  We show that systematic full configuration-interaction (FCI) calculations
enable prediction of the energy spectra and the intrinsic spatial and spin
structures of the many-body wave functions as a function of the detuning
parameter for the case of three-electron hybrid qubits based on GaAs asymmetric
double quantum dots. Specifically, in comparison with the case of weak
interactions and treating the entire three-electron double-dot hybrid qubit as
an integral unit, it is shown that the predicted spectroscopic patterns,
originating from strong electron correlations, manifest the formation of Wigner
molecules (WMs). Signatures of WM formation include: (1) a strong suppression
of the energy gaps relative to the non-interacting-electrons modeling, and (2)
the appearance of a pair of avoided crossings arising between states associated
with two-electron occupancies in the left and right wells. The Wigner molecule
is a physical entity associated with electron localization within each well and
it cannot be captured by the previously employed independent-particle or
two-site-Hubbard theoretical modeling of the hybrid qubits. The emergence of
strong WMs is investigated in depth through the concerted use of FCI-adapted
diagnostic tools like charge and spin densities, as well as conditional
probability distributions. Furthermore, the energy spectrum as a function of
the strength of the Coulomb repulsion (at constant detuning) is calculated in
order to complement the thorough analysis of the factors contributing to WM
emergence. We report remarkable agreement with recent experimental
measurements. The present FCI methodology for multi-well quantum dots can be
straightforwardly extended to treat valleytronic two-band Si/SiGe hybrid
qubits, where the central role of the WMs was confirmed recently.
