銣87原子之玻色-愛因斯坦凝聚體的製備以及其拉曼躍遷之研究

The Production of 87Rb Bose-Einstein Condensates and Their Raman Transitions

于金業(Chin-Yeh Yu)

指導教授 ： 劉國平

共同指導教授 ： 林育如(Yu-Ju Lin)

國立臺灣大學/理學院/物理研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.02013

摘要

我們實現光阱中的銣87原子的玻色-愛因斯坦凝聚體並對其做拉曼躍遷(Raman transition)。在實驗中，我們每14秒為一個週期可以產生一個凝聚體，實驗程序如下。首先，我們在磁光阱中(magneto-optical trap)收集冷原子並利用光學糖蜜(optical molasses)來做冷卻。接著，我們將這些原子轉移到磁阱中壓縮以提高密度並利用一個射頻磁場來做蒸發式冷卻。接著我們再轉移原子到一個交叉光偶極阱(crossed optical dipole trap)中並藉由降低雷射光強及增加位能梯度來減少光阱深，並藉此做進一步的蒸發冷卻。最後，我們可以產生原子數 N=2x10^5 的純玻色-愛因斯坦凝聚體。在產生凝聚體後，我們利用兩道拉曼雷射光對原子產生一個雙光子的拉曼躍遷，並利用此躍遷將原子F=1 中的三個mF自旋態給耦合起來。透過觀察原子在三個自旋態間的轉換，我們發現我們系統有個大小約38 kHz的失諧雜訊(detuning noise)，部分是由磁場的雜訊造成的。在未來，我們會將其中一道拉曼光轉換成攜帶有軌道角動量的拉蓋爾-高斯光(Laguerre-Gaussian beam)。如此一來，透過拉曼躍遷我們就可以將軌道角動量從光子轉移給原子。此外我們會進一步將原子轉移到一個環形位能阱中。如此便可在一個環形凝聚體中產生持續性的原子流。

關鍵字

玻色-愛因斯坦凝聚體 ； 雷射冷卻 ； 蒸發式冷卻 ； 磁光阱 ； 光偶極阱 ； 拉曼躍遷

並列摘要

We perform Raman transitions in Bose-Einstein condensates (BECs) of Rubidium 87 atoms in an optical dipole trap. We first build a system to produce BECs every 14 s. This is started by first collecting cold atoms in a magneto-optical trap (MOT) and cooling them in the optical molasses. We then load the atoms to a compressed magnetic quadrupole trap to increase density and to perform forced rf-evaporation. We next transfer the atoms to a crossed optical dipole trap and do a further evaporation by lowering the dipole beams power and increasing the potential gradient by quadrupole current and thus achieve BECs with N= 2x10^5 atoms. Once the BEC is produced, we give these atoms a two-photon Raman pulse, and observe the atoms flopping in three mF spin states of the F=1 hyperfine manifold. We find that the flopping in these states is affected by a ~38 kHz detuning noise, which possibly comes from our magnetic field noise. In the future, we will load the atoms to a ring shape potential and transfer angular momentum to the atoms by making one of the Raman beams into the Laguerre-Gaussian beam. With the angular momentum, we expect to generate azimuthal vector potentials for atoms in the ring trap, and produce persistent atomic currents.

並列關鍵字

Bose-Einstein condensate (BEC) ； laser cooling ； evaporative cooling ； magneto-optical trap ； magnetic quadrupole trap ； optical dipole trap ； Raman transition

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