Probing two-qubit capacitive interactions beyond bilinear regime using dual Hamiltonian parameter estimations

We report the simultaneous operation and two-qubit coupling measurement of a pair two-electron spin qubits that are actively decoupled from quasistatic nuclear noise in GaAs quadruple quantum dot array. Coherent Rabi oscillations both (decay time $\approx$2 {\mu}s; frequency few MHz) achieved by continuously tuning drive using rapidly converging real-time Hamiltonian estimators. By state conditional exchange oscillation measurements, we also observe strong capacitive interaction (> 190 MHz). show scaling with respect to intra-qubit energies is stronger than bilinear form, consistent recent theoretical predictions. high ratio (>16) between coherence phase-flip time, which supports possibility generating high-fidelity fast entanglement encoded simple interaction.