directly measured path integrals.

Zhu Shiliang commented:

> "Given the extraordinary success of Feynman's formula over nearly eighty years, this result itself is not surprising. But seeing it work… is still stunning. The familiar phrase 'sum over all paths' is no longer just a symbolic instruction in textbooks—we can see its results emerge directly from experimental data."

Significance for Chinese basic physics

SCNU is not among the traditional Chinese physics powerhouses (CAS, Peking, Tsinghua), yet the team spent six years on a single technical route (GRIN + single-photon imaging) to resolve an 80-year-old question on a tabletop-scale apparatus.
It showcases a "precision + ingenuity" route in Chinese basic physics, complementary to the "mega-facility + resources" route.

What to watch next

1. Extending from 5 time segments to 10, 20, or N segments—toward multi-particle systems. 2. Repeating the experiment with electrons or atoms instead of photons. 3. Moving from measuring the path integral to directly *imaging* it. 4. Diffusion into quantum field theory, quantum gravity, and quantum information groups at upcoming conferences.

References

1. NetEase Tech, *Feynman's 80-year-old quantum hypothesis directly verified by experiment for the first time*, 2026-08-27. 2. Wen Yongli, Wang Yunfei, Tian Liman, Yan Hui, Zhu Shiliang. *Experimental demonstration of the quantum least action principle*. Nature Photonics (2023). DOI: 10.1038/s41566-023-01212-1. 3. School of Physics, SCNU announcement, 2023-05-31. 4. Zhu Shiliang team. *Direct Experimental Verification of the Feynman Path Integral via Photon Propagator Measurement*. Nature journal (2026-08); details pending the officially published version.

> *Disclaimer: Core facts are cross-checked against NetEase Tech reports (2026-08-27), SCNU official announcements, and the 2023 Nature Photonics paper. Specific details of the 2026 work are subject to the officially published paper.*