Quantum optics with free-electrons is a promising field emerging based on the technology of electron microscopy 1. One part of the framework of free-electron quantum optics is the resonant and coherent interaction of free-electrons with bound-electron systems 2. Experimental access to this interaction is particularly challenging, since the bound-electron state has to couple directly to the near-field of the free electrons. With our experimental setup we are aiming to show this interaction for the first time 3. In this proof of principle experiment (see Fig. 1), we utilize a modulated free-space electron beam in a customized scanning electron microscope, to coherently drive electron spins. The near-field of this spatially modulated beam excites Zeeman levels in a α,γ-Bisdiphenylen-β-phenylally (BDPA) sample placed in a magnetic field. These quantum transitions couple inductively to a micro-coil. The signal in the micro-coil is measured with a lock-in amplifier, sensitive down to the thermal noise floor. A successful implementation of the proposed experiment will lay the foundation for coherently coupling modulated electron beams to bound-electron transitions. Realizing it in a electron microscope, allows to exploit the nano-scopic spatial resolution of electron microscopy. Apart from new spectroscopic methods utilizing the modulated near-field of an electron beam, higher order transitions may also be excited.
Weigner et al. (Wed,) studied this question.