0_4355 Due to their significant momentum mismatch, swift electrons and light do not interact directly. However, free electrons can couple to the optical fields of nanostructures, e.g., surface plasmon polaritons, stimulated by intense light pulses. Such physical phenomena are at the heart of the Photon-induced Near Field Electron Microscopy (PINEM) technique. The spatially confined optical excitations modify the wavefunction of free electrons that lose and gain discrete amounts of energy quantized by the electron-photon interactions, producing characteristic sidebands in the EELS spectra spaced at energies corresponding to the wavelength of the stimulating light pulses. Such interactions provide a quantum coherence in which the transverse and longitudinal electron wavefunctions of the electron pulses can be coherently manipulated. Recent research on the coherent quantum interaction of PINEM in Ultrafast TEMs has led to the development of new techniques, like Ramsey-type holography, which allows for the dynamical reconstruction of the quantum state of an electron beam interacting with a material of interest. These PINEM approaches demonstrate the potential for a combined attosecond-nanometer-millielectronvolt resolution in time-resolved spectroscopic imaging. Beyond the imaging application, it also allows the implementation of optically-controlled and spatially-resolved quantum measurements in parallel, providing an efficient and versatile tool for electron quantum optics and quantum coherent spectroscopy.Here, we demonstrate a new method using PINEM interactions for obtaining an absolute energy calibration of the entire range of electron spectrometer (up to three decades) with an accuracy reaching 10 µeV with a single acquisition and correcting the dispersion nonlinearities. We use high-finesse (Q~10⁶) microresonators that enable continuous wave (CW) PINEM and can provide phase-matched interaction between the 120 keV electrons and light circulating in a microring resonator. The inelastic interaction with electrons leads to the exchange of an integer number of photons and results in a broad electron spectrum, e.g., 100 eV wide, consisting of equally-spaced side peaks. Performing the Fourier transform of the measured electron provides a precise measure of the spectrometer’s dispersion. This calibration method is universal and readily applicable to any modern electron microscope. Near-term experiments on an aberration-corrected TEM aim to extend this approach to monochromated 200kV and 80kV electrons in which the improved energy resolution produces spectra with discrete, well-deconvolved side peaks. Vibrational and Valance EEL spectroscopy as oxidation state and chemical shift analysis can benefit from the precise calibration enabled by this approach.