An important step towards the quantum internet

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 (Image: Pixabay CC0)
(Image: Pixabay CC0)

The cornerstone for 1000-fold improvement in communication rates for bridging long distances.

Diamond is of great importance for future technologies such as the quantum internet. Special defect centers can be used as quantum bits (qubits) and emit single photons. To enable data transmission with practicable communication rates over long distances in the quantum network, all photons must be collected in optical fibers and transmitted without being lost. It must also be ensured that these photons all have the same color, i.e. the same frequency. This has been impossible until now.

Researchers in the research group ,,Integrated Quantum Photonics" led by Tim Schröder at Humboldt-Universität zu Berlin have succeeded for the first time worldwide in generating and detecting photons with stable photon frequencies emitted from quantum light sources, or, more precisely, from nitrogen defect centers in diamond nanostructures. This was made possible by a careful choice of diamond material, sophisticated nanofabrication methods carried out at the Joint Lab Diamond Nanophotonics of the Ferdinand Braun Institute, Leibniz Institute for Highest Frequency Technology, and special experimental control protocols. By combining the methods, the noise of the electrons, which previously disturbed data transmission, can be significantly reduced and the photons are emitted at a stable (communication) frequency.

In addition, the Berlin researchers show that, in perspective, the current communication rates between spatially separated quantum systems can be increased more than 1000-fold with the help of the developed methods, so that they have come an important step closer to a future quantum Internet.

The scientists have integrated individual qubits into optimized diamond nanostructures. These structures are 1000 times thinner than a human hair and make it possible to direct individual emitted light particles into optical fibers. However, during the fabrication of the nanostructures, the material surface is damaged at the atomic level and free electrons create uncontrollable noise for the generated light particles. Noise comparable to an unstable radio frequency causes fluctuations in the photon frequency and thus prevents successful quantum operations, such as entanglement.

A special feature in the diamond material used is that there are relatively many impurity atoms (nitrogen) in the crystal lattice. These may shield the quantum light source from spurious electrons at the surface of the nanostructure. "However, the exact physical processes need to be investigated in more detail in the future," explains Laura Orphal-Kobin, who conducts research on the quantum systems together with Tim Schröder. The conclusions drawn from the experimental observations are supported by statistical models and simulations, which Dr. Gregor Pieplow from the same research group is developing and implementing together with the experimenters.

Publication Optically Coherent Nitrogen-Vacancy Defect Centers in Diamond Nanostructures.

Laura Orphal-Kobin, Kilian Unterguggenberger, Tommaso Pregnolato, Natalia Kemf, Mathias Matalla, Ralph-Stephan Unger, Ina Ostermay, Gregor Pieplow, and Tim Schröder Physical Review X (2023) : 10.1103/PhysRevX.13.011042.