  Their intrinsic properties render single quantum systems as ideal tools for
quantum enhanced sensing and microscopy. As an additional benefit, their size
is typically on an atomic scale which enables sensing with very high spatial
resolution. Here, we report on utilizing a single nitrogen vacancy center in
nanodiamond for performing three-dimensional scanning-probe fluorescence
lifetime imaging microscopy. By measuring changes of the single emitter's
lifetime information on the local density of optical states is acquired at the
nanoscale. This technique to gather information on the local density of optical
states is important for the understanding of fundamental quantum optical
processes as well as for the engineering of novel photonic and plasmonic
devices.
