The interaction of qubits via microwave frequency photons enables
long-distance qubit-qubit coupling and facilitates the realization of a
large-scale quantum processor. However, qubits based on electron spins in
semiconductor quantum dots have proven challenging to couple to microwave
photons. In this theoretical work we show that a sizable coupling for a single
electron spin is possible via spin-charge hybridization using a magnetic field
gradient in a silicon double quantum dot. Based on parameters already shown in
recent experiments, we predict optimal working points to achieve a coherent
spin-photon coupling, an essential ingredient for the generation of long-range
entanglement. Furthermore, we employ input-output theory to identify observable
signatures of spin-photon coupling in the cavity output field, which may
provide guidance to the experimental search for strong coupling in such
spin-photon systems and opens the way to cavity-based readout of the spin
qubit
