Quantum Light Sources Realized Via Site-Selective Defect Engineering

When photons can be used to transmit information at the speed of light, quantum light sources, connected with quantum fiber optic cables and detectors, will be a requirement. An international team led by physicists at the Technical University of Munich (TUM) has taken a step toward providing light sources for quantum technologies, by creating quantum light sources in atomically thin material layers and positioning them within the material with nanometer-scale accuracy.

The ability to precisely control the placement of the light sources is critical to their value, the researchers said. While it is possible to create quantum light sources in conventional 3D materials such as diamond or silicon, the light sources cannot be precisely placed within these materials.

The TUM physicists irradiated a monolayer of the semiconductor molybdenum disulfide MoS₂ using a sub-nm focused helium ion beam to deterministically create defects. They irradiated the MoS₂ layer, which was three atoms thick, in a highly controlled manner, focusing the beam on a surface area of less than 1 nm.

To generate optically active defects — their desired quantum light source — the researchers precisely hammered molybdenum or sulfur atoms out of the semiconductor layer. Subsequent encapsulation of the ion-exposed MoS₂ flake with high-quality hexagonal boron nitride (hBN) revealed emission lines that produced photons in the visible spectral range.

By bombarding thin molybdenum sulfide layers with helium ions, physicists at the Technical University of Munich succeeded in placing light sources in atomically thin material layers with an accuracy of a few nanometers. The new method allows for a multitude of applications in quantum technologies. Courtesy of Christoph Hohmann/MCQST.

Based on their calculations the researchers surmised that these emission lines came from the recombination of highly localized electron-hole complexes at defect states generated by exposure to the helium ion. These defects served as traps for electron-hole pairs, which then emitted the desired light.

Researchers at TUM, the Max Planck Society, and the University of Bremen together developed a model to theoretically describe the energy states observed at the defect locations.

Since the light sources always had the same underlying defect in the material, they were theoretically indistinguishable. This would allow for applications based on the quantum-mechanical principle of entanglement, the researchers hypothesized.

“It is possible to integrate our quantum light sources very elegantly into photon circuits,” researcher Julian Klein said.

Looking ahead, the researchers want to create more complex light source patterns, in lateral 2D lattice structures, for example, in order to research multiexciton phenomena or exotic material properties.

“This constitutes a first key step toward optical quantum computers,” Klein said. “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 research was published in Nature Communications (https://doi.org/10.1038/s41467-019-10632-z).