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陳數是表征物理系統拓撲相的不變量，最近由東京工業大學的研究人員以受控方式進行了調整。他們在電子核自旋系統（即金剛石中的氮空位中心）中實現了這一壯舉，觀察了從零到三的陳數。這項工作為探索奇異拓撲及其在拓撲量子信息中的應用打開了大門。
陳數是一種不變的屬性（或“不變量”），它表征了各種物理系統中稱為“拓撲相”的獨特狀態。簡而言之，陳數提供了對材料內部電子行為及其集體屬性的深入了解。
科學家試圖通過調整陳省身數來研究不同拓撲相之間的轉變，以進一步闡明物質的性質。然而，系統拓撲對外部干擾的魯棒性使其在實驗上具有挑戰性。盡管已經建立了理論基礎，但在凝聚態物質系統中很少通過實驗觀察到更高的陳數。然而，材料科學和實驗技術的最新進展開辟了新的可能性。 最近，包括美國馬里蘭大學物理系 Walsworth 教授和日本東京工業大學電氣與電子工程系 Keigo Arai 副教授在內的國際研究團隊進行了探索， 與金剛石氮空位 (NV) 中心相關的電子核自旋系統中陳數的躍遷。他們的研究成果發表在 npj Quantum Information 期刊上。 “NV 中心是金剛石晶格中的缺陷，由一個氮原子和一個空晶格位點組成。該系統由于其可控的電子和核自旋自由度，為研究拓撲相提供了一個獨特的平臺?！?/span> 研究人員通過自旋控制微波改變控制哈密頓量（用于解決動力系統最優控制問題的函數）的參數，以操縱陳數，該數代表控制哈密頓參數球內包含的簡并數。因此，可以通過調整該球體的半徑和偏移來引起不同拓撲相之間的轉變。
接下來，該團隊采用實驗技術和數值模擬相結合的方式來表征系統的最終行為，觀察從零到三的陳數。此外，測量的拓撲相圖與數值模擬一致，并且可以映射到相互作用的三量子位系統上。最后，研究人員證明，NV 系統可以獲取更高的陳數，為探索更復雜的拓撲相鋪平道路。 不過，這項工作的新穎之處不僅在于觀察陳數的轉變，還在于證明其可控性。在單個系統內調整陳數的能力為實際應用開辟了途徑?！袄纾孔臃闯；魻柦^緣體中的高陳數相有望實現低功耗電子產品。隨著陳數的增加，普通金屬電極和手性邊緣通道之間的接觸電阻會降低，這使其成為未來電子產品的一個有吸引力的平臺 ”，Arai 博士指出。 事實上，NV中心系統內陳數的可調性為探索奇異拓撲及其在拓撲量子信息中的應用提供了令人興奮的可能性。這可能會推動量子計量學、下一代電子學、自旋電子學和量子計算領域的發展。 Tuning the Chern number in the nitrogen-vacancy center in diamond by Tokyo Institute of Technology The Chern number, an invariant quantity that characterizes topological phases in physical systems, was recently tuned in a controlled fashion by researchers from Tokyo Tech. They achieved this feat in an electronic-nuclear spin system, namely the nitrogen-vacancy center in diamond, observing Chern numbers from zero to three. This work opens doors to exploring exotic topology and its applications in topological quantum information. The Chern number is a non-changing property (or an "invariant") that characterizes unique states called "topological phases" in various physical systems. Put simply, Chern numbers provide insights into the behavior of electrons and their collective properties inside the material. Scientists try to investigate transitions between different topological phases by tuning the Chern number to shed further light on the properties of matter. However, the robustness of the system topology to external disturbances makes it experimentally challenging. Despite an established theoretical groundwork, higher Chern numbers have rarely been observed experimentally in condensed matter systems. Nevertheless, recent advancements in materials science and experimental techniques have opened up new possibilities. Recently, an international team of researchers, including Professor Walsworth from the Department of Physics at University of Maryland in the U.S. and Associate Professor Keigo Arai from the Department of Electrical and Electronic Engineering at Tokyo Institute of Technology (Tokyo Tech) in Japan, has explored the transitions of the Chern number in an electronic-nuclear spin system associated with the nitrogen-vacancy (NV) center in diamond. Their work is published in the npj Quantum Information journal.
"The NV center, a defect in the diamond lattice, consists of a nitrogen atom coupled with a vacant lattice site. This system provides a unique platform for investigating topological phases owing to its controllable electronic and nuclear spin degrees of freedom," explains Dr. Arai. The researchers varied the parameters of a control Hamiltonian (a function used to solve a problem of optimal control for a dynamical system) through spin-control microwaves to manipulate the Chern number, which represented the number of degeneracies enclosed in a control Hamiltonian parameter sphere. Consequently, transitions between different topological phases could be induced by adjusting the radius and offset of this sphere.
The team next employed a combination of experimental techniques and numerical simulations to characterize the system's resulting behavior, observing Chern numbers from zero to three. Additionally, the measured topological phase diagram was in agreement with the numerical simulations and could be mapped onto an interacting three-qubit system. Finally, the researchers demonstrated that the NV system could enable access to even higher Chern numbers, paving the way for exploring more complex topological phases. The novelty of this work, however, lies not only in observing the transitions of the Chern number but also in demonstrating its controllability. The ability to tune the Chern number within a single system opens up avenues for practical applications. "For instance, high Chern number phases in quantum anomalous Hall insulators hold promise for low-power-consumption electronics. As the Chern number increases, the contact resistance between normal metal electrodes and chiral edge channels decreases, making it an attractive platform for future electronics," points out Dr. Arai. Indeed, the tunability of the Chern number within the NV center system offers exciting possibilities for exploring exotic topologies and their applications in topological quantum information. This could potentially advance the fields of quantum metrology, next-generation electronics, spintronics, and quantum computation.
來源 Source：https://phys.org/news/2023-09-tuning-chern-nitrogen-vacancy-center-diamond.html
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