This thesis addresses four key contributions. First, we investigated how tem-poral correlations and dephasing in a quantum dot photon-pair source affecttwo-photon interference, analyzing these effects through multiple approaches,including the sensor method, temporal post-selection, and temperature varia-tion under constant temporal correlations. Second, we implemented a methodbased on low-quality factor micropillar cavities to enhance collection effi-ciency for photon pairs. This approach is based on relatively simple fabrica-tion method and achieves high collection efficiency without solely relying onstrong Purcell enhancement, in contrast to conventional concept. Third, wepresented non-Gaussianity criteria that explicitly account for optical losses,enabling reliable characterization of photon states, and we experimentally ver-ified the presence of non-Gaussianity using a quantum dot photon-pair source.Finally, we introduced a deep-learning-based super-resolution imaging tech-nique that accurately reconstructs images of quantum dots with high resolu-tion without prior system knowledge, even in high-density and noisy environ-ments.Together, these results offer practical solutions for advancing quantum dotphoton sources suitable for emerging quantum technologies.

The visibility of two-photon interference is limited by the indistinguishability of the photons. In the cascaded emission of a three-level system, such as a single quantum dot, the indistinguishability of each photon in the pair is primarily affected by two main factors: the temporal correlation between paired photons and dephasing. Investigating the individual effects of these factors on photon indistinguishability is challenging, as both factors affect it simultaneously. In this study, we investigate the temperature-dependent two-photon interference visibility of the biexciton and exciton photons emitted from a single quantum dot under two-photon resonant excitation, while keeping the temporal correlation between the paired photons intact. Finally, we simultaneously extract the coherence times of the biexciton and exciton photons as a function of temperature.

Two-photon interference is an indispensable resource in quantum photonics, but it is not straightforward to achieve. The cascaded generation of photon pairs contains intrinsic temporal correlations that negatively affect the ability of such sources to perform two-photon interference, thus hindering applications. We report on how such correlation interplays with decoherence and temporal postselection, and under which conditions temporal postselection could improve two-photon interference visibility. Our study identifies crucial parameters and points the way to a source with optimal performance.

Quantum non-Gaussian states of light have fundamental properties that are essential for a multitude of applications in quantum technology. However, many of these features are difficult to detect using standard criteria due to optical losses and detector inefficiency. As the statistics of light are unknown, the loss correction on the data is unreliable, despite the fact that the losses can be precisely measured. To address this issue, we employ a loss-mitigated verification technique utilizing quantum non-Gaussian witnesses, which incorporate the known optical losses and detector inefficiency into their derivation. This approach allows us to address the considerable challenge of experimentally demonstrating unheralded quantum non-Gaussian states of single photons and photon pairs.

Quantum non-Gaussianity is a stricter criterion and the next step after non-classicality in the classification of quantum states. We will present our experimental results on the certification of quantum non-Gaussian coincidences through a noisy channel.

Entangled photon pairs play a crucial role in quantum information processing such as quantumkey distribution, linear optical quantum computation, quantum teleportation, entanglement swapping, etc. Quantum dots have gathered much attention due to their deterministic generation of the photon pairs.

 The emission process of the photon pair in a quantum dot relies on the cascaded spontaneous emission of a three-level system. Time correlations between emitted photons arise because of their time-ordered emissions. The time correlations lead to low purities of photonic states, resulting in deteriorated photon indistinguishability. We explore the visibility of the two-photon interference depending on the coherence times. Furthermore, we investigate the photon indistinguishability in the cascade emissions by executing the temporal post-selection from a triggered two-photon interference.

 Low photon collection efficiency calls for a photonic structure such as a micropillar cavity. The usual technique is to adopt the Purcell enhancement. However, this benchmark is not suitable for biexciton-exciton photon pairs because biexciton and exciton photons have different energies. We developed a new paradigm to increase the collection efficiency based on suppression of the emission into the outside of cavity mode using the low-quality factor cavity, which contradicts the conventional method relying on the Purcell enhancement.

Quantum dots are promising semiconductor sources of nonclassical light. Quantum dots can be used to, on-demand, generate single photons [1] and photon pairs [2]. They can generate pairs of photons entangled in polarization [3] and time bin [4]. Furthermore, the photons emitted by a quantum dot can be employed in a broad range of applications including optical quantum computation, long-distance quantum communication, and protocols like quantum state teleportation. In particular, the possibility to excite the quantum dots resonantly is one of the most important assets of this system. The resonant excitation is the process responsible for the high photon generation efficiency. In combination with photonic structures that enables high photon extraction efficiency, quantum dots can lay the basis for accomplishing very high efficiency sources of quantum light [5].

The Hong-Ou-Mandel (HOM) effect is a direct consequence of the bosonic nature of photons [1], as such, it occurs maximally in the presence of truly indistinguishable photons. Measuring the magnitude of the HOM effect is required to determine the reliability with which a photon source is able to produce indistinguishable photons. Using this method, single photon sources have shown the ability to provide photons with remarkable levels of indistinguishability [2].

The generation of photon pairs in quantum dots is in its nature deterministic. However, efficient extraction of photon pairs from the high index semiconductor material requires engineering of the photonic environment. We report on a micropillar device with 69.4(10)% efficiency that features broadband operation suitable for extraction of photon pairs. Opposing the approaches that rely solely on Purcell enhancement to realize the enhancement of the extraction efficiency, our solution exploits a suppression of the emission into the modes other than the cavity mode. Furthermore, the design of the device can be further optimized to allow for an extraction efficiency of 85%.