Zi xuan ku zhong zhong xin zi xuan tui xiang gan de li lun yan jiu

Ph.D.The quantum science and technologies, including quantum computing, quantum communication, and quantum sensing, have recently attracted considerable attention, from not only the scientific community but also the industrial world and the general public. The quantum features of qubits especially superposition and entanglement are the basis of quantum technologies. However, the quantum coherence of the qubits is fragile since the qubits are inevitably interacting with environments. Therefore, studying the decoherence mechanisms and developing methods to protect the coherence are of vital importance.In order to understand the mechanisms of decoherence, many methods have been developed. For environments that can be approximated as Gaussian and Markovian, the semi-classical model, which utilizes the properties of Gaussian noise, can be used to efficiently study the decoherence. However, for more general quantum systems, quantum feedbacks from the central qubits to the environments are important, which makes the semi-classical model invalid and full-quantum models necessary. The straightforward way to simulate a quantum system is the exact numerical simulation, such as diagonalization and time-integration. However, the number of qubits that can be exactly simulated is limited, which is often insufficient for realistic systems. In order to tackle this many-body-physics difficulty, several theories have been developed, such as, the pair-correlation approximation, cluster expansion (CE), linked cluster expansion (LCE) and cluster correlation expansion (CCE), among which CCE is the most successful one.However, the same as other cluster expansion based theories, straightforward CCE does not work in the regime near clock transitions (CTs). The CTs occur where the transition frequencies are insensitive to the first order of field fluctuations. Therefore, CTs have relatively long coherence times. However, the mediated long-range interactions induced by the second order effect of field fluctuation make the CCE diverging. In this thesis we modify the CCE theory so that the decoherence of central spin near CTs can be efficiently simulated.Another way to prolong the coherence is to reduce the broadening of noises from the environment by dynamically polarize the nuclear spins in the bath (DNP). Here we propose a new DNP mechanism which can polarize the nearby nuclear spins in NV centers near zero field and lock the spin transition to CTs. The DNP channel is established by sequential weak measurements of the nuclear spins followed by entropy dumping through initialization of the electron spin.We also develop a theory to study the decoherence of multi-level central spins in nuclear spin bath. The theory is applied to study the electron spin echo envelope modulation (ESEEM) of Bi:Si in low field, which can be used to measure the gyromagnetic ratio and abundance of nuclear spins. We find that the effects of pair-correlations (CCE-2) are negligible in the experimental regime. By comparing with experimental results and utilizing the multi-level CCE-1 models, we further find that the close strongly coupled nuclear spins can only induce fast oscillations and a negligible initial drop which are undetectable in the experiments.近些年量子科學與技術（比如量子計算和量子傳感）引起了不僅包括科學界，還包括工業界和大眾在內的廣泛關注。量子技術之所以有很大的前景，主要是因為量子比特相比傳統比特有一定的優勢。這一優勢很大程度上來源於量子比特具有的相干性。然而，因為量子比特不可避免的與環境糾纏在一起導致這一特性非常脆弱，很容易發生退相干。因此，搞清退相干的來源以及發展保護相干性的方法對量子研究與應用來說是至關重要的。人們已經發展了多個理論來研究退相干的來源。比如說，對於高斯與瑪律可夫環境，我們可以用半經典模型（利用高斯雜訊的性質）來方便的研究其退相干。然而對於更一般的量子系統，比如固體缺陷，中心比特對環境的反作用很重要，這使得半經典模型不再適用，我們就必須使用全量子的模型。最簡單直接的方法是使用嚴格數值模擬，比如嚴格對角化或時間積分算法。然而，嚴格數值模擬只能處理很少數的量子比特，對實際的系統是遠遠不夠的。為了解決這一問題，幾種近似理論被發展起來，包括對關聯近似，集群展開（CE），連結集群展開（LCE）和集群關聯展開（CCE）。然而，與其他基於集群擴展的理論一樣，直接的CCE 不適用於接近鐘躍遷（CTs）的情況。在CTs 處，躍遷頻率對一階噪聲不敏感。因此，CT 對於保護中心自旋的一致性很重要。然而，超精細介導的二階效應引起的長程相互作用使CCE 發散。本文對CCE 理論作了進一步的發展，以有效地模擬CTs 附近中心自旋的退相干。另一種延長相干性的方法是通過動態極化核自旋（DNP）來減少環境噪聲的展寬。在這裡我們提出了一種新的DNP 機制，它可以在不知道具體耦合的情況下極化NV 中心附近的核自旋並且可以將外場鎖定在CTs 上。DNP通道是由核自旋的連續弱測量和通過電子自旋初始化的熵傾銷建立的。我們還發展了適用於多能級的CCE 模型，並應用到低場下Bi:Si 的電子自旋回波包絡調製（ESEEM），它可以用來測量自旋的迴旋比和豐度。我們發現，在實驗條件下，對關聯（CCE-2）的影響可以忽略不計。通過與實驗結果的比較，利用CCE-1 多能級模型，我們進一步發現，緊密強耦合的核自旋只會引起實驗中無法檢測到的快速振盪和可忽略的初始下降。Zhang, Gengli = 自旋庫中中心自旋退相干的理論研究 / 張耿立.Thesis Ph.D. Chinese University of Hong Kong 2020.Includes bibliographical references (leaves 111-124).Abstracts also in Chinese.Title from PDF title page (viewed on November 4, 2021).Zhang, Gengli = Zi xuan ku zhong zhong xin zi xuan tui xiang gan de li lun yan jiu / Zhang Gengli