  Tremendous progress in few-qubit quantum processing has been achieved lately
using superconducting resonators coupled to gate voltage defined quantum dots.
While the strong coupling regime has been demonstrated recently for odd charge
parity flopping mode spin qubits, first attempts towards coupling a resonator
to even charge parity singlet-triplet spin qubits have resulted only in weak
spin-photon coupling strengths. Here, we integrate a zincblende InAs nanowire
double quantum dot with strong spin-orbit interaction in a magnetic-field
resilient, high-quality resonator. In contrast to conventional strategies, the
quantum confinement is achieved using deterministically grown wurtzite tunnel
barriers without resorting to electrical gating. Our experiments on even charge
parity states and at large magnetic fields, allow to identify the relevant spin
states and to measure the spin decoherence rates and spin-photon coupling
strengths. Most importantly, we find an anti-crossing between the resonator
mode in the single photon limit and a singlet-triplet qubit with an electron
spin-photon coupling strength of $g/2\pi=139\pm4$ MHz. Combined with the
resonator decay rate $\kappa/2\pi=19.8\pm0.2$ MHz and the qubit dephasing rate
$\gamma/2\pi=116\pm7$ MHz, our system achieves the strong coupling regime in
which the coherent coupling exceeds qubit and resonator linewidth. These
results pave the way towards large-scale quantum system based on
singlet-triplet qubits.
