A single-photon source emits exactly one photon per trigger event, and it is the missing component that photonic quantum computing and quantum networks are currently designed around rather than with: 2026 experiments deliver antibunching across many material platforms, but purity, brightness, indistinguishability and telecom-band integration have still not been achieved together in one device.
Summary
A single-photon source (SPS) is a device that, when triggered, emits one and only one photon into a defined optical mode. The standard figure of merit is the second-order correlation at zero delay, g²(0), which is 1 for a laser and 0 for a perfect single-photon emitter; values below roughly 0.5 confirm a single quantum emitter, and state-of-the-art work targets 0.1 and below. Three further parameters decide usefulness: brightness or emission efficiency (what fraction of triggers yield a collected photon), indistinguishability (whether successive photons interfere perfectly, required for fusion and Bell-state measurements), and wavelength (telecom bands for fibre, or matched to a spin qubit). A fifth, usually decisive in practice, is integration: whether the emitter can be excited electrically, stabilised against charge noise, and coupled into a waveguide or fibre without free-space alignment and post-selection.
The physics comes in two families. Saturable emitters are single quantum objects that can hold only one excitation at a time: epitaxial semiconductor quantum dots, colour centres in hexagonal boron nitride, silicon carbide and silicon, localised excitons in transition metal dichalcogenides, single organic molecules such as dibenzoterrylene, and acceptor-bound excitons in II-VI materials. Photon blockade is the alternative: a nonlinear cavity-emitter system whose energy ladder is anharmonic enough that absorbing a second photon is forbidden. Conventional and unconventional blockade schemes suffer a purity-brightness trade-off, which recent theory attacks using three-body interactions between one photonic mode and two qubits, or by numerical multi-objective optimisation of the open-system dynamics. A third route, heralded pair generation, is probabilistic by construction and carries multiphoton emission errors.
The parameter that actually decides the field is whether a true SPS is needed at all. In quantum key distribution it largely is not: decoy-state protocols were invented precisely to overcome the limitations of imperfect single-photon sources and are the most widely adopted approach in deployed systems. Verification protocols for delegated quantum computation have likewise been rewritten to replace single-photon sources with trusted local rotations or weak coherent pulses. Where the substitution fails is measurement-based photonic computing: schemes that build caterpillar and lattice graph states directly from single-photon sources, linear optics and heralded measurements need l-2 fewer photons and achieve a 2^(l-2) higher success rate than fusion-based assembly for a state of length l ≥ 3. That exponential is the real economic case for the technology.
Evidence base
Open questions
Assessment drafted 2026-08-31 from up to 18 KB sources using the technology-scorecard framework; scores are a draft read pending review.