  It is demonstrated that exact diagonalization of the microscopic many-body
Hamiltonian via systematic full configuration-interaction (FCI) calculations is
able to predict the spectra as a function of detuning of three-electron hybrid
qubits based on GaAs asymmetric double quantum dots. It is further shown that,
as a result of strong inter-electron correlations, these spectroscopic
patterns, including avoided crossings between states associated with different
electron occupancies of the left and right wells, are inextricably related to
the formation of Wigner molecules. These physical entities cannot be captured
by the previously employed independent-particle or Hubbard-type theoretical
modeling of the hybrid qubit. We report remarkable agreement with recent
experimental results. Moreover, the present FCI methodology for multi-well
quantum dots can be straightforwardly extended to treat Si/SiGe hybrid qubits,
where the central role of Wigner molecules was recently experimentally
confirmed as well.
