我們用磁光聚(Magneto-Optical Trap, MOT)產生一個低溫銫原子團，有1.49×108
個原子，溫度約為162μK，密度約1010 個/cm3，在低溫銫原子團中，利用兩道雷射光，
Red and Blue laser，Red laser 將低溫銫原子從基態|62S1/2,F=4＞激發至中間態|62P3/2,F=5
＞， Blue laser 將低溫銫原子從|62P3/2,F=5＞激發至最終態|82S1/2,F=4＞，當這兩道雷
射光與銫原子的躍遷頻率產生共振時，銫原子會從|62S1/2,F=4＞態躍遷至|82S1/2,F=4＞
態，然後做階梯式的輻射躍遷，其中從|6281/2,F=4＞衰變至|62P1/2＞ 波長760nm 的螢
光可以被偵測到。
實驗上，我們以chop mod 方式讓實驗在磁光聚光場關閉時進行，利用聲光晶體
將Red and Blue laser 從cw 雷射變為pulse 打入低溫銫原子中，改變兩pulses 打入低
溫銫原子的先後時間差，觀測760nm 螢光的強度，進而可得知原子從|62S1/2,F=4＞態
轉換至|82S1/2,F=4＞態的居量轉換，結果發現在Blue pulse 先Red pulse 190ns 時可得
到較強的銫原子居量躍遷。

Two laser lights, Red and Blue lasers, were used to excite the cold cesium atom from
ground state |62S1/2,F=4＞ to the intermediate state |62P3/2,F=5＞(red laser), and from the
intermediate state |62P3/2,F=5＞ to the final state |82S1/2,F=4＞(blue laser). Cold cesium
atom (temperature ~162　K, density~1010atoms/cm3) was prepared by a cell
magneto-optical trap (MOT). When these two lasers are on resonance, the cold cesium
atom will be transferred from the ground state|62S1/2,F=4＞ to the final state |82S1/2,F=4＞.
The population of the |82S1/2,F=4＞ state can be monitored by the fluoresces from
|82S1/2,F=4＞ decay to |62P1/2＞ at 760nm.
In our experiment, the chopping mod allows us to probe the atoms while the MOT
lasers filed are temporary off. An acoustic optical modulator was used to convert the cw
lasers into pulse lasers to interact with cold cesium atom. Changing the time difference
between the red and blue pulses interact with cold cesium atoms monitoring the intensity
of fluoresces at 760nm, we found that the efficiency of atomic population transformation
from the ground state|62S1/2,F=4＞ to the final state |82S1/2,F=4＞ as a function of delay
time. The result is: to have the best population transformation, i.e. the maximum of
fluorescence at 760 nm, the blue pulse need to be earlier about 190ns before the red pulse
interacting with cold Cesium atoms.

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