By metal-organic vapor-phase epitaxy, we have fabricated InAs quantum dots (QDs) on InGaAs/GaAs metamorphic buffer layers on a GaAs substrate with area densities that allow addressing single quantum dots. The photoluminescence emission from the quantum dots is shifted to the telecom C-band at 1.55 μm with a high yield due to the reduced stress in the quantum dots. The lowered residual strain at the surface of the metamorphic buffer layer results in a reduced lattice mismatch between the quantum dot material and growth surface. The quantum dots exhibit resolution-limited linewidths (mean value: 59 μeV) and low fine-structure splittings. Furthermore, we demonstrate single-photon emission (g(2)(0)=0.003) at 1.55 μm and decay times on the order of 1.4 ns comparable to InAs QDs directly deposited on GaAs substrates. Our results suggest that these quantum dots can not only compete with their counterparts deposited on InP substrates but also constitute an InAs/GaAs-only approach for the development of non-classical light sources in the telecom C-band.

We demonstrate the emission of polarization-entangled photons from a single semiconductor quantum dot in the telecom C-band (1530 nm–1565 nm). To reach this telecommunication window, the well-established material system of InAs quantum dots embedded in InGaAs barriers is utilized with an additional insertion of an InGaAs metamorphic buffer to spectrally shift the system to the desired wavelengths. For the observation of polarization-entangled photon pairs, the biexciton-exciton cascade of a quantum dot displaying an intrinsically low fine-structure splitting is investigated by means of polarization-dependent cross-correlation measurements. A complete set of tomography measurements enables us to reconstruct the two-photon density matrix and therefore to calculate a corresponding fidelity f+ to the maximally entangled Bell state Ψ+ of 0.61 ± 0.07, a concurrence of 0.74 ± 0.11, a tangle of 0.55 ± 0.14, and a negativity of 0.63 ± 0.12, clearly proving the entanglement of the states. Finally, the development of the concurrence is studied in dependency of the post-selected time-gate of the emission events and the progression of the time-delay dependent fidelity to distinct Bell states is displayed.

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...