  In a long-held preconception, photons play a central role in present-day
quantum technologies. But what are sources producing photons one by one good
for precisely? Well, in opposition to what many suggest, we show that
single-photon sources are not helpful for point to point quantum key
distribution because faint laser pulses do the job comfortably. However, there
is no doubt about the usefulness of sources producing single photons for future
quantum technologies. In particular, we show how single-photon sources could
become the seed of a revolution in the framework of quantum communication,
making the security of quantum key distribution device independent or extending
quantum communication over many hundreds of kilometers. Hopefully, these
promising applications will provide a guideline for researchers to develop more
and more efficient sources, producing narrowband, pure and indistinguishable
photons at appropriate wavelengths.
