About ten years ago, Richard Warburton, lead author of the study, and his team at the University of Basel, Switzerland, started out on the quest for an SPS, which would, at the same time, be efficient and deliver pure and coherent single photons at a high repetition rate. They settled for an approach borrowed from the atom optics community; the researchers placed the quantum dot in a miniaturized version of an open Fabry–Pérot cavity (Figure). The curved top mirror has a radius of 10 μm and a depth of only 500 nm. For years, the researchers grappled with numerous experimental problems, such as charge noise from the semiconductor environment, unwanted absorption within the structure or from the surface, and the question of how to optically excite the quantum dot. Warburton recalls: “We faced many hurdles and wondered many times if we were on a good track. Yet, we pressed on.”
After years of hard work and in cooperation with Arne Ludwig and Andreas Wieck at Ruhr-University Bochum who realised the low-noise semiconductor heterostructure, the researchers had mitigated all the problems. Their source produced highly coherent single photons with two-photon interference visibility of 97.5% at a repetition rate of 1 GHz. Importantly, Tomm et al. realized an end-to-end efficiency of 57% (N. Tomm et al. Nat. Nanotechnol. 16, 399–403; 2021). When sending in a single excitation pulse with a laser, in more than half of the cases, a single photon would exit the optical fibre, which collects the photons from the semi-transparent top mirror. This achievement is the result of a careful optimization of all optical components involved, rather than of a single innovation.
This is a preview of subscription content, access via your institution