Since its inception, research of cavity quantum electrodynamics (CQED) 1,2 has greatly extended our understanding of light-matter interactions and our ability to utilize them. Thus far, all the work in this field has been focused on light interacting with bound electron systems -such as atoms, molecules, quantum dots, and quantum circuits. In contrast, markedly different physical phenomena and applications could be found in free-electron systems, the energy distribution of which is continuous and not discrete, implying tunable transitions and selection rules. In addition to their uses for electron microscopy 3,4 , the interaction of free electrons with light gives rise to important phenomena such as Cherenkov radiation, Compton scattering, and free-electron lasing 5,6 . Advances in the research of ultrafast electron-light interactions have also enabled the development of powerful tools for exploring femtosecond dynamics at the nanoscale 7,8 . However, thus far, no experiment has shown the integration of free electrons into the framework of CQED, because the fundamental electron-light interaction is limited in strength and lifetime. This limit explains why many phenomena have remained out of reach for experiments with free electrons. In this work, we developed the platform for studying CQED at the nanoscale with free electrons and demonstrated it by observing their coherent interaction with cavity photons for the first time. To demonstrate this concept, we directly measure the cavity photon lifetime via a free electron probe and show more than an order of magnitude enhancement in the electron-photon interaction strength. These capabilities may open new paths toward using free electrons as carriers of quantum information, even more so after strong coupling between free electrons and cavity photons will have been demonstrated.Efficient electron-cavity photon coupling could also allow new nonlinear phenomena of cavity opto-electro-mechanics and the ultrafast exploration of soft matter or other beam-sensitive materials using low electron current and low laser exposure.

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 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

We present the first observation of the quantum nature of the Cherenkov effect, by phasematching light & electron waves. Interacting coherently along hundreds of microns, each electron simultaneously absorbs and emits hundreds of photon quanta.