Physicists of the University of Bonn have taken
one more important hurdle on the path to what is
known as a quantum computer: by using 'laser tweezers'
they have succeeded in sorting up to seven atoms
and lining them up. The researchers filmed this
process. After the end of the embargo the result
can be seen on the University of Bonn's homepage
( www.uni-bonn.de ).
They report on their breakthrough in the next issue
of the prestigious journal Nature (13th July 2006).
the experiment the research team headed by Dr.
Arno Rauschenbeutel and Professor Dieter Meschede
decelerated several caesium atoms for a period
of several seconds so that they were hardly moving,
then loaded them onto a 'conveyor belt' consisting
of lasers. This conveyor belt is made up of a
standing light wave composed of many peaks and
troughs – possibly
comparable to a piece of corrugated iron. 'Unfortunately
it cannot be predicted which trough precisely the
atoms will land in,' Arno Rauschenbeutel explains.
'It's rather like pouring several eggs from a big
dish into an egg carton – which section each egg
rolls into is a matter of chance.'
However, anyone wishing to calculate with atoms
must be able to place them exactly. 'All the atoms
on the conveyor belt have to have the same distance
from each other,' is how Arno Rauschenbeutel sketches
the challenge. 'Only then can we get them to interact
in a controlled way in what is called a quantum
gate.' By lining up gate operations like these it
would already be possible to carry out simple quantum
Bonn physicists therefore subsequently 'sorted'
the atoms in their experiment on the conveyor
belt. They did this by first taking a photo to
record their positions. They next set the conveyor
belt moving – and with it the caesium atoms 'trapped'
in the troughs. In this way they transported the
wrongly placed atoms to their 'laser tweezers' – this
is basically nothing more than another conveyor
belt consisting of laser beams which is oriented
orthogonally to the first conveyor belt. 'When we
set the tweezers' light wave in motion, we can lift
the wrongly placed atoms off the conveyor belt,'
Arno Rauschenbeutel explains. 'Then we move the
conveyor belt to the desired position and simply
pop the atom back in.'
The film shows how well this works: the tweezers
select two atoms consecutively from the belt and
put them back on again in such a way that they are
exactly the same distance from each other and from
a third atom. 'Sorting seven atoms in this way takes
us about two seconds,' Dr. Rauschenbeutel says.
The next aim of the Bonn physicists is to construct
a quantum gate. For this purpose they want to 'write'
quantum information onto two caesium atoms and then
place them between two tiny mirrors. The intention
is that they should interact there with each other,
i.e. exchange information by emitting and absorbing
fluorescent light. If this is successful, it will
be the next milestone for the Bonn researchers on
their way to the quantum computer.
atom-sorting machine. Yevhen Miroshnychenko, Wolfgang
Alt, Igor Dotsenko, Leonid Förster,
Mkrtych Khudaverdyan, Dieter
Meschede, Dominik Schrader, Arno Rauschenbeutel. Nature, 13th July 2006 (Vol.
442, No 7099)