Engineering quantum light-matter interactions and photon statistics in the low Q cavity-coupled QDs

Abstract

In quantum information technologies like quantum communication, quantum computing, quantum sensing, and quantum metrology, the generation and manipulation of qubits are essential. Among numerous qubits, such as superconductor qubits, spin qubits, and photonic qubits, photonic qubits have been in the spotlight due to their irreplaceable advantages. Photonic qubits are decoherence-robust due to the photon's feeble interaction with their surroundings. Moreover, a single photon can concurrently store numerous degrees of freedom, including polarization, time-bin, phase, angular momentum, and frequency. Long-distance qubit transfer is achievable at the speed of light via free-space or traditional optical fiber technologies. However, the quantum photonic system suffers from significant quantum loss, the accuracy of quantum operations on photonic qubits is inadequate, and there are currently no reliable and perfect quantum light sources available. The epitaxially grown quantum dot (QD) of nanoscale scale is a promising contender for future quantum light sources. The QDs exhibit discrete exciton levels, enabling them to function as a two-level system (TLS) akin to an atom. Because the QDs have a unitary internal quantum efficiency, they are luminous quantum emitters that can generate single photons with a high purity level. The diverse exciton combinations in QDs provide us with a wide range of options, such as spin-photon entanglement or polarization-entangled photon pairs. However, generating highly indistinguishable photons necessitates substantial material engineering and specialized excitation techniques. Further, to increase light extraction efficiency from host materials of high refractive index, we must integrate QDs with cavities. To construct a photonic-qubit-based scalable system, high efficiency of photon-participating interactions, such as two-photon interference (TPI), is important. The Gaussian mode profile is desirable due to its symmetric profile and the highest tolerance for mode mismatching. However, intrinsically single photons from QDs do not have Gaussian profiles. Because of the challenges in interacting with photonic qubits, emitter-level engineering is required to modify photons from QDs. A low Q cavity coupling with quantum emitters enhances light-matter interactions, which is a way to engineer proper photonic qubits. Furthermore, a large mode volume and broad spectral linewidth of the low Q cavity can be easily coupled with quantum emitters and provide a collectively interacting system. Additionally, because of the lack of other quantum mechanical functionality, there are no universal quantum emitters able to independently construct large-scale quantum systems. For example, compared with bright and pure single photons from QDs, the difficulty of long-term storing of quantum states and intrinsic spectral randomness of the QDs pose significant obstacles to achieving a large-scale quantum system. Atomic emitters can store quantum states for a long time due to their long coherence and have well-defined physical properties, which is suitable for an absolute reference of the system. However, they have low tunability and integration potential, which is not desirable for small integrated systems. Atomic defects can serve as quantum emitters at room temperature and have spin states. Nonetheless, they too strongly interact with their environments, leading to significant decoherence. However, building a hybrid system with various quantum emitters presents a promising alternative for scalability. In such systems, QDs serve as bright single-photon sources, atomic emitters store quantum states for extended periods, and defects provide entangled spin-photons. Here, we implemented research for a scalable system using QDs. First, we modified the optical properties of the photons via quantum light-matter interactions in the low Q cavity-coupled QDs system. We changed the modes of single photons into Gaussian profiles by engineering the exciton dynamic, which is controlled by cavity coupling and an existing multi-exciton complex in the QD. This made the quantum interactions of photons more efficient and made the mode mismatching more stable. Furthermore, we recovered single-photon purity together with minimizing single-photon loss. Next, we experimentally showed that two QDs can interact with each other in a steady-state through a shared radiative field of the coupled low Q cavity. We optically accessed the dominant long-lived entangled state and indirectly observed it by measuring photon statistics changes in the emission. Our approach suggests a new method for forming a collectively interacting system, and the entangled states can be a tool for quantum information technologies. Finally, we investigated TPI from two different quantum emitters with similar optical properties. Both the QDs and warm atomic ensembles can generate quantum lights of similar wavelengths and coherence. A QD generates single photons, while a warm atomic ensemble simultaneously produces both thermal light and heralded photon-pairs. In this thesis, we achieved significant advancements in the dynamical engineering of light-matter interactions, paving the way for future scalable photonic systems. By improving optical coherence and enabling interactions between diverse quantum emitters, our findings lay a strong foundation for the next generation of quantum information technologies.