Researchers led by professor Tim Schröder at Humboldt-Universität zu Berlin have generated and detected photons with stable frequencies emitted from from nitrogen-vacancy (NV) defect centers in diamond nanostructures. The achievement holds implications for data transmission with feasible communication rates over long distances — for example, in a quantum network. For this process to take place, all photons must be collected and transmitted without being lost. In addition, all photons must maintain a common frequency. According to the researchers, the demonstration is the first generation and detection of photons with stable photon frequencies emitted from quantum light sources. NV defect centers in diamond act as quantum memories and can be interfaced with coherent photons as demonstrated in entanglement protocols. However, particularly in diamond nanostructures, the effect of spectral diffusion leads to optical decoherence, hindering entanglement generation. The researchers investigated the spectral properties of single NVs in diamond nanostructures by performing photoluminescence excitation spectroscopy. They examined spectral diffusion in three excitation regimes that are relevant for prospective quantum applications such as entanglement generation, according to the researchers. Comparison of experimental results showed the relation of spectral diffusion dynamics to the type and number of ionized defects, the researchers said. Via quantum operations (entanglement), quantum information can be stored in emitted single photons and transmitted in optical fibers in the future quantum internet. Courtesy of HU Berlin/AG Integrierte Quantenphotonik. Also in the work, the researchers showed that the current communication rates between spatially separated quantum systems can prospectively be increased more than a thousandfold with the help of the developed methods. The research was published in Physical Review X (www.doi.org/10.1103/PhysRevX.13.011042).