Alexander Holleitner and Jonathan Finley, physicists at the Technical University of Munich (TUM), with their team have 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.
»This constitutes a first key step towards optical quantum computers,« says Julian Klein, lead author of the study. »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 to place the light sources exactly and precisely controlled. 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 used a three-atom thick layer of molybdenum disulfide (MoS2) as their starting semiconductor material. 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 were precisely hammered out of the layer. The imperfections are traps for so-called 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.
On the Road to New Light Sources
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 generate more complex light source patterns, for example in lateral two-dimensional lattice structures of excitons, in order to investigate many-particle 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 so-called Bose-Hubbard model which seeks to account for complex processes in solids.
However, progress could be made not only in theory, but also with regard to possible technical developments. Since the light sources are always based on the same defect in the material, they are basically indistinguishable. This enables applications based on the quantum mechanical principle of entanglement.
»You can integrate our quantum light sources very elegantly into photonic circuits«, says Klein. »Because of their high sensitivity, quantum sensors could be built into smartphones, for example, and extremely secure encryption technologies developed for data transmission.«
J. Klein, et al., Site-selectively generated photon emitters in monolayer MoS2 via local helium ion irradiation, Nature Communications, 10, 2755 (2019) – DOI: 10.1038/s41467-019-10632-z