  We report the observation of room-temperature superradiance from single,
highly luminescent diamond nanocrystals with spatial dimensions much smaller
than the wavelength of light, and each containing a large number (~10^3) of
embedded nitrogen-vacancy (NV) centres. After excitation of the nanodiamonds
with an off-resonant, green laser pulse, we observe i) ultrafast radiative
lifetimes down to ~ 1 ns, and ii) super-Poissonian photon bunching in the
autocorrelation function of the light emitted from the fastest nanodiamonds. We
explain our findings with a detailed theoretical model based on collective
Dicke states and well-known properties of NV centres. Using a minimal set of
fit parameters, the model captures both the wide range of different lifetimes
and the nontrivial photon correlations found in the experiments. The results
pave the way towards a systematic study of superradiance in a well controlled,
solid-state quantum system at room temperature. Ultimately, quantum engineering
of superradiance in diamond has the potential for advancing applications in
quantum sensing, energy harvesting, and efficient photon detection.
