Cosmological relic neutrinos are a fundamental ingredient of modern cosmology, but until today they escaped direct detection because of their extremely low interaction probability. Detecting the cosmological neutrino background is the goal of the PTOLEMY project. In PTOLEMY neutrinos are captured by tritium and electrons are emitted with a kinetic energy of a fraction of electronvolt (eV) above the tritium beta decay endpoint at 18keV. Electrons are then decelerated to few eVs, by removing a precisely known amount of kinetic energy with an time-varying electro-magnetic filter.