We study the energy level structure of the Tavis-Cumming model applied to an ensemble of independent magnetic spins $s=1/2$ coupled to a variable number of photons. Rabi splittings are calculated and their distribution is analyzed as a functin of photon number $n_{rm max}$ and spin system size $N$. A sharp transition in the distribution of the Rabi frequency is found at $n_{rm max}approx N$. The width of the Rabi frequency spectrum diverges as $sqrt{N}$ at this point. For increased number of photons $n_{rm max}>N$, the Rabi frequencies converge to a value proportional to $sqrt{n_{rm max}}$. This behavior is interpreted as analogous to the classical spin resonance mechanism where the photon is treated as a classical field and one resonance peak is expected. We also present experimental data demonstrating cooperative, magnetic strong coupling between a spin system and photons, measured at room temperature. This points towards quantum computing implementation with magnetic spins, using cavity quantum-electrodynamics techniques.
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