Generation of bright and coherent single photons from a cavity-coupled quantum dot

Abstract

Since Max Planck discovered the quantization of light, nonclassical lights have been widely researched. Among them, single photons have significant attraction. In particular, single-photon interference experiments show the weirdness of quantum physics and reveal the fundamental nature of superposition and quantum interference. Later single-photon experiments continue to creation of quantum entanglements between correlated multiple photons. Using their various degrees of freedom of photons, such as polarizations, paths, time-bin, and orbital angular momentums, we can easily encode quantum information into photons, so called photonic quantum bits. Moreover, since photons do not interact with environment easily, they can be transmitted over long distance without loss. Additionally, their quantum states can be easily manipulated by linear optics. These properties make them candidates for quantum applications such as quantum computing, quantum communication and quantum simulator. Furthermore, these systems can be extended and scaled up to form quantum networks. However, these applications demand a source of scalable and efficient single photons and quantum gates based on quantum interference. In other words, bright single-photon sources with long coherence time for interconnectivity of quantum systems are required. For such achievements, various quantum emitters have been proposed and researched, and we focused on semiconductor-based quantum dots. They are easily fabricated and have high single-photon purity. Despite these advantages, there are some issues with using quantum dots as scalable and efficient single-photon sources. Quantum dots in bulk exhibits a photon collection efficiency of only 1~2% due to a large difference in refractive index at their interfaces. In addition to low collection efficiency, quantum dots suffer from the dephasing issue by unwanted interaction with phonons and charge in a crystal. Ideally, the temporal extension of wave-packets (coherence time) is limited by a lifetime of quantum dots, determined by Fourier transformation. However, additional dephasing effects such as spectral diffusion or time jitter occur, leading to a reduction in coherence time. In this thesis, I have studied methods to enhance the collection efficiency and increase the coherence time of photons from quantum dots. To enhance collection efficiency, I designed and fabricated a photonic crystal cavity. This cavity adjusts a directionality of lights vertically, enhancing the collection efficiency of photons from quantum dots. To increase coherence time, we approached in two different ways: one is using a narrow bandwidth filter, and the other using quasi-resonant pumping. The first approach increases the coherence time of quantum dots through spectral filtering. However, due to spectral filtering, this approach leads to intensity loss. Therefore, I approached alternative way, using a different pumping method that reduces the dephasing effects such as time jitter induced by phonon. Lastly, I will show the integration of cavity embedding quantum dots and fiber, which not only enhance the collection efficiency, but also does acquire any alignment. I enhanced photons extraction efficiency by designing and fabricating the cavity, and improved coherence time using two different approaches. Comparing two approaches, quasi-resonant pumping has more advantages than The Etalon filter with respect to loss. However, the quasi-resonant pumping still relies on the phonon-assisted excitation, and for scalable and efficient single photon platform, I integrated quantum dots with fiber. HOM interference depending on pumping method can be further extended by resonant pumping, which we expect will increases coherence time even more, and could be conducted in pulse laser to generate nearly pure N00N states for quantum gate, quantum sensing. Additionally, it is expected that implementing these features in the integration of a cavity with fiber will enable the construction of a scalable and efficient platform for providing indistinguishable single photons.