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Researchers from Paderborn University, the University of Basel, and Ruhr University Bochum have developed a method to generate nearly indistinguishable photons for quantum communication. In their paper published in *Physical Review Letters*, they show that special semiconductor nanostructures can produce individual photons and pairs that are almost perfectly identical. These particles form the basis for quantum entanglement and interference, which are essential for complex calculations using light.

Previously, generated photons suffered from temporal correlations or being out of focus, reducing their quality. The new team solved this by using a process called "biexciton decay" within semiconductor quantum dots inside an optical resonator. A biexciton is a molecule with two bound excitons, where each exciton consists of an electron and an electron hole. When this structure decays, it leaves behind a single exciton and a photon. As lead author Timon Baltisberger explains, this "biexciton cascade" emits photons easily, generating one after the other.

By integrating the quantum dot into a specialized optical cavity similar to a laser, the researchers accelerated and controlled the light emission. Professor Richard Warburton notes that this results in photons that are 90% indistinguishable, compared to merely 60% without this effect. Dr. Klaus Jöns adds that photon purity can be optimized using the resonator, with limitations currently set by vibrations in the semiconductor’s crystal lattice known as phonons. This phenomenon, called "cavity feeding," must be minimized in future designs.

The more indistinguishable and pure the photons are, the lower the error rate in data processing. Quantum dots are seen as a promising technology for mass-producing these high-quality particles. While this study focuses on pair generation, initial findings on single-photon sources were published separately in *Physical Review Applied*. The work demonstrates that biexciton decay can produce very high-quality photons provided the system is properly controlled using a cavity.