  The last two decades experimentally affirmed the quantum nature of free
electron wavepackets by the rapid development of transmission electron
microscopes into ultrafast, quantum-coherent systems. In particular, ultrafast
electron pulses can be generated and timed to interact with optical
near-fields, yielding coherent exchange of the quantized photon energy between
the relativistic electron wavepacket and the light field. So far, all
experiments have been restricted to the physically-confining bounds of
transmission electron microscopes, with their small, millimeter-sized sample
chambers. In this work, we show the quantum coherent coupling between electrons
and light in a scanning electron microscope, at unprecedentedly low electron
energies down to 10.4 keV, so with sub-relativistic electrons. Scanning
electron microscopes not only afford the yet-unexplored electron energies from
~0.5 to 30 keV providing optimum light-coupling efficiencies, but they also
offer spacious and easily-configurable experimental chambers for extended and
cascaded optical set-ups, potentially boasting thousands of photon-electron
interaction zones. Our results unleashes the full potential of quantum
experiments including electron wavepacket shaping and quantum computing with
multiple arithmetic operations and will allow imaging with low-energy electrons
and attosecond time resolution.
