Theory Predicts Near-unity Polarized Emission from Excitons in Asymmetric Cavities, Overcoming 50% Light Loss

Single-photon sources, crucial components for quantum technologies and simulators, increasingly rely on excitons within semiconductor quantum dots. Luca Vannucci and Niels Gregersen, from DTU Electro at the Technical University of Denmark, investigate a significant limitation of current designs, namely the unavoidable loss of half the emitted photons when collecting light in a specific polarization. Their theoretical work explores how embedding an exciton within an asymmetric vertical cavity, designed to favour emission of a particular polarization, overcomes this challenge. The researchers identify optimal configurations for preparing the exciton’s quantum state and demonstrate a pathway towards achieving near-unity polarized emission efficiency, offering a substantial improvement for practical quantum devices.

Exciton-Cavity Coupling and Single-Photon Emission

Single-photon sources utilizing excitons within semiconductor quantum dots are promising for photonic quantum computers and simulators. Creating reliable quantum systems requires indistinguishable photons, demanding precise control over the emission process. This work presents a detailed theoretical investigation into single-photon emission from both neutral and charged excitons within a carefully designed cavity, focusing on how the strength of exciton-cavity interaction and polarization selection rules impact performance. Results demonstrate that strong exciton-cavity coupling significantly boosts the single-photon emission rate and improves photon indistinguishability, while polarization selection effectively suppresses unwanted emission pathways. The analysis reveals distinct emission characteristics for neutral and charged excitons, offering opportunities to tailor single-photon sources for advanced quantum applications.

Asymmetric Cavity Boosts Polarized Photon Emission

Scientists developed a theoretical model to enhance the efficiency of single-photon sources using semiconductor quantum dots, addressing a limitation in vertically emitting devices where half of emitted photons are typically lost due to polarization. The study focused on an asymmetric vertical cavity designed to favor light emission in a specific polarization, enabling near-unity efficiency. Researchers analyzed the quantum dynamics of an exciton within the cavity, calculating coupling constants, loss rates, and background emission rates to establish a precise electromagnetic environment. They discovered that maximum emission into one polarization and zero emission into the orthogonal polarization occurred when the cavity axes aligned with the exciton axes and the fine-structure splitting was zero.

Introducing a non-zero fine-structure splitting and rotating the cavity axes induced a precession of the exciton state, reducing emission into the original polarization and increasing it into the orthogonal polarization. To further enhance efficiency, scientists introduced an energy splitting between the cavity modes, mimicking elliptical micropillars, and achieved emission levels exceeding 0. 8. Detailed analysis revealed that a large fine-structure splitting is crucial for a precession period shorter than the exciton lifetime, and narrowing the cavity bandwidth and detuning one mode could further suppress unwanted polarization, reaching a maximum of 0. 94 under optimized conditions. This work establishes a pathway toward highly efficient, polarization-controlled single-photon sources for quantum technologies.

Quantum Dots and Cavity Enhancement of Emission

A substantial body of research focuses on single-photon sources based on quantum dots embedded within optical cavities, aiming to enhance light-matter interaction, control emission properties, and purify emission from background noise. Researchers commonly utilize micropillar and nanowire cavities, exploring strategies to control polarization and achieve indistinguishable photons, with increasing efforts towards on-chip integration for scalability. Some research explores using electrical fields to control the quantum dot’s emission properties, such as fine structure and polarization, and advanced excitation techniques are being developed to improve source performance. Theoretical modeling plays a crucial role in understanding and optimizing device performance, encompassing various material systems for both quantum dots and cavities. This comprehensive body of work highlights the vibrant and rapidly evolving field dedicated to developing high-performance single-photon sources for quantum technologies, emphasizing the importance of quantum dots, cavity quantum electrodynamics, and advanced control techniques.

Asymmetric Cavities Enhance Photon Polarization Efficiency

This research presents a theoretical study of single-photon emission from a quantum dot embedded within a specially designed microcavity, demonstrating a pathway towards highly efficient and indistinguishable single-photon sources. Scientists have shown that utilizing an asymmetric cavity shape significantly enhances the polarization of emitted photons, approaching near-unity efficiency, a substantial improvement over the 50% limit found in conventional symmetric cavities. This enhancement stems from carefully controlling the interaction between the quantum dot and the cavity, splitting the degeneracy of the light modes to favor emission in a specific polarization. The team developed both a comprehensive numerical model and an analytical solution, valid under weak coupling conditions, to accurately describe the quantum dynamics of this system.

Their findings reveal that aligning the cavity axes at 45 degrees with respect to the quantum dot axes, alongside a sufficiently large fine-structure splitting, is crucial for maximizing polarized emission from a neutral exciton. Notably, these specific alignment requirements do not apply when working with charged excitons, offering flexibility in device design. This work represents a significant step towards building the robust, high-performance single-photon sources essential for advancements in quantum technologies.