On the day of the Nobel Prize, Chinese scientists make progress in quantum nonlocal properties

Three scholars were awarded the Nobel Prize in Physics this year for experimentally verifying the theory of quantum nonlocality, which is still an important topic. On the same day the Nobel Prize in Physics was awarded, the team of academician Guangcan Guo of the Chinese University of Science and Technology (CSU) made important progress in quantum localizability, which was published on October 4 in the internationally renowned journal Physical Review Letters and was chosen as the cover article of the issue.

The team, Chuanfeng Li and Yunfeng Huang, in collaboration with Spanish theoretical physicists, experimentally verified true many-body nonlocality based on the theoretical framework of local operation and shared randomness (LOSR, Local operation and shared randomness), and the results showed that all correlations arising in nature cannot be explained by two- or three-body nonlocal correlations.

Quantum mechanics allows for the existence of nonlocal associations between particles, i.e., quantum nonlocality. Quantum nonlocality is an important resource for realizing various quantum information processes. For many-body systems, true many-body nonlocality is considered to be the strongest nonlocal correlation that can be exhibited in many-body systems. The true many-body nonlocality test usually relies on the violation of inequalities in the framework of local operation and classical communication (LOCC, Local operation and classical communication). However, in the test of many-body nonlocality, we usually need to assume that the probability distribution of joint measurements between different observers obeys the non-signaling condition, and classical communication is forbidden, so the true many-body nonlocality based on LOCC is not well defined.

In quantum resource theory, LOCC is not the only free operation that can be used to define entangled resources; after removing classical communication, a more natural definition is based on the theoretical framework of LOSR. In the experiment, the group first verified the non-local nature of the three-body entanglement structure. The LOSR local model of this three-body system allows shared randomness among the three bodies and allows no restrictions on the association between any two bodies, i.e., it allows the association to be quantum or to have a wider range of associations that do not rely on the generalized probabilistic theory described by quantum mechanics.

The group, together with Marc-Olivier Renou and others, improved the LOSR non-local inequality proposed in previous theoretical work, and proposed a more robust inequality for the white noise commonly found in multiphoton experiments, and used the self-developed "sandwich" high-fidelity multiphoton The non-local nature of the true three-body LOSR was verified with a 26.3 standard deviation violation using the self-developed "sandwich" high-fidelity multi-photon entanglement source [PRL115, 260402 (2015)]. The group further experimentally violated the true four-body LOSR inequality in the four-body case, and the experiments excluded the model of localizability based on shared randomness and three-body generalized correlations, demonstrating true four-body nonlocalizability. The results of these experiments show that only two- or three-body nonlocal correlations cannot explain all the correlations arising in nature. The inequalities presented in this work can be extended to the arbitrary N-body case, and the method of preparing high-fidelity entangled sources lays the foundation for experimentally demonstrating nonlocality of a larger number of subsystems.

The co-first authors of the article are Cao Huan, a Ph.D. student at the Key Laboratory of Quantum Information, CAS, Chao Zhang, an associate researcher, and Marc-Olivier Renou at the Institute of Photonic Sciences, Spain. the work was supported by grants from the National Natural Science Foundation of China, CAS, and Anhui Province.