  Since its inception, research of cavity quantum electrodynamics (CQED) has
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
are 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, the interaction of free
electrons with light enables important phenomena such as Cherenkov radiation,
Compton scattering, and free-electron lasing. However, 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. We
also 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.
