Large-scale, distributed quantum states are the basis for novel applications in quantum communication, quantum remote sensing or distributed quantum computing. The necessary infrastructure will be provided by distributed quantum networks [1], allowing for quantum bit transmission, processing and storage at single nodes. Sources of single photons and entangled photon states are important constituents of such networks. Epitaxially grown quantum dots (QDs) show great potential for the deterministic generation of such photon states. Here we utilize the emerging family of epitaxially grown GaAs/AlGaAs quantum dots obtained by droplet etching and nanohole infilling. They are outstanding sources of indistinguishable single photons and polarization-entangled photon pairs, with a unique combination of metrics (brightness, purity, coherence, emission rate) [2,3]. Under pulsed resonant two-photon excitation, we observe emission of polarization-entangled photon pairs with high purity of 0.99 and high entanglement fidelities up to 0.94. Bright entangled photon emission is realized by tailoring the dielectric environment around the QDs, allowing for the first implementation of a core element in quantum communication: Entanglement swapping between two pairs of photons emitted by a single quantum dot is demonstrated with a fidelity up to 0.81 [2].

Coupling these emitters to quantum memories with long coherence times enables the development of hybrid nanophotonic devices for quantum communication applications. GaAs/AlGaAs QDs exhibit very narrow wavelength distributions at rubidium based quantum memory transitions. We apply strain tuning via piezoelectric actuators to allow for reversible fine-tuning of the emission frequency. Employing active feedback then allows to stabilize the frequency of single photons to an atomic rubidium standard [4]. Another promising quantum memory candidate is Silicon-vacancy centers in diamond, as they show strong interactions with single photons via their zero phonon line at around 737nm. We report the first GaAs/AlGaAs quantum dots emitting single photons at this wavelength. Polarization entangled photons are generated via the biexciton-exciton cascade decay with a fidelity of 0.73. High single photon purity above 89% is maintained up to 80K (above liquid nitrogen temperature), rendering this system attractive for real-world implementations.

To ensure excellent quantum optical characteristics, epitaxial QDs have to be buried under thick barrier layers to maintain a distance from detrimental surface states. However, to unlock their full potential, QDs need to be integrated into scalable (hybrid) nanophotonic devices that require a vanishing distance to the surface. For instance, investigating strong coupling to optical cavity fields requires ultra-small mode volumes, and efficient interaction with plasmonic or dielectric nanoparticles can only be achieved within a few nanometers, therefore requiring surface proximity of QDs. Although single epitaxial QDs very close to the surface have been studied for over two decades. Weak and broad emissions were the result, rendering advanced quantum optical experiments infeasible. We show the complete restoration of optical properties from quantum dots grown directly on a semiconductor surface, leading to bright, ultra-stable, coherent and blinking-free single photon emission [5]. Under quasi-resonant excitation, single photons are generated with 98.8% purity, 77% indistinguishability, linewidths down to 4µeV and 99.7% persistency across 11 orders of magnitude in time. The emission is stable even after two years and when being subjected to nanomanufacturing processes. Some long-standing stumbling blocks for surface-dominated quantum dots are thereby removed, unveiling new possibilities for hybrid nano-devices and applications in quantum communication or sensing.