Research Category: Conveyor System
Sponsored Links:

Sponsored Link:


Researchers: Dr. Artur Widera
Location: Institut für Angewandte Physik,Sekretariat Zimmer 307 ,53115 Bonn, Germany
Research Details
Let's assume we carried out the following experiment: we put a coin in the
hand of a test person. We'll simply call this person Hans. Hans's task is now to
toss the coin several times. Whenever the coin turns up 'heads', his task is to
take a step to the right. By contrast, if it turns up 'tails', he takes a step
to the left. After 10 throws we look where Hans is standing. Probably he won't
have moved too far from his initial position, as 'heads' and 'tails' turn up
more or less equally often. In order to walk 10 paces to the right, Hans would
have to get 10 'heads' successively. And that tends not happen that often.
Now, we assume that Hans is a very patient person. He is so patient that he
does this experiment 1000 times successively. After each go, we record his
position. When at the end we display this result as a graph, we get a typical
bell curve. Hans very often ends up somewhere close to his starting positions
after 10 throws. By contrast, we seldom find him far to the left or right.
The experiment is called a 'random walk'. The phenomenon can be found in many
areas of modern science, e.g. as Brownian motion. In the world of quantum
physics, there is an analogy with intriguing new properties, the 'quantum walk'.
Up to now, this was a more or less a theoretical construct, but physicists at
the University of Bonn have now actually carried out this kind of 'quantum
walk'.
A single caesium atom held in a kind of tweezers composed of laser beams
served as a random walker and coin at the same time. Atoms can adopt different
quantum mechanical states, similar to head and tails of a coin facing upwards.
Yet at the microcosmic level everything is a little more complicated. This is
because quantum particles can exist in a superposition of different states.
Basically, in that case 'a bit of heads' and 'a bit of tails' are facing
upwards. Physicists also call this superposition.
Using two conveyor belts made of laser beams, the Bonn physicists pulled
their caesium atom in two opposite directions, the 'heads' part to the right,
the 'tails' part to the left. 'This way we were able to move both states apart
by fractions of a thousandth of a millimetre,' Dr. Artur Widera from the Bonn
Institute of Applied Physics explains. After that, the scientists 'threw the
dice once more' and put each of both components into a superposition of heads
and tails again.
After several steps of this 'quantum walk' a caesium atom like this that has
been stretched apart is basically everywhere. Only when you measure its position
does it 'decide' at which position of the 'catwalk' it wants to turn up. The
probability of its position is predominantly determined by a second effect of
quantum mechanics. This is due to two parts of the atom being able to reinforce
themselves or annihilate themselves. As in the case of light physicists call
this interference.
As in the example of Hans the coin thrower, you can now carry out this
'quantum walk' many times. You then also get a curve which reflects the atom's
probability of presence. And that is precisely what the physicists from Bonn
measured. 'Our curve is clearly different from the results obtained in classical
random walks. It does not have its maximum at the centre, but at the edges,'
Artur Widera's colleague Michal Karski points out. 'This is exactly what we
expect from theoretical considerations and what makes the quantum walk so
attractive for applications.' For comparison the scientists destroyed the
quantum mechanical superposition after every single 'throw of the coin'. Then
the 'quantum walk' becomes a 'random walk', and the caesium atom behaves like
Hans. 'And that is exactly the effect we see,' Michal Karski says.
Professor Dieter Meschede's group has been working on the development of
socalled quantum computers now for many years. With the 'quantum walk' the team
has now achieved a further seminal step on this path. 'With the effect we have
demonstrated, entirely new algorithms can be implemented,' Artur Widera
explains. Search processes are one example. Today, if you want to trace a single
one in a row of zeros, you have to check all the digits individually. The time
taken therefore increases linearly with the number of digits. By contrast, using
the 'quantum walk' algorithm the random walker can search in many different
places simultaneously. The search for the proverbial needle in a haystack would
thus be greatly speeded up.
Contact persons
Team
The realization of a controlled lightmatter coupling on the singleparticle
level, i.e. single atoms and single photons, opens unique prospects.
Furthermore, it strikes fundamental questions in quantum mechanics, e.g. the
quantum mechanical measurement process and the inevitable influence of the
"observer". An intriguing property of a coupled quantum mechanical system is the
formation of nonclassical correlations between the components of the system. It
has been demonstrated that these quantum mechanical correlations, such as
entanglement,
can be used to solve certain problems of information processing considerably
faster than on any "classical" computer. The most important "quantum algorithms"
are related to the
factorization of huge numbers and
searching databases.
The experiment
In our experiment we couple single neutral cesium atoms to the field of a
high finesse optical resonator. Single photons can be stored for a certain time
between the mirrors of our resonator: A single photon is reflected 300,000 times
on avarage before it gets lost! Moreover, the confinement of the electric field
to a small volume results in a high atomcavity coupling strength, i.e. the rate
of energy exchange between atoms and the cavity field.
A possible goal is to couple several atoms via the cavity field and to create
correlated (e.g. entangled) atomic states.
Check for full research
http://agmeschede.iap.unibonn.de
