  The atom sets an ultimate scaling limit to Moores law in the electronics
industry. And while electronics research already explores atomic scales
devices, photonics research still deals with devices at the micrometer scale.
Here we demonstrate that photonic scaling-similar to electronics-is only
limited by the atom. More precisely, we introduce an electrically controlled
single atom plasmonic switch. The switch allows for fast and reproducible
switching by means of the relocation of an individual or at most -- a few atoms
in a plasmonic cavity. Depending on the location of the atom either of two
distinct plasmonic cavity resonance states are supported. Experimental results
show reversible digital optical switching with an extinction ration of 10 dB
and operation at room temperature with femtojoule (fJ) power consumption for a
single switch operation. This demonstration of a CMOS compatible, integrated
quantum device allowing to control photons at the single-atom level opens
intriguing perspectives for a fully integrated and highly scalable chip
platform -- a platform where optics, electronics and memory may be controlled
at the single-atom level.
