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Microscopic processes can also be very complex.
Quantum mechanics rules out the idea that the world of atoms has 'deterministic
chaos'. Among other reason for this, quantum mechanical systems
develop non-deterministically from many simultaneous initial states.
In quantum chaos research physicists are looking for similarities, in
the quantum world, to the deterministic chaos of the everyday world.
In this way, scientists at the Max Planck Institute of Quantum Optics
have been investigating chaos in quantum mechanical systems that would
be deterministically chaotic according to the rules of macroscopic
physics.
Scientists working with Gernot Stania and Herbert
Walther have now succeeded in finding the first experimental evidence
of quantum chaos in a system in which the components, during the
experiment, in principle can disperse in any direction. They harked
back to an historical experiment: demonstrating the photoelectric
effect by releasing electrons onto metal when light is projected on
them.
In the classical experiment, electric voltage is
created across two metal plates, one of them covered with an alkali
metal. The experimenter hits the alkali metal with light at a
particular frequency (and thus energy). As soon as the energy moves
above a certain amount, the light frees the electrons from the metal,
which is observable as electric current. Albert Einstein published his
explanation for this effect a hundred years ago, which was important
for the development of quantum theory and recognised with a Nobel
Prize in 1921.
The scientists from the Max Planck Institute of
Quantum Optics adapted the classical experiment to their needs. In the
modern version, the alkali metal is not applied to a metal plate, but
is replaced in the experimental setup by a flying beam of rubidium
atoms (compare with image 1). The atoms are then exposed to both an
electrical field and a strong magnetic field. As in the historical
experiment, the atoms are only hit with a light of a particular
frequency which is able to cause them to release electrons. This
electron beam is measured subject to the light frequency.
Between the magnetic field, the electric field, and
the electrostatic forces in the atom (the attraction of protons and
electrons), three different forces are acting on the electrons in the
rubidium atoms, each of which provokes very different electron
movements. As long as one of these forces outweighs the others, the
movement of the electrons is simple and not chaotic. That is the case,
for example, when the electron has not yet absorbed laser light and
finds itself near the atomic nucleus. However, in the moment in which
the electron takes up a light particle, it changes to a high energy
state and thus falls more under the influence of the external
electromagnetic field. Its movement then becomes chaotic. In the
process of this movement, the electron moves farther and farther from
the nucleus, until it is free.
The chaos in the movement is demonstrated through
the fact that the electron beam fluctuates in a particular way which
matches the energy of the light particles. These fluctuations are
called 'Ericson fluctuations'. The researchers were not only able to
demonstrate the Ericson fluctuations, they were also able to adjust
the initial state of the strength of the electric and magnetic field,
and thus how chaotically the system behaved, according to the rules of
macroscopic physics. In this way, they were able to show the
connection between deterministic chaos and the fluctuations of the
photocurrent. The more chaotically the system reacted, according to
the rules of macroscopic physics, the stronger the measured
fluctuations. |
