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On-demand single-photon sources are devices capable of emitting individual photons upon external triggering. This capability is crucial for numerous quantum technologies, including quantum communication, quantum computing, and quantum sensing. Unlike spontaneous emission, which occurs randomly, on-demand sources provide precise control over the timing of photon generation. This controllability is essential for synchronizing quantum operations and establishing secure quantum key distribution.

Several physical systems are being explored for realizing on-demand single-photon sources. Quantum dots, which are semiconductor nanocrystals, exhibit strong light-matter interactions and can be engineered to emit single photons. Superconducting circuits, particularly those employing the transmon qubit architecture, can also be manipulated to produce single photons. Other promising approaches include color centers in diamond, trapped atoms and ions, and parametric down-conversion processes when carefully controlled.

The performance of on-demand single-photon sources is typically evaluated based on several key metrics. Photon indistinguishability, which measures how identical the emitted photons are, is vital for interference experiments required in many quantum protocols. High purity, meaning minimal probability of emitting more than one photon, is also critical. Furthermore, high efficiency, referring to the probability of successfully emitting a photon when triggered, and a short emission time are desirable for practical applications. Ongoing research focuses on improving these parameters to enable robust and scalable quantum systems.