Diamond material is of great importance for future techno­logies such as the quantum internet. Special defect centers can be used as quantum bits and emit single light particles that are referred to as single photons. To enable data trans­mission with feasible communi­cation rates over long distances in a 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 frequency. Ful­filling these require­ments has been impossible until now.

Researchers in the “Inte­grated Quantum Photonics” group 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-vacancy defect centers in diamond nanostructures. This was enabled by carefully choosing the diamond material, sophis­ticated nano­fabrication methods carried out at the Joint Lab Diamond Nanophotonics of the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, and specific experimental control protocols. By combining these 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 researchers show that the current communication rates between spatially separated quantum systems can prospec­tively be increased more than 1000-fold with the help of the developed methods – an important step closer to a future quantum internet. The scientists have integrated individual qubits into optimized diamond nano­structures. These structures make it possible to transfer emitted photons in a directed manner into glass 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, preventing successful quantum operations such as entanglement.

A special feature of the diamond material used is its relatively high density of nitrogen impurity atoms in the crystal lattice. These possibly shield the quantum light source from electron noise at the surface of the nano­structure. “However, the exact physical processes need to be studied in more detail in the future,” explains Laura Orphal-Kobin, who inves­tigates quantum systems together with Tim Schröder. The conclusions drawn from the experi­mental obser­vations are supported by statistical models and simulations, which Gregor Pieplow from the same research group is developing and imple­menting together with the experi­mental physicists. (Source: HU Berlin / FBH)

Reference: L. Orphal-Kobin et al.: Optically Coherent Nitrogen-Vacancy Defect Centers in Diamond Nanostructures, Phys. Rev. X 13, 011042 (2023); DOI: 10.1103/PhysRevX.13.011042

Links: Diamond Nanophotonics, Ferdinand-Braun-Institute, Leibniz-Institut für Höchstfrequenztechnik, Berlin, Germany • Integrated Quantum Photonics Group, Humboldt University Berlin, Berlin, Germany