Single-photon sources are devices that emit one photon at a time, which is crucial for applications in quantum communication and quantum computing. These sources can be based on various technologies, including quantum dots, color centers in diamonds, and atomic ensembles. By controlling the emission process, researchers can ensure that the photons are produced with specific properties, such as polarization or wavelength. The ability to generate single photons is essential for secure communication protocols like quantum key distribution. In these systems, the unique characteristics of single photons help prevent eavesdropping, as any attempt to intercept the photons would disturb their state. This makes single-photon sources a key component in advancing quantum technologies.