Quantum mechanics

Algorithm

Quantum key distribution

Process (computing)

Statistics

Nondeterministic algorithm

Computer security

Operating system

Computer science

Brightness

Spontaneous parametric down-conversion

Quantum optics

Photonics

Photon

Physics

Photon entanglement

Quantum entanglement

Mathematics

Optics

Quantum

Parametric statistics

Key (lock)

Advanced Quantum Technologies

Entangled photon sources are desirable and key elements for photonic quantum information processing. The spontaneous parametric down‐conversion (SPDC) process is a convenient way to produce entangled photon pairs. It has been widely used and has already made great contributions to quantum information science. SPDC sources have been thoroughly studied, and a detailed review, previously lacking, is now appropriate. In this article, different types of SPDC sources suitable for multiphoton experiments are reviewed. A comparison of their respective performances is presented, focusing on the parameters of indistinguishability, collection efficiency, and brightness, from which it is possible to see how the spatial and spectral properties of the SPDC process can be tailored, step by step, to achieve near‐perfect photon pairs. Recent progress on multiplexed heralded single‐photon sources, which effectively mitigate the nondeterministic nature of the SPDC process, is also described.

Spontaneous Parametric Down‐Conversion Sources for Multiphoton Experiments