雷射誘發透明冷卻銣-87

Abstract

本實驗旨在導入並進一步發展電磁誘導透明冷卻（Electromagnetically Induced Transparency Cooling，EIT Cooling）技術，以提高玻色–愛因斯坦凝聚（Bose–Einstein Condensation，BEC）的製備效率；同時系統性探討利用純雷射冷卻實現玻色–愛因斯坦凝聚的可行性及其冷卻極限。本實驗首先利用磁光阱（magneto-optical trap, MOT）收集冷原子，並將其溫度降至接近都卜勒極限溫度（Doppler temperature, \\(T_D\\)）。接著以次都卜勒冷卻（sub-Doppler cooling）技術在自由空間中將原子進一步冷卻至約 \\(6~\\mu\\text{K}\\)。隨後，我們將約 \\(2\\times10^{6}\\) 顆冷原子載入光偶極阱（optical dipole trap, ODT）；在維持約 \\(10~\\mu\\text{K}\\) 的低溫下，成功將原子密度提升至 \\(3\\times10^{13}\\,\\text{cm}^{-3}\\)。

在次都卜勒冷卻階段，本實驗引入 \\(\\Lambda\\)-型增益灰稠漿冷卻（\\(\\Lambda\\)-enhanced gray molasses cooling, GMC）與進階電磁誘導透明冷卻（EIT cooling）技術。透過光場中原子與激發態之去耦合效應形成暗態（dark state），避免冷原子持續散射光子，進而實現甚至突破反衝溫度（recoil temperature, \\(T_r\\)）之極限。對銣原子而言，\\(T_r \\approx 0.36~\\mu\\text{K}\\)。此類雙光子冷卻技術可減少對高功率雷射以產生光偶極阱及效率較差之蒸發致冷的依賴，為以純雷射冷卻實現玻色–愛因斯坦凝聚（BEC）奠定實驗基礎。

This experiment aims to introduce and further develop Electromagnetically Induced Transparency (EIT) cooling to improve the efficiency of producing Bose–Einstein condensation (BEC); it also systematically explores the feasibility and ultimate cooling limits of achieving BEC solely through direct laser cooling.

The experiment begins by collecting cold atoms in a magneto-optical trap (MOT) and cooling them to near the Doppler temperature (\\(T_D\\)). Subsequent sub-Doppler cooling in free space lowers the atomic temperature further to about \\(6~\\mu\\text{K}\\). Approximately \\(2\\times10^{6}\\) of these atoms are then loaded into an optical dipole trap (ODT); while maintaining a temperature of roughly \\(10~\\mu\\text{K}\\), the atomic density is increased to \\(3\\times10^{13}\\,\\text{cm}^{-3}\\).

During the sub-Doppler cooling stage, we implement \\(\\Lambda\\)-enhanced gray molasses cooling (GMC) and advanced Electromagnetically Induced Transparency (EIT) cooling. These two-photon techniques create dark states by decoupling atoms from their excited states, thereby suppressing photon scattering and enabling temperatures at or even below the recoil limit (\\(T_r\\)); for rubidium, \\(T_r \\approx 0.36~\\mu\\text{K}\\). This approach lowers our reliance on high-power lasers for optical dipole trapping and on the relatively inefficient evaporative cooling, thus laying the groundwork for achieving Bose–Einstein condensation (BEC) purely via laser cooling.