  Single-photon sources are subjected to a fundamental limitation in the speed
of operation dictated by the spontaneous emission rate of quantum emitters
(QEs). The current paradigm of the rate acceleration suggests coupling of a QE
to a metal nanostructure, in particular, a metal nanoparticle (MNP). Here, we
demonstrate that, in contrast to this approach, a MNP itself can behave as a
quantum emitter. We determine both the first- and second-order correlation
functions of light spontaneously emitted by a MNP strongly coupled to a QE and
show that this light should exhibit sub-Poissonian photon statistics and
perfect photon antibunching. This discovery opens a prospect to single-photon
sources with unprecedented generation rates up to 100 THz.
