Experimental proof of stochastic photon emission in quantum radiation reaction
In plain words
Quantum theory says an electron loses energy in random discrete jumps when it emits gamma rays, while simpler models treat the loss as a smooth drag. Experiments have now seen quantum radiation reaction but cannot yet tell the random-jump picture from the smooth one.
Precise statement
For $\chi \sim 0.1-1$ the quantum-stochastic description predicts electron energy straggling (growth of energy spread) and quantum quenching, both absent in quantum-continuous models (LL with a Gaunt-factor reduction of the emitted power). In head-on electron-laser collisions with $a0 \sim 10-100$ and $\gamma \sim 1e3-1e4$, measure post-collision electron spectra (energy spread evolution, high-energy tail) or photon spectra with precision sufficient to reject the quantum-continuous model at $5\,\sigma$, controlling shot-to-shot jitter in collision timing and overlap.
What would settle it
A laser-electron collision campaign with measured collision parameters per shot that distinguishes the two quantum models at $5\,\sigma$.
Status in the literature
Unverified note
Los et al. (Nature Communications, 2026) reported a $>5\,\sigma$ observation of radiation reaction with substantial quantum effects, favoring quantum over classical models, but the quantum-continuous and quantum-stochastic models performed comparably.