CSU realizes device-independent true many-body entanglement test
The first experimental implementation of device-independent true many-body entanglement test has been achieved by the team of academician Guangcan Guo at the Chinese University of Science and Technology. The team, including Chuanfeng Li, Yunfeng Huang and Geng Chen, in collaboration with Swiss scholars, has constructed a new method for testing the true multibody entanglement state, which can test the true entanglement property of multibody systems without making any assumptions about the measurement devices. This is the first international experimental work to test the true entanglement of an arbitrary many-body system, and the results were published in the internationally recognized academic journal Physical Review Letters on Oct. 31.
If a many-body system is arbitrarily divided into two parts, and there is entanglement between these two parts, then this property of the many-body system is true many-body entanglement. True many-body entanglement is the strongest existent form of quantum entanglement and is an important resource for realizing quantum information processes. To examine the true entanglement property of a many-body system, it is usually necessary to know the dimensionality of the system in advance and to ensure that the measurement equipment can accurately implement various measurements. However, it is possible that real physical systems have hard-to-know degrees of freedom and that errors in the actual measurement equipment can cause misclassifications. To resolve these difficulties, in principle, device-independent measurements can be used, and a test of the entanglement nature of the system can be made by analyzing only the violation of Bell's inequality by the measurement results without making any assumptions about the system. However, limited by the difficulty of constructing the multi-body Bell inequality, the device-independent test for arbitrary true multi-body entangled systems is still a pressing problem to be solved.
By resolving the internal structure of the multibody system, the research group delineates the minimum connected set and the complete connected set, respectively, and then applies the two-body Bell inequality to the test of multibody entangled systems. This method can test true many-body entanglement of arbitrary size and form, and measure the weight of true many-body entanglement in many-body entanglement, using only the conventional CHSH-type two-body Bell inequality. The experimental results show that a fully connected ensemble is very resistant to noise, and a minimally connected ensemble is robust to noise while achieving higher efficiency, i.e., the number of measurement devices does not grow exponentially with the system size. Experiments were conducted to examine several important forms of many-body entangled states, and the measurements of both connected sets can prove the existence of true many-body entanglement and estimate the weight of true many-body entanglement among them.
More importantly, the group experimentally examined a very weakly entangled state that was shown to be unable to violate the standard many-body Bell inequality, making it difficult to determine its true entanglement properties by device-independent methods until then. In the experiments of the research group using the new structure-resolving method, this weakly entangled state transcends the local criterion and is confirmed to have true many-body entanglement.
The co-first authors of the article are Chao Zhang, an associate researcher at the Key Laboratory of Quantum Information, CAS, and Wenhao Zhang, a postdoctoral fellow. This work was supported by grants from the National Natural Science Foundation of China, the Chinese Academy of Sciences and Anhui Province.
Figure: (a) Structural resolution of the four-body entanglement. The black dots represent particles and the connected lines represent entanglement. The four-body entanglement can be decomposed into four-body true entanglement, two two-body entanglement, etc.; (b) the smallest connected set; (c) the complete connected set.
Links to paper：
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.129.190503
