  A method for characterising the wave-function of freely-propagating particles
would provide a useful tool for developing quantum-information technologies
with single electronic excitations. Previous continuous-variable quantum
tomography techniques developed to analyse electronic excitations in the
energy-time domain have been limited to energies close to the Fermi level. We
show that a wideband tomography of single-particle distributions is possible
using energy-time filtering and that the Wigner representation of the
mixed-state density matrix can be reconstructed for solitary electrons emitted
by an on-demand single-electron source. These are highly localised
distributions, isolated from the Fermi sea. While we cannot resolve the pure
state Wigner function of our excitations due to classical fluctuations, we can
partially resolve the chirp and squeezing of the Wigner function imposed by
emission conditions and quantify the quantumness of the source. This tomography
scheme, when implemented with sufficient experimental resolution, will enable
quantum-limited measurements, providing information on electron coherence and
entanglement at the individual particle level.
