  We report the observation of a mechanism of maser generation in an ensemble
of inter-coupled, inhomogeneously broadened two-level systems, enhanced by high
quality factor electromagnetic cavity modes. In this previously unobserved form
of population inversion, an inseparable quantum system leads to cavity-enhanced
stimulated emission arising from interactions within an ensemble of two-level
systems, as opposed to a traditional ensemble of noninteracting identical three
level systems. The effect is observed in a cryogenically cooled whispering
gallery mode sapphire resonator containing dilute Fe$^{3+}$ impurity ions.
These ions exhibit strong spin-lattice interaction, leading to both electron
spin resonance broadening and phonon mediated spin-spin coupling. The maser
effect is due to a $\left|1/2\right\rangle \rightarrow \left|3/2\right\rangle$
energy transition in electron spin angular momentum observed at zero external
magnetic field. Both continuous and oscillating regimes are observed with
corresponding thresholds both in detuning frequency and incident power.
