  When an ensemble of quantum emitters couples to a common radiation field,
their polarizations can synchronize and a collective emission termed
superfluorescence can occur. Entering this regime in a free-space setting
requires a large number of emitters with a high spatial density as well as
coherent optical transitions with small inhomogeneity. Here we show that by
coupling nitrogen-vacancy (NV) centers in a diamond membrane to a high-finesse
microcavity, also few, incoherent, inhomogeneous, and spatially separated
emitters - as are typical for solid state systems - can enter the regime of
collective emission. We observe a super-linear power dependence of the emission
rate as a hallmark of collective emission. Furthermore, we find simultaneous
photon bunching and antibunching on different timescales in the second-order
auto-correlation function, revealing cavity-induced interference in the
quantized emission from about fifteen emitters. We develop theoretical models
for mesoscopic emitter numbers to analyze the behavior in the Dicke state basis
and find that the population of collective states together with cavity
enhancement and filtering can explain the observations. Such a system has
prospects for the generation of multi-photon quantum states, and for the
preparation of entanglement in few-emitter systems.
