First of its kind! Guo Guangcan's team at CSU verifies quantum nonlocality of star networks

Recently, academician Guangcan Guo's team at the Chinese University of Science and Technology (CSU) has made important progress in the study of nonlocality of quantum networks. The team, including Chuanfeng Li, Yunfeng Huang and Chao Zhang, collaborated with theoretical physicists from Spain and Switzerland to experimentally verify the full network nonlocality in star-shaped quantum networks for the first time.

 

The results were published in the internationally recognized journal Nature Communications on April 14.

 

 

Quantum technology promises interesting new approaches to computing, communication, sensing and high-precision measurements; one branch that is becoming increasingly interesting is quantum networks.

 

A quantum network is an infrastructure that interconnects distant quantum devices through a given network architecture. The connections can include quantum communication channels or the distribution of entangled particles between different devices. In recent years, quantum network technology has been developing rapidly, and potential future applications of quantum networks include a quantum Internet connecting quantum devices, which is closely related to the development of quantum repeaters for long-distance communication.

 

However, quantum networks are not only of technical interest. In the last decade, networks based on the distribution of entangled particles from multiple independent sources have become a relevant platform for the study of fundamental physics.

 

Networks consisting of independent sources of entangled particles connecting distant users are a rapidly evolving quantum technology and an increasingly promising testbed for fundamental physics. Here, the research team addresses the authentication of its post-classical properties by demonstrating full-network noncriticality. Full-network nonlocality goes beyond standard nonlocality in networks by falsifying any model in which at least one source is classical, even if all other sources are subject only to the no-signal principle.

 

 

Three-branch star networks. The binary physical system generated at the source Si is assigned to the yellow party, and measurements are performed in the received system. Branches can choose to perform measurements on the received system, indicated by the circle denoted by xi.

 

Ultimately, the team reports on the full network nonlocality observed in a star-shaped network with three independent optical quantum sources and three quantum bits entangled in exchange for joint measurements. This result shows that experimental observation of full network nonlocality beyond the double-localization is possible under current technological conditions.

 

Among them, Bell nonlocality has been a hot research topic in the field of quantum information.

 

In recent years, nonlocality in more complex quantum networks containing multiple independent sources has been explored. Due to the inclusion of multiple independent hidden variables, completely new quantum correlations can be generated in quantum networks that are distinct from the traditional Bell nonlocality. Among them, the Bilocal model is the simplest and most studied quantum network, i.e., two independent entangled sources assign entangled pairs to three observers, similar to the entanglement exchange scenario, where the intermediate node receives two particles and makes a Belgian measurement resulting in nonlocal correlation of the whole network.

 

However, the previously defined network nonlocality cannot portray the nonclassicality of all sources in the whole network, which in most cases can degenerate to the violation of the standard Bell inequality, and the intermediate nodes do not need to employ entanglement measurements. Therefore, physicists have proposed the concept of Full network nonlocality, which requires that all sources in the network distribute nonclassical resources that can be used to certify the nonclassical nature of the full connectivity in the network. Currently, full network nonlocality has been tested only in the simplest bilocal model.

 

In this work, the research group achieves a theoretical and experimental breakthrough to successfully verify the full network nonlocality in more complex star networks, respectively. Theoretically, the group proposes a new criterion for full network nonlocality using only Independence and No-signaling principles without requiring the network to follow quantum mechanical principles. Experimentally, the group built a star-shaped network with three peripheral nodes using a high-quality "sandwich" entangled light source, and achieved a high-quality Greenberger-Horne-Zeilinger state at the central node. Projection measurements (shown in Fig.)

 

Schematic diagram of the star-shaped quantum network and experimental setup

 

Detailed experimental setup

 

Experimental results

 

In the end, the experimental results violate the full network nonlocality criterion at a level of more than 10 standard deviations. This is the first verification of full-network nonlocality in a complex network, which is important for the application of large-scale quantum networks, especially in the security assurance of quantum communication protocols.

 

Reference links:

[1] https://news.ustc.edu.cn/info/1055/82499.htm

[2] https://www.nature.com/articles/s41467-023-37842-w#Fig3