Multiple images of two objects located between two parallel mirrors illustrate the principle of electromagnetically induced transparency of atomic nuclei: The interaction of x-rays with two layers of iron within such a system of mirrors (an optical resonator) leads to a quantum mechanical superposition state of iron and its mirror images that causes the iron atomic nuclei to appear transparent. |
At
the high-brilliance synchrotron light source PETRA III, a team of DESY
scientists headed by Dr. Ralf Röhlsberger has succeeded in making atomic
nuclei transparent with the help of X-ray light. At the same time they
have also discovered a new way to realize an optically controlled light
switch that can be used to manipulate light with light, an important
ingredient for efficient future quantum computers.
The research results are presented in the current edition of the scientific journal Nature (DOI: 10.1038/nature10741).
The
effect of electromagnetically induced transparency (EIT) is well known
from laser physics. With intense laser light of a certain wavelength it
is possible to make a non-transparent material transparent for light of
another wavelength. This effect is generated by a complex interaction of
light with the atomic electron shell. At DESY’s X-ray source PETRA III,
the Helmholtz research team of Röhlsberger managed to prove for the
first time that this transparency effect also exists for X-ray light,
when the X-rays are directed towards atomic nuclei of the Mössbauer
isotope iron-57 (which makes up 2% of naturally occurring iron). Quite
remarkably, only very low light intensities are needed to observe this
effect, in contrast to standard EIT experiments.
How
does the experiment work? The scientists positioned two thin layers of
iron-57 atoms in an optical cavity, an arrangement of two parallel
platinum mirrors that reflect X-ray light multiple times. The two layers
of iron-57 atoms, each approximately three nanometres thick, are
precisely kept in position between the two platinum mirrors by carbon,
which is transparent for X-ray light of the wavelength used. This kind
of sandwich with a total thickness of only 50 nm is irradiated
under very shallow angles with an extremely thin X-ray beam from the
PETRA III synchrotron light source. Within this mirror system, the light
is reflected back and forth several times, generating a standing wave, a
so-called resonance. When the light wavelength and the distance between
both iron layers are just right in proportion, the scientists can see
that the iron becomes almost transparent for the X-ray light. In order
for this effect to occur, one iron layer must be located exactly in the
minimum (node) of the light resonance, the other one exactly in the
maximum. When the layers are shifted within the cavity, the system
immediately becomes non-transparent. The scientists attribute this
observation to a quantum-optical effect, caused by the interaction of
atoms in the iron layers. Unlike single atoms, the atoms in an optical
cavity together absorb and radiate in synchrony. In the geometry of this
experiment their oscillations mutually cancel each other, as a result
of which the system appears to be transparent. In contrast to previous
experiments in the optical regime, only few light quanta are necessary
to generate this effect.
“Our
result of achieving transparency of atomic nuclei is virtually the EIT
effect in the atomic nucleus,” Röhlsberger describes the experiments.
“Undoubtedly, there is still a long way to go until the first quantum
light computer becomes reality. However, with this effect, we are able
to perform a completely new class of quantum-optical experiments of
highest sensitivity. With the European XFEL X-ray laser, currently being
built in Hamburg, there is a real chance to control X-ray light with
X-ray light.”
This
experiment definitely means considerable technical progress for quantum
computing: apart from the basic possibility to make materials
transparent with light, the intensity of light is decisive for a future
technical realisation as well. Every additional quantum of light
produces additional waste heat; this would be reduced by the use of the
presently discovered effect.
For
the continuation of these experiments and the optimal utilisation of
the extremely small X-ray beam size of the highly brilliant X-ray source
PETRA III, a new coating facility will be installed at DESY for the
production and optimisation of these optical cavities.
The
experiments of the DESY scientists also showed another parallel to the
EIT effect: the light trapped in the optical cavity only travels with
the speed of a few metres per second—normally it is nearly 300,000 km
per second. With further experiments, the scientists will clarify how
slow the light really becomes under these circumstances, and whether it
is possible to use this effect scientifically. A possible application
and at the same time an important building block on the way to
light-quantum computers is, for example, the storage of information with
extremely slow or even stopped light pulses.
Electromagnetically induced transparency with resonant nuclei in a cavity
