Researchers from the Kavli Institute of NanoScience in Delft are the first
to have successfully captured a single electron in a highly tunable carbon nanotube
double quantum dot. This was made possible by a new approach for producing ultraclean
nanotubes. Moreover, the team of researchers, under the leadership of Spinoza
winner Leo Kouwenhoven, discovered a new sort of tunnelling as a result of which
electrons can fly straight through obstacles. The results of the research were
published by Nature Nanotechnology on 5 April 2009.
A quantum dot can be viewed as a small 'box' which traps a controllable number
of electrons. This box is coupled to one or more gate electrodes with which
the number of electrons on the dot can be varied. The researchers developed
a new technology to make extremely clean nanotube quantum dots. This makes it
possible to capture a single electron in a nanotube. Moreover, the researchers
succeeded in making the first highly-controllable single electron double dot.
Controlling quantum dots
One of the pipe dreams within quantum mechanics is the construction of a super-powerful
quantum computer. In order to do this, it must be possible to manipulate the
electron spin of the quantum dots. That would enable quantum information to
be stored and read again. However, up until now it has proved impossible to
accurately control double quantum dots in nanotubes (two quantum dots linked
together) that capture only a single electron.
The researchers used silicon electrodes positioned close to the ultraclean
nanotube to accurately control the number of electrons of the quantum dot. Three
electrodes were used in the research, although more electrodes can be incorporated.
The ultraclean tube ensures that no disruption occurs in the manipulation of
the electrons.
Tunnelling
Whilst studying the double quantum dot, the researchers discovered a new type
of tunnelling that is analogous to tunnelling according to the Klein paradox.
Tunnelling is an effect in which rapidly moving electrons can fly straight through
obstacles. The particle goes straight through a barrier even though it does
not have enough energy to go over the barrier. Normally tunnelling ceases as
soon as the barrier is too large. The famous Klein paradox predicts that if
the barrier is made even bigger still, tunnelling can once again take place
due to the influence of relativistic quantum mechanics.
In the case of normal tunnelling, electrons can only move from one quantum
dot to another due to the tunnel coupling of the wave functions on both sides
of the energy barrier within the double quantum dot. Researchers used the silicon
gate electrodes to manipulate the barrier and observed tunnelling could become
enhanced even though the barrier was increasing, as predicted in the Klein paradox.
This method of tunnelling emphasises the close relationship between the physics
of semiconductors, such as those in this research, and high-energy physics.
The research took place at the Kavli Institute for Nanoscience of Delft University
of Technology. The first author of the article in Nature Nanotechnology is Gary
Steele. Gary Steele, Georg Götz and Leo Kouwenhoven carried out the research
with the aid of a grant from the Foundation for Fundamental Research on Matter
(FOM) and NWO. Leo Kouwenhoven
received the NWO/Spinoza Award in 2007.
Posted May 14th, 2009
