In this talk, I will start by discussing experiments in which ultrafast methods are used to prepare and study individual spins in single III-V quantum dot (QD) nanostructures. Results confirm the long held belief that electron spin dynamics are governed by the hyperfine field of the nuclear spin bath and its long-time evolution due to quadrupolar interactions between nuclear spins [1,2]. For electrons, we obtain T2 ~1ns consistent with the findings of other groups, due to the slow dephasing of the electron spin that occurs over microsecond timescales [2-4]. In contrast, for holes, we measure much longer timescales (several hundred ns) due to the weaker hyperfine interaction with the nuclear spin system. Our focus will shift to the on-demand generation of quantum light. Here, we have recently shown that for resonantly excited two-level systems, emission of the photon during the presence of the excitation laser pulse and subsequent re-excitation results in a degradation of the obtainable single-photon purity [4,5]. While this can be exploited to generate photon pairs on demand, we investigate a scheme based on two-photon excitation of the biexciton and demonstrate that it yields superior performance with an ultra-low multi-photon probability [5-7]. We show how coherent quantum state preparation [8] can be combined with coherent triggering of photon emission to suppress unwanted re-excitation processes, that lead to multi-photon errors, while precisely timed stimulation pulses reduce the timing jitter of emitted photons [6]. In this way, we simultaneously obtain 𝑔(0) < 10 and photon indistinguishability exceeding ~90% by coherently triggering single photon emission. Finally, our attention will turn from quantum light sources to superconducting nanowire single-photon detectors (SNSPDs) that nowadays provide near-unity detection quantum efficiency, negligible dark count rates and picosecond timing resolution. We have developed single and multi-pixel SNSPDs and explored how local He-ion irradiation can be used to significantly enhance photon detection efficiency [9]. Such methods provide much perspective for multi-pixel and focal-plane detectors with photon number resolving capabilities.

[1] A. Bechtold et al., Nature Physics (2015). DOI: 10.1038/nphys3470;

[2] A. Bechtold et al. Phys. Rev. Lett. 116, 027402 (2016);

[3] K. A. Fischer et al. New J. Phys. 18, 113053 (2016);

[4] K. A. Fischer et al. Nature Physics doi:10.1038/nphys4052, (2017); 

[5] L. Hanschke et al. Physical Review Letters 125 (17), 170402, (2020); 

[6] F. Sbrezny et al. Phys. Rev. Lett. 128, 093603 (2022);

[7] E. Schöll et al. Phys. Rev. Lett. 125, 233605 (2020);

[8] V. Villafane et al. Phys. Rev. Lett. 130, 083602, (2023); 

[9] S. Strohauer et al. arXiv:2305.14175, to appear in Adv. Q. Tech (2023);