  Single photon sources, which are compatible with quantum memories are an
important component of quantum networks. In this article, we show optical
investigations on isolated GaAs/Al$_{0.25}$Ga$_{0.75}$As quantum dots grown via
droplet epitaxy, which emit single photons on resonance with the Rb-87-D$_2$
line (780 nm). Under continuous wave resonant excitation conditions, we observe
bright, clean and narrowband resonance fluorescence emission from such a
droplet quantum dot. Furthermore, the second-order correlation measurement
clearly demonstrates the single photon emission from this resonantly driven
transition. Spectrally resolved resonance fluorescence of a similar quantum dot
yields a linewidth as narrow as 660 MHz ($ 2.7~\mu eV $), which corresponds to
a coherence time of 0.482 ns. The observed linewidth is the smallest reported
so far for strain free GaAs quantum dots grown via the droplet method. We
believe that this single photon source can be a prime candidate for
applications in optical quantum networks.
