A technique that makes atomic nuclei transparent to light at certain wavelengths was extended to the x-ray regime and could have important implications in the fields of lasing and quantum information processing. At the high-brilliance synchrotron radiation 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 discovered a new way to realize an optically controlled light switch that can use light to manipulate light –an important mechanism for future efficient quantum computers. 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 DESY’s x-ray source PETRA III, the Helmholtz research team led by 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). They 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. (Image: Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany) The researchers 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 the element carbon, which is transparent for the x-ray wavelength used. With a thickness totaling 50 nm, the layers were irradiated under very shallow angles with an extremely thin x-ray beam from the PETRA III synchrotron. 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 right 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, 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 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 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,” he said. 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. The DESY experiments also unearthed another parallel to the EIT effect: The light trapped in the optical cavity travels with the speed of only a few m/s — 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-quantum computers is, for example, the storage of information with extremely slow or even stopped light pulses. The research appeared in Nature. For more information, visit: www.desy.de