超高光學深度量測之二能階吸收光譜與光學前驅波技術之比較研究

Abstract

在量子光學與量子資訊領域中，冷原子介質的高光學深度（Optical Depth,OD）是實現高效能光記憶體與量子通訊的關鍵。然而，傳統以二能階吸收光譜為基礎的頻域掃描方法，在高OD 冷原子團的量測上效率低下，且易受透鏡效應影響。為解決此問題，本研究提出一種以時間域超輻射（Superradiance, 又稱前驅波Precursor）現象為基礎的新型OD 量測技術。該方法僅需注入一個短脈衝並記錄其時域響應，即可經由Bessel 函數解析擬合反推出OD，省去傳統掃頻過程。本論文建構出結合磁光阱（MOT）與壓縮技術的冷原子平台，成功產生OD∼ 103 的原子團，並設計具有奈秒等級解析度的探測光路。透過理論推導與實驗比對，證實時間域超輻射法與傳統二能階吸收光譜法在中高OD 條件下具有良好的一致性，且可顯著縮短量測時間。此技術未來可應用於即時監測、動態調控，並可望與量子記憶體、單光子源等元件整合，拓展其在量子科技中的應用潛力。In quantum optics and quantum information science, cold atomic media with high optical depth (OD) are essential for realizing efficient quantum memories and long-distance quantum communication. Traditional OD measurement methods based on two-level absorption spectroscopy suffer from low efficiency and distortion in the high OD regime due to lens effects. To overcome these limitations, this thesis introduces a time-domain technique based on the superradiant (precursor) response of cold atoms under a single probe pulse.We establish an experimental platform combining a high-OD cold cesium atomic ensemble (OD ∼ 103) via a magneto-optical trap (MOT) with compression beams and a nanosecond-scale probe pulse system. A theoretical model is developed using Maxwell–Bloch equations, yielding an analytical expression for the superradiant waveform via Bessel functions. By fitting the temporal response, the OD can be accurately retrieved without frequency scanning. Experimental results show strong agreement with theoretical predictions and demonstrate a linear correspondence with conventional spectroscopy, validating the proposed method’s effectiveness and speed. This approach offers promising applications in real-time monitoring, system optimization, and integration with single-photon sources and quantum memory devices.