Single photon sources: theory, practice, and applications in quantum technologies

Quantum technologies such encryption, communication, computing, imaging, and metrology are active areas of research and development. A successful commercialization of these endeavors depends on availability of reliable, efficient, and cost-effective hardware in the form of single photon sources and photodetectors capable of photon number resolution. An ideal single photon source produces deterministically and efficiently indistinguishable photons, one (or two) at a time, at a desired rate and propagating in a chosen direction. Somewhat surprisingly, an attenuated laser does not meet all of these criteria, therefore, alternative sources need to be developed. A few decades of research yielded several sources that, although still not ideal, perform better than an attenuated laser. This presentation is a tutorial on these sources. It begins with a discussion why single photon sources are needed using examples from quantum encryption and imaging, and why an attenuated laser will not suffice. After reviewing physical characteristics of an ideal source, an in-depth and objective discussion of the major sources follows, in particular those based on spontaneous parametric down conversion, four wave mixing, quantum dot, color center, isolated system, and ensemble. The presentation ends with a brief discussion of generating entangled photon pairs and their use in quantum communication and imaging.