  We provide a comprehensive study of the capabilities of modulated electron
wavefunctions for the preparation and readout of the quantum state of the
quantum emitters (QEs) they interact with. First, we consider perfectly
periodic electron combs, which do not produce QE-electron entanglement,
preserving the purity of the QE while inducing Rabi-like dynamics in it. We
extend our findings to realistic, non-ideally modulated electron wavepackets,
showing that the phenomenology persists, and exploring their use to prepare the
emitter in a desired quantum state. Thus, we establish the balance that
electron comb size, emitter radiative decay, and electron-emitter coupling
strength must fulfil in order to implement our ideas in experimentally feasible
platforms. Finally, moving into the limit of small electron combs, we reveal
that these wavefunctions allow for quantum state tomography of their target,
providing access not only to the populations, but also the coherences of the QE
density matrix. We believe that our theoretical results showcase modulated
free-electrons as very promising tools for quantum technologies based on
light-matter coupling.
