‘Shield Up!' - Light Defines Its Own Protected Path

Scientists at the University of Rostock have developed a new type of photonic circuit in which high-energy light beams can define their own path - and shield themselves from external interference. This discovery was recently published in the renowned journal “Science”.

“By nature, photons are difficult to tame,” explains Professor Alexander Szameit, physicist from the University of Rostock. With his group he carried out the groundbreaking experiments. “As soon as you have collected them in a defined place and at a defined point in time, they become independent again.” In fact, researchers have been looking for ways to direct light into desired paths for centuries: lenses and curved mirrors concentrate sunlight on a tiny focal point. Powerful lasers generate coherent beams and short, intense light pulses. And optical fiber optic cables transport the gigantic amounts of data from the Internet around the world. Still, light waves are amazingly sensitive. Just a little crack in a lens

Topological insulators are solids, the interior of which is impervious to electrons. At the same time, however, their surface conducts electrical currents completely unhindered. These amazing materials were first produced in 2007 by Laurens Molenkamp and his team at the University of Würzburg. Professor Szameit has long been fascinated by their photonic counterparts. "Since we first succeeded in realizing a topological insulator for light, we have been working on new ways to make these unique materials usable," the physicist remembers.

Photonic topological isolators guide light along precisely defined paths. The mathematical principles on which they are based ensure enormous resistance to manufacturing errors and external interference. But it is precisely these outstanding properties that are also a hurdle for their technological use: “Once light is trapped in a topological channel, it no longer experiences any scattering losses. This also means, however, that it can no longer be controlled from the outside without destroying the protection that has just been created, ”says co-author Dr. Matthias Heinrich, a member of Professor Szameit's team, gets to the heart of the problem.

The solution seems to be found quickly on paper: “In principle, it's actually quite simple. You only need a switch with which you can switch the topological properties of the system on or off between two successive light pulses, ”jokes Szameit. However, the topology of a system is directly related to the global spatial arrangement of its components, while the length of ultrashort laser pulses is measured in femtoseconds (the trillionth fraction of a millionth of a second) - many orders of magnitude beyond the reach of the fastest control electronics.

In close collaboration with theorists from Rostock, Barcelona, ​​Lisbon and Moscow, the young Rostock researchers developed a material in which the light pulses can decide whether they activate topological protection or whether they spread like a conventional material. "Depending on their peak intensity, optical pulses can behave fundamentally differently," explains Lukas Maczewsky, doctoral student and first author of the work. "The magic word is non-linearity: in photonics two plus two is sometimes significantly more than four."

Two years of intensive research and countless hours in the laboratories of the Rostock Institute for Physics have now been crowned with success. The non-linear topological insulator - a new type of synthetic material - allows light pulses above a certain power threshold to induce a short-lived topological domain in their direct environment. "Like the shields of the USS Enterprise, this self-generated cocoon follows the pulses on their way," is how the self-confessed Star Trek fan Szameit sums up the complex physics.

The result of this successful international cooperation represents a significant advance in basic research in the field of quantum optics and topological photonics. Even if there are still some hurdles to overcome before they become the building blocks of a functional optical quantum computer, there are hardly any limits to the imagination it is about innovative applications of the developed materials. Photonic data processing and light-based neural networks may sound like science fiction, but the rapid pace of scientific progress could soon make them a reality

The work was funded by the German Research Foundation (DFG) and the Alfried Krupp von Bohlen und Halbach Foundation.

The original publication in "Science" is available at DOI: 10.1126 / science.abd2033 .