  Triple quantum dots (TQDs) are promising semiconductor spin qubits because of
their all-electrical control via fast, tunable exchange interactions and
immunity to global magnetic fluctuations. These qubits can experience strong
transverse interaction with photons in the resonant exchange (RX) regime, when
exchange is simultaneously active on both qubit axes. However, most theoretical
work has been based on phenomenological Fermi-Hubbard models, which may not
fully capture the complexity of the qubit spin-charge states in this regime.
Here we investigate exchange in Si/SiGe and GaAs TQDs using full configuration
interaction (FCI) calculations which better describe practical device
operation. We show that high exchange operation in general, and the RX regime
in particular, can differ significantly from simple models, presenting new
challenges and opportunities for spin-photon coupling. We highlight the impact
of device electrostatics and effective mass on exchange and identify a new
operating point (XRX) where strong spin-photon coupling is most likely to occur
in Si/SiGe TQDs. Based on our numerical results, we analyze the feasibility of
a remote entanglement cavity iSWAP protocol and discuss design pathways for
improving fidelity. Our analysis provides insight into the requirements for TQD
spin-photon transduction and demonstrates more generally the necessity of
accurate modeling of exchange in spin qubits.
