  Coulomb blockade is a fundamental phenomenon in physics enabling transfer of
individual electrons one by one into electrically isolated nanostructures such
as nanowires or quantum dots and thereby creation of sources of single
electrons. Nowadays, solid-state single-electron sources are key elements of
the emerging new technologies of quantum information processing and
single-electron electronics. Moreover, advanced research in free-electron
quantum optics and developments in electron microscopy require the point
sources of free electrons in vacuum, which can be controlled on a single
electron level. However, up to now, single-electron vacuum guns were not
realized in practice. The problems to be solved include the formation of stable
tip-shaped heterostructured emitters and control of liberated electrons in time
and energy domain. Here we overcome these challenges by creating a field
emission (FE) electron source based on a carbon nanowire coupled to an
ultra-sharp diamond tip by a tunnel junction. Using energy spectroscopy, we
directly observe Coulomb oscillations of the electron Fermi level in the
nanowire at room temperature and FE currents up to 1 $\mu$A. We reveal that the
oscillations are suppressed at high FE current either by Joule heating or due
to the high tunneling resistance, depending on the nanowire charging time,
ranging from about 25 fs to 1 ps. We anticipate that the combination of
introduced carbon single-electron sources with laser-induced gating is highly
promising for the creation of coherent ultrashort free-electron bunches of
interest for low-energy electron holography and ultrafast electron or X-ray
imaging and spectroscopy.
