Topological insulators are quantum materials, which, due to their exotic electronic structure, on surfaces and edges conduct electric current like metal, while acting as an insulator in bulk. Scientists from the Max-Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) have demonstrated for the first time how to tell apart topological materials from their regular—trivial—counterparts within a millionth of a billionth of a second by probing it with ultra-fast laser light. Their method could open the way for such materials to be used as logic elements in light-controlled electronics able to process information tens of thousands times faster as currently possible. Their study appeared in Nature Photonics.

The most common illustration of the topology concept involves an elastic pretzel, which can be stretched, bent, or twisted in any way; no matter the deformation, it is impossible to make a bagel out of a pretzel or add holes to it, without tearing it apart. The number of holes in a pretzel is thus invariant and provides topological information about the pretzel shape.

In a solid material, quantum-mechanical laws restrict which energies electrons can have, leading to the formation of bands with either allowed or forbidden energies. Using the concept of topology, physicists can describe complex shapes of allowed energy bands and assign them a specific topological number. A special topology of the band structure in a material system manifests itself in exotic properties that can be observed—such as the surface conductivity in topological insulators.