singleemitter

Single emitter refers to a quantum system capable of emitting light one quantum at a time, a key resource in quantum optics and quantum information. After excitation, such a system relaxes by emitting a single photon into a defined decay channel. A defining experimental feature is photon antibunching, where the likelihood of detecting two photons in quick succession is reduced compared with classical light. This is quantified by the second-order correlation function g^(2)(τ); for an ideal single emitter, g^(2)(0) = 0, and real devices strive for g^(2)(0) < 0.5.

Common physical realizations include isolated atoms or ions, semiconductor quantum dots, and color centers in diamond (for example, NV and SiV centers). Defects in two-dimensional materials such as hexagonal boron nitride, organic molecules, and, in some cases, superconducting qubits emitting microwave photons, also serve as single emitters. In photonic applications, the emitter is often placed in an optical cavity or coupled to a waveguide to enhance emission into a preferred mode via the Purcell effect, and to improve collection efficiency for integration into photonic circuits.

Emission is typically driven by optical or electrical excitation. Pulsed pumping can produce photons on demand, while resonant excitation can improve photon indistinguishability. Performance is described by brightness, quantum efficiency, purity (g^(2)(0)), and the indistinguishability of emitted photons, which is essential for interference-based quantum information protocols. Challenges include spectral diffusion, dephasing, blinking, photobleaching in some molecules, and losses in coupling to the desired optical channel.

Single emitters underpin quantum technologies such as quantum key distribution, linear optical quantum computing, and quantum networks, with ongoing work aimed at stability, integration with nanophotonic devices, and scalable multi-emitter architectures.