As far as transistor size is concerned, it doesn't get any smaller than this. An international group of researchers from the Helsinki University of Technology, the University of New South Wales and the University of Melbourne have successfully built a fully working transistor that is just one atom in size, smashing previous records and, more importantly, creating a very unique venue to study phenomena to be exploited in the rapidly developing field of quantum computing.
The single-atom transistor dethrones the previous record holder, a one by ten atoms thick device manufactured by the University of Manchester using graphene. Another strong contender is Cornell University's single-atom transistor, which was developed back in 2002 but wasn't fully functional.
The work of the Australo-Finnish team relies on quantum tunneling, the phenomenon by which an object on the edge of a barrier could suddenly find itself past the barrier by mere matters of chance, even without owning the necessary energy to actually break through it. While the probability of this happening with everyday objects — say, a person suddenly finding him or herself on the other side of a wall — is next to zero, on the quantum scale this is a phenomenon that is not only common, but is also essential to life on Earth as it is what ultimately allows the Sun to shine.
As documented in the related paper published on the journal Nano Letters, the transistor developed by the team works by sequential quantum tunneling of single electrons between an atom of phosphorus and the source and drain leads of the transistor, which are made of silicon. The tunneling phenomenon can be suppressed or allowed by controlling the voltage on a nearby metal electrode which is a few tens of nanometers in size.
The catch here is that, while the core of the transistor is effectively just one atom in size, the complementary equipment, particularly the electrode, is very bulky (in atomic terms) and wouldn't let us pack a lot more transistors in an integrated circuit than we already can with current semiconductor technology.
However, as Dr. Möttönen explained, the team wasn't interested in building the tiniest transistor for a classical computer, but rather a better single-atom quantum bit which would be the heart of a quantum computer. In this, the team's discovery is very significant: all the electric current in the transistor passes through the same single atom, and this allows us to very effectively study the phenomena arising under such extreme size conditions.
For the first time, the researchers were able to observe "spin up" and "spin down" states — which will be translated into a "1" and a "0" respectively — for a single phosphorus atom. This is another crucial step towards the control of these states and, ultimately, the realization of a small and stable quantum bit that can be read, written and stored quickly and consistently.