單一量子點之磁光與糾纏態特性(I)

Abstract

在此為期兩年計畫中，我們將進行自組式單一量子點的磁光性質與其所產生的糾纏光子對之研 究，主題分為兩部分，第一部分基於先前對磁光性質的觀測中，negative trion 不同與傳統反磁性的 順磁化行為，我們將深入探討並建立模擬多體行為的物理模型，來了解此順磁化行為的成因。第二 部分為在量子點內因biexciton 與exciton 所形成的雙能階系統(two-level system)而產生的雙光子糾 纏態(entangled photon pair)研究。一般認為由於量子點幾何的不對稱性，造成exciton 能階分裂(fine structure splitting，FSS)，使其糾纏態特性被破壞。在我們的計畫中，將調變此能量分裂，探討其 與雙光子糾纏態的關係;我們將利用施加水平磁場與電場兩種方法來調變FSS，由於在水平磁場的 環境下，兩個分裂的exciton 能階位置移動速率不同，所以在某些量子點中及適當的磁場強度下， FSS 有機會消失，另一方面，利用水平電場控制量子點內載子波函數的幾何形狀對稱性，也可以達 到調變FSS 的目的，且更具應用潛力。上述兩種方式都可以調變FSS，不過顯然其物理成因並不 相同，我們將建立糾纏態光子對的量測系統，來探討並比較這兩種方式在無FSS 時，量子點雙光 子糾纏態的情況，並據以評估在量子資訊處理應用時，產生糾纏態光子對的較佳方式。另外，我們 也將嘗試在非正向基板上成長量子點，來控制量子點的對稱性並探討其與糾纏態之間的關聯性。

In this two-year project, we shall study the magneto-optics in single quantum dots (QDs) and the entangled photon pairs generated by them. The first topic will be based on our previous magneto-photoluminescence observation in single QDs. Interestingly, the negative trion shows paramagnetic behavior, which is very different from conventional results. We shall investigate the phenomena further and build up a many-body model considering the inter-particle interaction and their response to the external magnetic field. The lager second part will focus on the entanglement of two-level system formed by exciton and biexciton in QDs. People believe that the shape asymmetry of QDs causes the energy level splitting of exciton (fine structure splitting, FSS) and then destroys the entanglement property. In our plan, we shall modulate the splitting energy and study the relationship between it and the entanglement. Two methods, applying in-plane magnetic field and gated lateral electric field, will be used to tune the FSS. By applying in-plane magnetic field, two split levels shift with different rate so, in some QDS, the FSS goes to zero at a proper magnetic intensity. On the other hand, even a more promising way in future application, we can modulate the carrier wavefunctions in QDs with a lateral electric field by metal gates to reduce the FSS. Finally, we shall setup a measurement system to characterize the entanglement of generated photon pairs to discuss the situation of zero FSS in these two methods, also to evaluate the better way to generate the entangled photon pairs for applications in quantum information processing. In addition, we shall also grow the QDs on an off-angle substrate to control its anisotropy and study the connection between the shape asymmetry and the FSS.