Solid-state single-photon emitters

631 Photonic technologies are becoming increasingly prevalent in our daily lives. After decades of rapid advances, light sources — especially lasers and light-emitting diodes — have become high-performance, yet low-cost and reliable components, driving the Internet and lighting cities. A new frontier of research is the development of non-classical light sources: sources that produce streams of photons with controllable quantum correlations. A central building block, in particular, is a single-photon emitter (SPE) — a fundamental resource for many scalable quantum information technologies1–6. The ideal on-demand SPE emits exactly one photon at a time into a given spatiotemporal mode, and all photons are identical so that if any two are sent through separate arms of a beam-splitter, they produce full interference (a signature of indistinguishability). Such SPEs play a central role in a range of proposed quantum computing schemes, including linear7,8 quantum simulation9, quantum walks10 and boson sampling11, and precision measurement12. SPEs are also useful or necessary in many quantum secure communication schemes13,14 and light flux metrology applications (such as defining the quantum candela — the SI base unit of luminous intensity) that do not require indistinguishability15,16. Over the years, various processes have been studied to generate single photons. The first demonstration of a SPE used an atomic transition of sodium atoms17, though its reliability and efficiency were low. Today, it is possible to control cold atoms to efficiently produce single photons on-demand with near-identical wave packets18. Such sources, however, still require complex set-ups, and the loading of atoms or ions can be intermittent. The dynamics of atombased sources is also relatively slow, leading to very low operation rates. Alternatively, single photons can be generated by heralding one of two photons produced using a nonlinear process such as spontaneous parametric down-conversion19,20 or spontaneous fourwave mixing21. To overcome the unpredictable generation times of heralded photons, multiplexing schemes are being developed to rearrange them into regular intervals, though more work is needed to improve the single-photon purity and efficiency of such schemes, which are currently limited largely by losses in switching, photon storage and detection. One of the most promising types of single-photon sources today are solid-state SPEs based on atom-like emitters — including Solid-state single-photon emitters