  The particle-like nature of light becomes evident in the photon statistics of
fluorescence of single quantum systems as photon antibunching. In
multichromophoric systems, exciton diffusion and subsequent annihilation
occurs. These processes also yield photon antibunching but cannot be
interpreted reliably. Here, we develop picosecond time-resolved antibunching
(psTRAB) to identify and decode such processes. We use psTRAB to measure the
true number of chromophores on well-defined multichromophoric DNA-origami
structures, and precisely determine the distance-dependent rates of
annihilation between excitons. Further, psTRAB allows us to measure exciton
diffusion in mesoscopic H- and J-type conjugated-polymer aggregates. We
distinguish between one-dimensional intra-chain and three-dimensional
inter-chain exciton diffusion at different times after excitation and determine
the disorder-dependent diffusion lengths. Our method provides a new lens
through which excitons can be studied at the single-particle level, enabling
the rational design of improved excitonic probes such as ultra-bright
fluorescent nanoparticles, and materials for optoelectronic devices.
