Applications of single photons in quantum metrology, biology and the foundations of quantum physics

With the development of photonic quantum technologies, single photons have become key for various applications including quantum communication and quantum computing, discussed in an accompanying Review. Here we overview the applications of single photons in quantum metrology, biology and experiments probing the foundations of quantum physics. For each of these applications, we outline the main milestones reached so far, the remaining challenges, and the improvements that could be made in the future. We conclude with a wish list for future single-photon sources. This Review overviews the application of single photons in quantum metrology, quantum biology and the foundations of quantum physics, discussing specific needs and requirements, achieved milestones and an outline for future improvements.