High-quality sources of single photons are of paramount importance for quantum communication, sensing and metrology. To these ends, resonantly excited two-level systems have recently generated widespread interest. Nevertheless, for resonantly excited two-level systems, emission of a photon during the presence of the excitation laser pulse and subsequent re-excitation results in a degradation of the obtainable single-photon purity . Here, we investigate a two-photon excitation scheme based on a three-level system formed by the bi-exciton - exciton cascade in a self-assembled quantum dot and demonstrate that it improves the multi-photon error rate by several orders of magnitude . We support our experiments with a new theoretical framework and simulation methodology to understand few-photon sources.
For a resonantly excited two-level system the multi-photon error rate scales linear with the pulse length . In contrast, the two-photon excitation scheme exhibits a quadratic dependence, improving the obtainable multi-photon error rate by several orders of magnitude for short pulses. Moreover, the scheme is easy to implement and facilitates fast repetition rates in contrast to schemes involving three-level lambda-type systems that require re-pumping. Unlike resonant excitation of a two-level system, this scheme does not require the measurement technique of cross-polarized suppression to reject the excitation laser and, thus, enables a higher source brightness. Finally, the scheme is directly compatible with increasing the emission rate by Purcell enhancement.
 K.A. Fischer et al. Nature Physics 13, 649-654 (2017)
 L. Hanschke, et al. arXiv:1801.01672 (2018)
 K.A. Fischer, et al. Quantum Science and Technology 3, 1 (2017)