Final Report Summary - EXQUISITE (External Quantum Control of Photonic Semiconductor Nanostructures) The EXQUISITE project explored the external quantum control of photonic semiconductor nanostructures. The research was performed at the forefront of science and technology in the non-classical regime of external control where quantum fluctuations govern the nonlinear dynamics of feedback coupling, optical injection, chaos and synchronization. In order to address this regime at the crossroads between nonlinear dynamics, nanophotonics and photonic quantum technology, the EXQUISITE team realized and applied nanophotonic light sources, which take advantage of cavity quantum electrodynamics (cQED) effects. These devices include bright single-photon sources and high-beta microlasers based on only a few quantum emitters in the active medium. An important technological outcome is the first deterministic fabrication of microlasers with a well-defined number of about 1-20 site-controlled quantum emitters, i.e. semiconductor quantum dots (QDs), spatially and spectrally aligned to the laser mode. This break-through ensures optimum coupling of the small ensemble of emitters to the lasing mode, which is the basis for the exploration of external control and non-linear dynamics in the quantum regime of few-photon – few-emitter interaction.The experimental and tehoretical work of EXQUISITE focused on controlling the emission features of nanophotonic systems by optical injection, time-delayed optical feedback and by their mutual coupling. These studies have in common that they aim at pushing the external optical control to its quantum limit, where for instance single photons back couple into the optical mode of a microcavity. This specific widely unexplored regime of non-linear dynamics is highly attractive as it connects classical and quantum physics to answer important questions appearing in the transition region between the two limiting cases. In this regard, the EXQUISITE team studied nano- and microlasers close to the ultimate limit of thresholdless lasing. Supported by microscopic modelling, they highlighted for instance possible pitfalls in the interpretation of experimental data of high-beta nanolasers and established methods to unambiguously identify lasing in such devices by quantum optical spectroscopy. Of particular interest were bimodal micropillar lasers subject to time delayed feedback and optical injection at ultra-low light levels down to the nanowatt regime. The emission dynamics of such high-beta microlasers is governed by intrinsic instabilities leading to abnormally high bunching in their feedback-controlled photon statistics. Moreover, the project revealed novel non-classical effects under optical injection, where a QD-microlaser is only partially locked to the master laser – in contrast to full locking in the classical regime.Moving further into the realm of quantum optics, EXQUISITE focused on important problems like the coherent control of two-level emitters. In a pioneering experiment, the EXQUISITE team succeeded in driving a quantum dot for the first time by the light of a high-beta microlaser which represents the first “real-world” application of such a cQED-enhanced laser, demonstrating their high application potential in the field of quantum nanophotonics. By mutually coupling and synchronizing two cQED-enhanced microlasers with on the order of 10 intracavity photons the project also undertook groundbreaking work towards resolving the question how zero-lag synchronization of classical oscillators relates to entanglement of quantum systems at ultra-low light levels close to the quantum regime of single-photon coupling. Overall, the research performed in EXQUISITE project uncovered intriguing phenomena at the crossroads between classical and quantum physics. Based on the gained technological, experimental and conceptual knowledge it set the ground for further exciting research towards the quantum regime
