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Illustration of the principle of optical resonance, where thin sheets of iron can be made transparent using X-ray lasers.
Transparency is generally a property of a material's density or crystal structure, and varies depending on the wavelength of light. However, transparency can also be achieved by exploiting quantum interference between energy level transitions in atoms. Up until now, such transparency has been confined to optical wavelengths, due to the typical energies of atomic transitions.
Transitioning between energy levels within atomic nuclei (instead of electron transitions) involves much higher energies, corresponding to hard X-ray frequencies. Ralf Röhlsberger, Hans-Christian Wille, Kai Schlage, and Balaram Sahoo of the Deutsches Elektronen-Synchrotron (DESY) in Germany have induced transparency in iron-57 nuclei, using an X-ray laser to drive the nuclei to resonance. The experiment not only made the iron nuclei nearly vanish, but also slowed the X-ray photons to a small fraction of their usual speed.
This result holds out the tantalizing possibility of quantum optics in the nuclear regime, providing us new ways of manipulating light at far higher energies than have previously been possible. The basic technique is termed electromagnetically induced transparency (EIT). It involves balancing the absorption of light by an atom or nucleus with a corresponding emission, which makes it appear as though the material is nearly absent.
Much as a plucked string has places of total constructive interference (antinodes) and total destructive interference (nodes), the standing light wave will have points where the the photons reinforce each other or cancel each other out. By arranging materials at these points of interference, the transition between energy levels can be managed in such a way to ensure that the emission and absorption of photons are balanced.
The iron itself is of the relatively rare isotope 57Fe, which has a special nuclear transition that makes it suitable for this work. The cooperation between the layers is also critical; one must be precisely at the node of the standing wave of laser light, and the other at the antinode. Shifting or removing one layer even slightly makes the effect disappear, ruining the transparency. Airplanes and adolescent wizards will not be made to vanish by this technique, in other words.