  Majorana bound states are putative collective excitations in solids that
exhibit the self-conjugate property of Majorana fermions - they are their own
antiparticles. In iron-based superconductors, zero-energy states in vortices
have been reported as potential Majorana bound states, but the evidence remains
controversial. Here, we use scanning tunneling noise spectroscopy to study the
tunneling process into vortex bound states in the conventional superconductor
NbSe2, and in the putative Majorana platform FeTe0.55Se0.45. We find that
tunneling into vortex bound states in both cases exhibits charge transfer of a
single electron charge. Our data for the zero-energy bound states in
FeTe0.55Se0.45 exclude the possibility of Yu-Shiba-Rusinov states and are
consistent with both Majorana bound states and trivial vortex bound states. Our
results open an avenue for investigating the exotic states in vortex cores and
for future Majorana devices, although further theoretical investigations
involving charge dynamics and superconducting tips are necessary.
