High-quality sources of single photons are of paramount importance forquantum communication, sensing and metrology. To these ends, resonantly excitedtwo-level systems based on self-assembled quantum dots have recently generatedwidespread interest. Nevertheless, we have recently shown that for resonantlyexcited two-level systems, emission of a photon during the presence of theexcitation laser pulse and subsequent re-excitation results in a degradation ofthe obtainable single-photon purity. Here, we investigate a new scheme forgenerating single photons from self-assembled quantum dots, based on two-photonexcitation of the biexciton, and demonstrate that it yields superiorperformance with an ultra-low multi-photon probability. In this method,re-excitation is strongly suppressed improving the obtainable multi-photonerror rate by several orders of magnitude. Specifically, the pulse-lengthdependence of the multi-photon error rate reveals a quadratic dependence incontrast to the linear dependence of resonantly excited two-level systems. Wesupport our experiments with a new theoretical framework and simulationmethodology to understand few-photon sources.
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