  We report energy selective tunneling readout-based Hamiltonian parameter
estimation of a two-electron spin qubit in a GaAs quantum dot array.
Optimization of readout fidelity enables a single-shot measurement time of 16
on average, with adaptive initialization and efficient qubit frequency
estimation based on real-time Bayesian inference. For qubit operation in a
frequency heralded mode, we observe a 40-fold increase in coherence time
without resorting to dynamic nuclear polarization. We also demonstrate active
frequency feedback with quantum oscillation visibility, single-shot measurement
fidelity, and state initialization fidelity up to 97.7%, 99%, and over 99.7%,
respectively. By pushing the sensitivity of the energy selective
tunneling-based spin to charge conversion to the limit, the technique is useful
for advanced quantum control protocols such as error mitigation schemes, where
fast qubit parameter calibration with a large signal-to-noise ratio is crucial.
