  We investigate spin states of few electrons in a double quantum dot by
coupling them weakly to a magnetic field resilient NbTiN microwave resonator.
We observe a reduced resonator transmission if resonator photons and spin
singlet states interact. This response vanishes in a magnetic field once the
quantum dot ground state changes from a spin singlet into a spin triplet state.
Based on this observation, we map the two-electron singlet-triplet crossover by
resonant spectroscopy. By measuring the resonator only, we observe Pauli spin
blockade known from transport experiments at finite source-drain bias and
detect an unconventional spin blockade triggered by the absorption of resonator
photons.
