Quantum dots for quantum networks

Single and entangled photon sources are fundamental building blocks of future light-based quantum technologies such as quantum communication, optical quantum computing, and, more in general, for quantum networks. Despite the high number of photon sources proposed over the last years, their exploitation in quantum optical technologies has been limited so far. One of the main reasons for this is that the envisioned applications set very stringent requirements on the properties of the photon source to be used. An ideal photon source should provide single and entangled photons deterministically, with high purity, high efficiency, high indistinguishability, and, in the case of entangled photons, a high degree of entanglement. Among the different photon sources available to date, semiconductor quantum dots (QDs) are arguably one of the most promising. A QD is a nanometric crystalline structure capable of confining the wavefunction of charge carriers in a semiconductor in all three dimensions. The discrete, atom-like states forming due to confinement can be exploited for the emission of single photons and, in specific conditions, entangled photon pairs. QDs can be grown in a variety of semiconductor combinations. This feature, together with the possibility to tune their optoelectronic properties by changing their physical dimensions and/or via the application of external fields, allows for the control of their light emission with high precision over a broad spectral range. Over the years, several demonstrations of a QD-based source meeting the requirements of the wish-list appeared in the literature. The best source is yet to be disclosed though, as not all the requirements were reached simultaneously in the same experiment. The work performed during my Ph.D. aims to test the suitability of QDs as sources of entangled photons for future quantum networks. The thesis focuses on the fabrication and study of near-ideal nanophotonic devices based on GaAs QDs fabricated by droplet etching and their exploitation in advanced quantum optics experiments. In the first part, I will show and discuss the demonstration of quantum teleportation and entanglement swapping using single and entangled photons generated quasi-deterministically from a single GaAs QDs. More specifically, in the teleportation experiment, a single quantum state is transferred from one photon to another via an entangled photon pair. In the entanglement swapping instead, the interference of two photons coming from two different entangled pairs is exploited to transfer entanglement to the remaining two, previously uncorrelated, photons. These two experiments represented the first benchmark to test the suitability of QDs for quantum communication, as the two quantum protocols lay at the base of a fundamental element of a quantum network, i.e., the quantum repeater. The obtained results highlighted that additional improvement of the photon source is still needed to optimize the fidelity of the protocols, especially for what concerns photon-indistinguishability and -extraction efficiency. For this reason, in the second part of the thesis, great efforts have been devoted to the fabrication of photonic devices. The photonic structure we investigated, i.e., a circular Bragg resonator, consists of a single QD in the center of a central cylindrical cavity surrounded by a circular Bragg grating. It features a modest Purcell enhancement with a broadband resonance and an improved extraction efficiency. Record-high values of indistinguishability and brightness were recently reported in the literature using this structure. Building upon these results, we decided to make an additional step and integrate this photonic device onto micro-machined piezoelectric actuators. This is needed to achieve full control over the QD electronic structure and generate entangled photons with near-unity fidelity and tunable energy. The fabrication of the full device requires several steps: It starts from the epitaxial growth of the semiconductor sample containing the GaAs QDs which is reduced into a semiconductor membrane via wet-chemical etching. A patterned mask is then written on it using electron beam lithography and transferred on the membrane through reactive ion etching. In parallel, we also built an imaging setup to locate the position of the QDs across the wafer allowing us to position the cavity around a single QD with nanometric precision. We fabricated the first photonic cavities onto micro-machined piezo-actuators and, in the very last part of this thesis, we report the first experiments using them as sources of light.