Observation of 2D Cherenkov radiation

TL;DR

This paper reports the first experimental observation of 2D Cherenkov surface waves, revealing quantum effects and enhanced electron-photon interactions in reduced dimensions, opening new avenues in free-electron quantum optics.

Contribution

It demonstrates the existence of 2D Cherenkov surface waves and achieves unprecedented quantum coupling strengths with free electrons, confirming theoretical predictions.

Findings

Observation of narrow-bandwidth 2D Cherenkov surface waves.

Quantum coupling strength exceeds previous records by over two orders of magnitude.

Potential for free-electron-based quantum emitters of heralded Fock states.

Abstract

For over 80 years of research, the conventional description of free-electron radiation phenomena, such as Cherenkov radiation, has remained unchanged: classical three-dimensional electromagnetic waves. Interestingly, in reduced dimensionality, the properties of free-electron radiation are predicted to fundamentally change. Here, we present the first observation of Cherenkov surface waves, wherein free electrons emit narrow-bandwidth photonic quasiparticles propagating in two-dimensions. The low dimensionality and narrow bandwidth of the effect enable to identify quantized emission events through electron energy loss spectroscopy. Our results support the recent theoretical prediction that free electrons do not always emit classical light and can instead become entangled with the photons they emit. The two-dimensional Cherenkov interaction achieves quantum coupling strengths over two orders of magnitude larger than ever reported, reaching the single-electron-single-photon interaction regime for the first time with free electrons. Our findings pave the way to previously unexplored phenomena in free-electron quantum optics, facilitating bright, free-electron-based quantum emitters of heralded Fock states.