Light for the Nanoworld

Defects in thin molybdenum sulfide layers, generated by bombardment with helium ions, can serve as nano-light sources for quantum technologies. (Source: C. Hohmann, MCQST)

An international team headed up by Alexander Holleitner and Jonathan Finley, physicists at the Technical University of Munich TUM, has succeeded in placing light sources in atomically thin material layers with an accuracy of just a few nanometers. The new method allows for a multi­tude of applic­ations in quantum techno­logies, from quantum sensors and transistors in smartphones through to new encryption tech­nologies for data transmission. Previous circuits on chips rely on electrons as the information carriers. In the future, photons which transmit information at the speed of light will be able to take on this task in optical circuits. Quantum light sources, which are then connected with quantum fiber optic cables and detectors are needed as basic building blocks for such new chips.

“This constitutes a first key step towards optical quantum computers,” says Julian Klein. “Because for future applications the light sources must be coupled with photon circuits, waveguides for example, in order to make light-based quantum calcu­lations possible.” The critical point here is the exact and precisely control­lable placement of the light sources. It is possible to create quantum light sources in conventional three-dimen­sional materials such as diamond or silicon, but they cannot be precisely placed in these materials.

The physicists then used a layer of the semi­conductor molybdenum disulphide as the starting material, just three atoms thick. They irradiated this with a helium ion beam which they focused on a surface area of less than one nanometer. In order to generate optically active defects, the desired quantum light sources, molybdenum or sulfur atoms are precisely hammered out of the layer. The imper­fections are traps for excitons, electron-hole pairs, which then emit the desired photons. Techni­cally, the new helium ion microscope at the Walter Schottky Institute’s Center for Nano­technology and Nano­materials, which can be used to irradiate such material with an unparal­leled lateral resolution, was of central importance for this.

Together with theorists at TUM, the Max Planck Society, and the Uni­versity of Bremen, the team developed a model which also describes the energy states observed at the imper­fections in theory. In the future, the researchers also want to create more complex light source patterns, in lateral two-dimensional lattice structures for example, in order to thus also research multi-exciton phenomena or exotic material properties. This is the experi­mental gateway to a world which has long only been described in theory within the context of the Bose-Hubbard model which seeks to account for complex processes in solids.

And there may be progress not only in theory, but also with regard to possible tech­nological developments. Since the light sources always have the same underlying defect in the material, they are theo­retically indis­tinguishable. This allows for appli­cations which are based on the quantum-mechanical principle of ent­anglement. “It is possible to integrate our quantum light sources very elegantly into photon circuits,” says Klein. “Owing to the high sensitivity, for example, it is possible to build quantum sensors for smartphones and develop extremely secure encryption tech­nologies for data trans­mission.” (Source: TUM)

Reference: J. Klein et al.: Site-selectively generated photon emitters in monolayer MoS2 via local helium ion irradiation, Nat. Commun. 10, 2755 (2019); DOI: https://doi.org/10.1038/s41467-019-10632-z

Link: Walter Schottky Institute, Technical University Munich TUM, Garching, Germany

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