  Transition metal oxides (TMOs) host a wealth of exotic phenomena ranging from
charge, orbital, and magnetic order to nontrivial topological phases and
superconductivity. In order to translate these unique materials properties into
novel device functionalities, TMOs must be doped. However, the nature of
carriers in doped oxides and their conduction mechanism at the atomic scale
remain unclear. Recent angle-resolved photoelectron spectroscopy (ARPES)
investigations provided new insight into these questions, revealing that the
carriers of prototypical metal oxides undergo a transition from a polaronic
liquid to a Fermi liquid regime with increasing doping. Here, by performing
\textit{ab initio} many-body calculations of the ARPES spectra of TiO$_2$, we
show that this transition originates from non-adiabatic polar electron-phonon
coupling, and occurs when the frequency of plasma oscillations exceeds that of
longitudinal-optical phonons. This finding suggests that a universal mechanism
may underlie polaron formation in TMOs, and provides a new paradigm for
engineering emergent properties in quantum matter.
