Researchers from the Chinese Academy of Sciences and the University of Glasgow have discovered a way to potentially provide control over the emission of individual photons for quantum computing. Active emitters in photonic structures are essential for a coherent electron-photon interface in a quantum photonic network. However, multiple quantum emitters in active cavities are usually positioned in a random fashion, which can make backscattering very difficult to control. As a result, the researchers said, it is hard to achieve a coherent interface between electrons and photons. The researchers devised a system of coupled microdisks with embedded quantum dots (QDs). By exciting the QDs with a laser, the researchers were able to set up so-called diabolical points (DPs) in the coupled microdisks. These DPs allowed control over the backscattering. (a) The schematics of two pairs of reversal states with the backscattering. Red arrows refer to + while blue arrows refer to −. (b) Four eigenvalues with J(a,b) of different values. Pink lines refer to results with Ja = Jb. Green lines refer to results with Ja = −Jb. Courtesy of Jingnan Yang et al. According to the researchers, randomly positioned QDs and defects are hard to control and could result in symmetric backscattering. Macroscopic control of the backscattering was achieved based on a competition between defects and emitters, which solved the problem of low controllability originating from randomly positioned scatterers. Backscattering coupling strength with both negative and positive values was realized. The competition was balanced by an optimized microdisk size and experimentally demonstrated, providing the basis for the successful observation of DPs. The researchers experimentally demonstrated one pair of DPs in the spectra with two strongly coupled microdisks. (a) SEM images of single and double microdisks. The excitation laser is labeled by the green arrow. (b) The red shift of cavity modes with increasing excitation power. (c) The wavelengths, linewidths, and splitting between two peaks extracted from Lorentz multipeak fitting. (d) Statistics of linewidth differences between split modes. The resolution of the spectrometer is 0.1 nm. (e) Statistics of the splitting. The splitting of 1000 μeV corresponds to 0.80 nm at the wavelength of 1000 nm. (f) Distribution of the splitting and half Gaussian fitting. Courtesy of Jingnan Yang et al. The team believes that realizing DPs in active photonic structures could advance the implementation of quantum information processing and scaling of quantum networks. In the future, when the interaction between emitters and cavities is improved, the CAS system could play an important role in studying quantum DP behaviors and integrating photons at DPs into quantum networks. Excitation-power-dependent PL maps of coupled cavities and the fitted results with different Ja,b. The resonance ωa = ωb is marked by purple dash lines. (a) Ja = 0 and Jb ≠ 0. (b)-(c) Ja Jb > 0. (c) Ja = Jb. (d) JaJb a = −Jb. DPs occur at resonance.Courtesy of Jingnan Yang et al. The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-020-0244-9).