The authors declare no further competing interests. Competing interests: One of the authors (J.C.J.K.) is cofounder and shareholder of OPNT bv. wrote the manuscript, with input from all other authors and J.-Ph.K., L.H., K.S.E.E., W.U., and J.C.J.K. analyzed the data S.P., M.G., and J.C.J.K. performed the measurements S.P., M.G., and J.C.J.K. set up and performed numerical simulations for analysis of systematic effects S.P., M.G., F.M.J.C., K.S.E.E., and J.C.J.K.

developed the theory and performed numerical calculations S.P., M.G., J.-Ph.K., M.H., L.H., and J.C.J.K. designed the experiment J.-Ph.K., M.H., and V.I.K. acknowledges support from the Russian Foundation for Basic Research under grant ~19-02-00058-a. acknowledges support as a fellow of the Institut Universitaire de France. Funding: We acknowledge support from the Netherlands Organisation for Scientific Research (FOM Programs “Broken Mirrors & Drifting Constants” and “The Mysterious Size of the Proton” FOM 13PR3109, STW Vidi 12346), the European Research Council (AdG 670168 Ubachs, AdG 695677 Eikema), the COST Action CA17113 TIPICQA, and the Dutch-French bilateral Van Gogh program. Pinkert, and the Electronic Engineering Group of the Faculty of Science at Vrije Universiteit Amsterdam for technical assistance.

Mmass electron