Well maybe not smaller than one atom, but how about seven atoms?

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An image of the template of the quantum dot device showing a central hole where seven phosphorus atoms are incorporated. Running diagonally from top left to bottom right are the two electronic leads to connect to the dot.
(PhysOrg.com) -- Scientists have literally taken a leap into a new era of computing power by making the world's smallest precision-built transistor - a "quantum dot" of just seven atoms in a single silicon crystal. Despite its incredibly tiny size - a mere four billionths of a metre long - the quantum dot is a functioning electronic device, the world's first created deliberately by placing individual atoms.
http://www.physorg.com/news193896845.html