  An optical nanoantenna and adjacent atomic systems are strongly coupled when
an excitation is repeatedly exchanged between these subsystems prior to its
eventual dissipation into the environment. It remains challenging to reach the
strong coupling regime but it is equally rewarding. Once being achieved,
promising applications as signal processing at the nanoscale and at the single
photon level would immediately come into reach. Here, we study such hybrid
configuration from different perspectives. The configuration we consider
consists of two identical atomic systems, described in a two-level
approximation, which are strongly coupled to an optical nanoantenna. First, we
investigate when this hybrid system requires a fully quantum description and
provide a simple analytical criterion. Second, a design for a nanoantenna is
presented that enables the strong coupling regime. Besides a vivid time
evolution, the strong coupling is documented in experimentally accessible
quantities, such as the extinction spectra. The latter are shown to be strongly
modified if the hybrid system is weakly driven and operates in the quantum
regime. We find that the extinction spectra depend sensitively on the number of
atomic systems coupled to the nanoantenna.
