By Matthew Francis
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.
In practice, EIT is created in optical resonant cavities, in which laser light is reflected back and forth until it creates a standing wave. 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.
Nucleons (protons and neutrons) within an atomic nucleus have energy levels available, just as electrons do within the larger atom. Because of the much stronger nuclear forces involved, the nucleus is a tightly confined space. That has two consequences: the nucleons need a lot of energy to move to a new level, and those energy levels are widely separated. Unlike electrons, where the energies are in the neighborhood of visible wavelengths, nuclear transitions correspond to X-ray energies.
In the case of nuclear EIT, two very thin sheets layers of iron (which normally absorbs X-rays) are placed between two platinum mirrors, all within a cavity. The iron is fixed in place by a layer of carbon, which is already transparent to some wavelengths of X-ray light. One iron sheet is placed at a node and the second at an antinode within the cavity; this particular arrangement creates the conditions necessary to drive the emission/absorption pattern needed to make the iron vanish. The X-rays behave as if the two sheets aren’t there.
(As a bonus, the intensity of light needed for this to happen is far lower than what’s usually needed for atomic EIT, a boon given the energies needed to drive X-ray lasers.)
As with similar atomic EIT, the speed of the X-ray photons is slowed drastically by all the absorption and emission. It ends up around 30 meters per second (as opposed to the vacuum light speed of 300 million meters per second)
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.
Though exciting, the transparency itself is only one aspect of many, as there are many potential applications and experimental possibilities. By exploiting the resonance of the transition between energy levels in the nucleus, the researchers have achieved something new, opening up the possibility of manipulating X-ray light in the same way optical light is in standard quantum optics.