Now, the world's smallest transistor!

In a major breakthrough which may pave the way for superfast computers, scientists claim to have created the world's smallest transistor from a 'quantum dot' of just seven atoms.

An international team has claimed that despite its incredibly tiny size -- a mere four billionth of a metre long -- the quantum dot is a functioning electronic device, the world's first created deliberately by placing individual atom.

According to the scientists, it can be used to regulate and control electrical current flow like a commercial transistor but it represents a key step into a new age of atomic-scale miniaturisation and superfast computers.

"The significance of this achievement is that we are not just moving atoms around or looking at them through a microscope. We are manipulating individual atoms and placing them with atomic precision, in order to make a working electronic device.

"The team has been able to fabricate an electronic device entirely out of crystalline silicon where we have replaced just seven individual silicon atoms with phosphorus atoms. That is amazing exactness.

"This is a huge technological achievement and it is a critical step to demonstrating that it is possible to build the ultimate computer -- a quantum computer in silicon," team member Prof Michelle Simmons of University of New South Wales said.

The technology for placing individual atoms on a surface, the scanning tunnelling microscope, has existed for two decades.

But until now nobody has been able to use it to make atomic-precision devices capable of processing electronic inputs from the macroscopic world, say the scientists.

"We are testing the limits of how small an electronic device can be. Now we have just demonstrated the world's first electronic device in silicon systematically created on the scale of individual atoms," Prof Simmons said.

The findings have been published in the latest edition of the Nature Nanotechnology journal.