CSU experiment tests the non-local nature of quantum networks
The team of academician Guangcan Guo at the University of Science and Technology of China has made important progress in the study of nonlocal quantum networks. （The results were published on July 13.
The results were published on July 13 in Physical Review Letters, a leading international journal.
Bell nonlocality is an important foundation of quantum mechanics and quantum information science. In the last decade or so, the study of nonlocality is no longer limited to the two-body problem, but has turned to more complex structures. Such experiments involve multiple independent entangled sources, which are experimentally distributed and entangled in multiple directions and eventually form nonlocal quantum networks. Because the nonlocal nature of quantum networks demonstrated breaks the framework of traditional Bell experiments, there are significant challenges in both physical theory and experiment.
Current experimental entanglement measurements connecting independent entangled sources are mainly using standard Bell state measurements, and there is a lack of research on more general entanglement measurements and the nature of the nonlocal quantum networks they create. Symmetric joint measurements are an important class of entanglement measurements that are fundamentally different from Bell state measurements. It has elegant and natural symmetries that can be exploited as a quantum information resource. Arbitrary symmetric joint measurements require cascade operation of control non-gates and control phase gates, which poses a very big challenge to linear optical quantum networks that can only probabilistically implement control gates between different photons.
In this experiment, the group first prepares a super-entangled pair that produces two photons with states that are entangled in the polarization degree of freedom and the path degree of freedom, respectively. Then, the polarization and path degrees of freedom of the same photon are measured by a universal quantum line to achieve an arbitrary symmetric joint measurement. Since deterministic control gates can be realized between different degrees of freedom of the same photon, the group has achieved arbitrary symmetric joint measurements deterministically with a self-designed polarization-path exchange device. The experimental results show that the fidelity of the symmetric joint measurement achieved by the group is 97.4%. The group used the symmetric joint measurement to realize the entanglement exchange and investigated the Bilocal Bell inequality and Full network nonlocality. The experimental results demonstrate completely different properties from the standard Bell state measurement of nonlocal quantum networks.
This result is the first step beyond the standard Bell-state measurement-based nonlocal quantum networks, proving that different entanglement measurements will construct nonlocal quantum networks with different advantages, and providing a technical route to build nonlocal quantum networks with different structures.
The first authors of the article are Cenxiao Huang, a PhD student and Xiaomin Hu, PhD, an associate researcher at the Key Laboratory of Quantum Information, Chinese Academy of Sciences. This research was supported by the Ministry of Science and Technology, the State Foundation of China, the Hefei National Laboratory and the Chinese Academy of Sciences, and researcher Biheng Liu is a Chung-Ying Young Scholar at the University of Science and Technology of China.
Figure 1: Schematic diagram of the experimental principle with symmetric joint measurements acting on quantum bits 2 and 3.
Figure 2: Diagram of the experimental setup.
Figure 3: The left panel shows the experimental results of Bilocal Bell inequality and the right panel shows the experimental results of Full network nonlocality.
Link to article：
https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.129.030502
