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Due to their excellent optical properties, quantum dots are promising for applications in photonic quantum technologies. For on-demand single-photon generation, a two-level system given by an excitonic transition is typically excited with a resonant laser pulse of area π. This prepares the two-level system in its excited state from where it spontaneously emits a single photon. However, emission that occurs already during the presence of the laser pulse allows for re-excitation and, thus, multi-photon emission which limits the single-photon purity [1].
In contrast, when exciting the system with a pulse of area 2π, the system is expected to be returned to the ground state. However, in this case emission during the presence of the pulse is most likely to occur when the system is in its excited state – exactly after an area of π has been absorbed. This restarts the Rabi oscillation with a pulse area of π remaining in the pulse which leads to re-excitation with near-unity probability and the emission of a second photon within the excited state lifetime [2,3].
Finally, we present the generation of single photons with ultra-low multi-photon probability [4]. Using two-photon excitation of the bi-exciton suppresses re-excitation and improves the single photon purity by several orders of magnitude for short pulses.
[1] K. A. Fischer, et al., New J. Phys. 18, 113053 (2016)
[2] K. A. Fischer, et al., Nature Physics 13, 649-654 (2017)
[3] K. A. Fischer, et al., Quantum Sci. Technol. 3, 014006 (2017)
[4] L. Hanschke et al., arxiv: 1801.01672 (2018)
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