Jonas Gutsche, Askan Zand, Marek Bültel, Artur Widera

• Individual quantum emitters form a fundamental building block for emerging quantum technologies. Collective effects of emitters, such as superradiance, might improve the performance of applications even further. When scaling materials to larger sizes, however, the optical density of states is modified by the surrounding material, and the collective coupling in small domains might be covered by transitions to bulk properties due to the presence of multiple collectively emitting domains, which inhomogeneously add. Here, we probe the optical properties of nitrogen vacancy centers in agglomerates of nanodiamonds. We quantify the transition from individual emitters to bulk emission by fluorescence lifetime measurements, and find a transition to occur on a length scale of \(\sim \!3\) wavelengths around the emitter. While our lifetime measurements are consistent with superradiant decay, the second-order correlation function, which is a standard measure to reveal collective properties, fails to probe collective effects for our case of an ensemble of collectively contributing domains to the emission. Therefore, we propose and apply a new measure to trace collective effects based on the intensity fluctuations of the emitted light. Our work points toward systematically studying collective effects in a scalable solid-state quantum system, and using them for quantum optical applications in agglomerates of highly-doped nanodiamonds.