Artificial quantum-light swimmer moves upstream, challenging Newton's third law

A research team from the Institute of Photonic Sciences has demonstrated that an engineered microscale swimmer can travel upstream against a directed flow of photons, a phenomenon that appears to contravene the conventional interpretation of Newton’s third law. The device, fabricated from a dielectric nanostructure and immersed in a specially prepared quantum light field, exploits non‑reciprocal optical forces that arise from the quantum correlations of the photon stream. When the swimmer is illuminated by a coherent beam tuned to generate a standing‑wave pattern, the asymmetric scattering of photons produces a net thrust opposite to the direction of the light’s momentum flux, allowing the swimmer to move upstream without an externally applied counterforce.

The findings, detailed in a paper published in Nature Physics, indicate that the apparent violation stems from the breakdown of action‑reaction symmetry at the quantum level, where fluctuations and entanglement can mediate forces that do not have a classical counterpart. The authors note that the effect relies on precise control of the light’s quantum state and the swimmer’s geometry, and they suggest potential applications in targeted drug delivery, nanoscale transport, and the development of novel photonic engines. Further experiments are planned to explore scalability and to assess the robustness of the mechanism under varying environmental conditions.