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BIT makes significant progress on research into quantum walk and topological phenomena

release date :2018-09-12 02:33:00  |   [ close window ]ViewCount:

  Translator: News Agency of BIT  Miao Yufei

  Editor: News Agency of BIT


  Recently, professor Zhang Xiangdong’s research group from the school of physics, Beijing institute of technology (doctoral student Wang Bo, young teacher and associate professor Chen Tian and professor Zhang Xiangdong), constructed a two-dimensional quantum walk platform which is suitable for observing the topological interface state. It was in the first experiment on this platform that the topological protective bound state of zero was observed. Relevant research results were published in the recent Physical Review Letters [Phys. Rev. Lett. 121, 100501 (2018)], which was funded by the National Key R&D Plan and the National Natural Science Foundation of China (NSFC). Wang Bo, doctoral student, is mainly responsible for experimental content. Chen Tian,associate professor, is responsible for the theoretical part. They are co-lead authors, and professor Zhang Xiangdong and associate professor Chen Tian are co-corresponding authors.


  Although a series of topological phenomena such as the existence of topological protective bound states were firstly predicted in electronic systems, an increasing number of studies showed that optical systems can provide a platform for observing topological phenomena. Besides, the quantum walk platform based on optical system showed that quantum walk can be used as an effective quantum simulation tool to study the topological phase of time-driven system. However, some current experimental studies were limited to one-dimensional quantum walk. As for 2 d and higher dimensional quantum walk solution, due to the lack of effective control for quantum walk location in space and the inability to synchronously construct areas characterized by different topological phase in the quantum walk system, the topology phenomenon existing in the 2 d and higher dimensional quantum walk has not been experimentally observed.



  Figure 1 Two-bit quantum walk test program



  Figure 2 Experimental observation of robust state and its stability


  Recently, for the first time, a two-dimensional quantum walk system which is suitable for observing the state of the topological interface was constructed. We used both the orbital angular momentum of light and spatial position two degrees of freedom to map the two-dimensional position space of a quantum walk system. By using custom q-plate with multiple patterns and birefringence crystals, we can conduct conditional translation operation in orbital angular momentum of light and spatial position respectively. In order to conduct polarization rotation operation with half wave plate, we achieved two-dimensional quantum walk (as shown in fig.1) experimentally. Since it is convenient to perform different rotation operations for different spatial positions on the platform, we can construct uneven areas in the system to represent different topological phases. On the boundary of the two non-uniform regions, we first observed the anomalous topological protective bound state in the two-dimensional quantum walk and proved the robustness of the bound state under the condition of disturbance and disorder (experimental results shown in fig. 2). In general, it is believed that the bound state of topological protection will appear at the interface of two regions with different topological phases. However, our research results showed that the bound state of topological protection also exist at the regional interface where the corresponding topological invariants are the same (the Chern Number is 0). This phenomenon did not accord with the previous understanding of the Chern numbers in the static system, instead it was a special phenomenon in the periodic driving discrete walk. In the previous optical waveguide system, the researchers could observe the bound states of abnormal topological protection through delicate design. In the quantum walk system, we experimentally proved the existence of the bound state of the anomalous topological protection. Our results enriched people's understanding of the topological phase and furnished a new approach to study the topological phenomena of high-dimensional quantum walk.


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