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Quantum light sources pave the way for optical circuits


Light for the Nano World

By bombarding thin layers of molybdenum sulfide with helium ions, physicists at the Technical University of Munich (TUM) were able to place light sources in atomically thin layers with an accuracy of just a few nanometers. The new method allows for multiple applications in quantum technology. Credit: Christoph Homan / MCQST

An international team, led by Alexander Holleitner and Jonathan Finley, physicists at the Munich Technical University (TUM), was able to place light sources in atomically thin layers to within just a few nanometers. The new method enables multiple applications in quantum technology, from quantum sensors and transistors to smartphones to new encryption technologies for data transmission.

Previous chip schemes have relied on electrons as information carriers. In the future, photons that 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 to quantum optical cables and detectors, are needed as the basic building blocks for such new chips.

An international team, led by TUM physicists Alexander Holtiner and Jonathan Finley, was able to create such quantum light sources in atomically thin layers of material and place them with nanometer accuracy.

A first step towards optical quantum computers

"This is the first key step towards optical quantum computers," says Julian Klein, lead author of the study. "Because for future applications, light sources must be combined with photonic circuits, waveguides, for example, to make possible quantum light-based calculations."

The critical point here is the precise and precisely controlled positioning of light sources. It is possible to create quantum light sources in conventional three-dimensional materials such as diamond or silicon, but they cannot be accurately placed in these materials.

Defective defects

Physicists then used a layer of semiconductor molybdenum disulfide (MoS2) as starting material, only three atoms thick. They irradiate this with a helium ion beam that focuses on a surface smaller than one nanometer.

In order to generate optically active defects, the desired quantum light sources, molybdenum or sulfur atoms are precisely impacted by the layer. Imperfections are traps for the so-called excitons, pairs of electron holes that then emit the desired photons.

The technically new helium ion microscope at the Walter Schottky Institute for Nanotechnology and Nanomaterials, which can be used to irradiate such material with incomparable lateral resolution, was central to this.

On the way to new light sources

Together with the theorists at TUM, the Max Planck Society and the University of Bremen, the team developed a model that also describes the energy states observed in theoretical imperfections.

In the future, researchers also want to create more sophisticated models of light sources, such as in lateral two-dimensional lattice structures, in order to investigate phenomena with many exciton or exotic material properties.

This is an experimental portal to the world, long ago only described in theory in the context of the so-called Bose-Hubbard model, which seeks to account for complex processes in solids.

Quantum sensors, transistors and secure encryption

And there may be progress not only in theory but also in terms of possible technological developments. Because light sources always have the same fundamental defect in the material, they are theoretically indistinguishable. This allows applications based on the quantum-mechanical entanglement principle.

"It is possible to integrate our quantum light sources very elegantly into photonic circuits," says Klein. "Thanks to the high sensitivity, for example, it is possible to create quantum sensors for smartphones and to develop extremely secure encryption technologies for data transmission."

Localized excitons in two-dimensional materials for integrated quantum optics

More information:
J. Klein et al., Photon-Emitter Selectively Generated in Monolayer MoS2 by local irradiation with helium ions, Natural communications (2019). DOI: 10.1038 / s41467-019-10632-z

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