In the present work, the effect of resonant pumping schemes in improving the photon coherence is investigated on InAs/InGaAs/GaAs quantum dots emitting in the telecom C-band. The linewidths of transitions of multiple exemplary quantum dots are determined under above-band pumping and resonance fluorescence via Fourier-transform spectroscopy and resonance scans, respectively. The average linewidth is reduced from 9.74 GHz in above-band excitation to 3.50 GHz in resonance fluorescence underlining its superior coherence properties. Furthermore, the feasibility of coherent state preparation with a fidelity of 49.2 % is demonstrated, constituting a step towards on-demand generation of coherent, single C-band photons from quantum dots. Finally, two-photon excitation of the biexciton is investigated as a resonant pumping scheme. A deconvoluted single-photon purity value of gHBT (0) = 0.072 ± 0.104 and a degree of indistinguishability of V HOM = 0.894 ± 0.109 are determined for the biexciton transition. This represents an important step towards fulfilling the prerequisites for quantum communication applications like quantum repeater schemes at telecom wavelength.Over the past two decades, semiconductor quantum dots (QDs) have received unceasing attention from researchers in the field of quantum optics due to their outstanding properties in terms of non-classical light emission 1-5 , i.e. bright singlephoton emission, entanglement fidelity, indistinguishability and the simultaneous combination of the aforementioned 4,5 . This designates them as promising candidates for applications like quantum computing and quantum communication 6 . The best performances are currently achieved with GaAs-based dots emitting in the near infrared 7 . However, especially regarding quantum communication schemes, an emission wavelength around 1550 nm (Telecom C-band) is much soughtafter both for satellite-based quantum communication due to an atmospheric transmission window and the possibility to perform it in broad daylight 8 , as well as for its fiber-based counterpart due to the global absorption minimum and low dispersion of standard glass fibers forming the existing global fiber network 9 . However, to extend the range of quantum communication applications such as quantum key distribution 10,11 , quantum relays 12,13 or quantum repeaters 14-17 are needed. The ideal light source for such applications combines bright single-photon and entangled-photon pair emission with a high degree of indistinguishability at 1550 nm.The emission of single and entangled photons in the telecom C-band has been demonstrated in two material systems, namely InAs/InP 18,19 and InAs/InGaAs/GaAs 20,21 . The last requirement, i.e. the indistinguishability of photons, is of major importance because it is necessary for two-photon interference (TPI), enabling linear-optic Bell state measurements and, therefore, entanglement swapping 22,23 in quantum repeater schemes. An experimental demonstration at this wavelength has been elusive in both material systems up to now.Ho...

The combination of semiconductor quantum dots with photonic cavities is a promising way to realize nonclassical light sources with state-of-the-art performances regarding brightness, indistinguishability, and repetition rate. Here we demonstrate the coupling of InGaAs/GaAs QDs emitting in the telecom O-band to a circular Bragg grating cavity. We demonstrate a broadband geometric extraction efficiency enhancement by investigating two emission lines under above-band excitation, inside and detuned from the cavity mode, respectively. In the first case, a Purcell enhancement of 4 is attained. For the latter case, an end-to-end brightness of 1.4% with a brightness at the first lens of 23% is achieved. Using p-shell pumping, a combination of high count rate with pure single-photon emission (g(2)(0) = 0.01 in saturation) is achieved. Finally, a good single-photon purity (g(2)(0) = 0.13) together with a high detector count rate of 191 kcps is demonstrated for a temperature of up to 77 K.

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