Two new discoveries – extending a technique that makes atomic nuclei transparent to light at certain wavelengths up to the x-ray range, and using light to manipulate light via an optically controlled switch – could have important implications in the fields of lasing and quantum information processing. Electromagnetically induced transparency (EIT) is a quantum optical effect in which the interaction of light with matter can render an opaque medium transparent for light of a particular wavelength. This effect is generated by a complex interaction between light and the atomic electron shell. At PETRA III, the x-ray source at the German electron synchrotron DESY, a team headed by Dr. Ralf Röhlsberger proved for the first time that this transparency effect exists also for x-ray light when the x-rays are directed toward atomic nuclei of the Mössbauer isotope iron-57 (which makes up 2 percent of naturally occurring iron). The researchers observed that, in contrast to standard EIT experiments, only very low light intensities were needed to observe this effect. 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. Courtesy of DESY in Hamburg, Germany. They positioned two thin layers of iron-57 atoms, each approximately 3 nm thick, in an optical cavity, where they were kept precisely in position between the two platinum mirrors by carbon, which is transparent for the x-ray wavelength used. With a thickness of 50 nm, the layers were irradiated under very shallow angles with an extremely thin x-ray beam from PETRA III. Within the mirror system, light was reflected back and forth several times, generating a resonance. When the light wavelength and the distance between the two iron layers were aligned in proportion, the scientists could see that the iron became almost transparent to the x-ray light. For this to occur, one iron layer must be located exactly in the minimum (node) of the light resonance, and the other one exactly in the maximum. When the layers are shifted within the cavity, the system immediately becomes nontransparent. The scientists attributed 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 canceled each other; therefore, the system appeared to be transparent. In contrast to previous experiments in the optical regime, only a few light quanta are needed to generate this effect. “Our result of achieving transparency of atomic nuclei is virtually the EIT effect in the atomic nucleus,” Röhlsberger said. “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. “The European XFEL x-ray laser, which is currently being built in Hamburg, could give scientists the possibility to control x-ray light with x-ray light.” This experiment made considerable strides in technical progress for quantum computing. Apart from the basic possibility of making materials transparent with light, the intensity of light is decisive for a future technical realization as well. Every additional quantum of light produces additional waste heat; this would be reduced by the use of the discovered effect. A new coating facility will be installed at DESY to produce and optimize the optical cavities necessary for the experiments. “Quantum optical concepts and concepts of nonlinear optics are taking a slow pace with x-rays,” Röhlsberger said, attributing this to the low photon degeneracy of x-ray sources, as they are just thermal sources. “Except for the x-ray lasers coming online now,” he added. “And the interaction of x-ray with matter is very weak and is masked also by competing nonradiative processes like photoabsorption, Auger processes, etc. “For these reasons, it has been hard to establish quantum optical concepts in the x-ray regime. This may change soon in light of the upcoming x-ray lasers like LCLS or the European XFEL or SACLA in Japan. And, as we have shown, nuclear resonance is an almost ideal system to work with in this field.” The research appeared in the Feb. 9 issue of Nature (doi: 10.1038/nature 10741þ). “Although this result is unlikely to have an immediate application, new capabilities are expected to emerge from a detailed quantum-level control of x-rays,” wrote Bernhard W. Adams of the Argonne National Laboratory, who was not involved in the research, in a commentary piece in that issue. “Among these are types of spectroscopy to probe chemical dynamics or a drastic reduction in the radiation dose required for biological x-ray applications.” The experiments also unearthed another parallel to the EIT effect: The light trapped in the optical cavity travels with the speed of only a few meters per second – normally, it is nearly 300,000 km/s. With further experiments, the scientists hope to clarify how slow the light really becomes under these circumstances, and whether it is possible to use this effect scientifically. A possible application and an important building block on the way to light-based quantum computers is, for example, the storage of information with extremely slow or even stopped light pulses. “We want to study the propagation of x-ray pulses under EIT conditions in a cavity,” Röhlsberger said. “This is a largely unexplored field, and we expect many interesting findings. “Another step in our research is to establish control schemes that allow for fast switching on/off of the transparency. In our case, we have no external laser to switch the transparency, because this job is taken by the vacuum field of the static cavity. Instead, we plan to do this by application of external magnetic fields and use the high polarization sensitivity of the nuclear resonance if the iron-57 atoms are embedded in a ferromagnetic environment.”