  As an elementary particle the electron carries spin \hbar/2 and charge e.
When binding to the atomic nucleus it also acquires an angular momentum quantum
number corresponding to the quantized atomic orbital it occupies (e.g., s, p or
d). Even if electrons in solids form bands and delocalize from the nuclei, in
Mott insulators they retain their three fundamental quantum numbers: spin,
charge and orbital[1]. The hallmark of one-dimensional (1D) physics is a
breaking up of the elementary electron into its separate degrees of freedom[2].
The separation of the electron into independent quasi-particles that carry
either spin (spinons) or charge (holons) was first observed fifteen years
ago[3]. Using Resonant Inelastic X-ray Scattering on the 1D Mott-insulator
Sr2CuO3 we now observe also the orbital degree of freedom separating. We
resolve an orbiton liberating itself from spinons and propagating through the
lattice as a distinct quasi-particle with a substantial dispersion of ~0.2 eV.
