title: Semiconductor quantum-dot–cavity systems as sources of highly nonclassical states of light
Creators: name: Cosacchi, Michael
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: quantum dot
subject: quantum optics
subject: phonons
subject: path integral
subject: 530

description: At the turning point from the digital to the quantum era, research on technologically viable quantum systems is active as never before. Many innovative ideas in all areas of quantum sciences and technologies, ranging from quantum metrology, quantum communication and quantum cryptography to quantum computation, are based on the exchange of information between different systems. As photons, being the quanta of light, travel at the highest possible speed, using them is an attractive pathway to encode information. Semiconductor systems are particularly interesting candidates for sources of such quanta of light, since they are expected to be directly integrable into existing semiconductor technologies. This cumulative thesis theoretically explores the suitability of self-assembled semiconductor quantum dots (QDs) in microcavities as sources of nonclassical photonic states and thus contributes to this fast-growing research area. In contrast to atomic devices, a QD is embedded in a solid-state bulk material and is therefore prone to its temperature-dependent vibrations called phonons once they are quantized. Even at cryogenic temperatures of a few kelvin, at which QDs are commonly operated, the phonon influence is known to have a profound impact on the QD state dynamics. Understanding the effect of this environment of longitudinal acoustic phonons on the joint QD electronic and cavity photonic dynamics is a prerequisite for harnessing these systems to generate specific photon states on demand. To this end, a numerically exact path-integral formalism is employed that does not rely on any further approximations, once the model is stated. Thus, all system--environment correlations are fully accounted for, thereby clearly going beyond a Born--Markov treatment of the phonon environment. This thesis is organized following the increasing complexity of exemplary photonic states as measured by the contributing photon number. Starting out with single photons, a phonon enhancement of their quality is reported and a protocol to store single photons in a QD--cavity system is proposed. Entanglement, being one of the most notorious consequences of quantum theory, is investigated in terms of polarization-entangled photon pairs. The generation of different types of Bell-state entanglement is discussed and a protocol to switch between them in a time-dependent manner is suggested. The analysis of the preparation of higher-order Fock states and $N$-photon bundles in QD--cavity systems showcases the different impact phonons might have on a protocol's success. The influence of phonons on the shape-changing of photon number distributions due to varying excitation conditions of the QD is discussed. The investigation is closed by a proposal to generate Schrödinger-cat states in QD--cavity systems. These studies are framed by two methodological considerations. While the analysis of the accuracy of the quantum regression theorem applied to photonic figures of merit in QD--cavity systems provides a groundwork, the discussion of a new numerically exact method to simulate arbitrary open quantum systems gives an outlook for future studies.