Chinese University of Science and Technology uses quantum measurement to achieve quantum orientation

[ Instrument Network Instrument R & D ] Guo Guangcan, an academician of the Chinese Academy of Sciences and a professor at the University of Science and Technology of China, made new progress in quantum orientation research. The team Li Chuanfeng and Xiang Guoyong's research team collaborated with Zhu Danjun from Fudan University and Shang Jiangwei from Beijing Institute of Technology to achieve efficient quantum orientation based on experiments with quantum entanglement measurement technology. The research results were published online February 13 in the international journal Physical Review Letters.

The quantum directional task refers to the sender Alice using quantum resources to send an arbitrary direction in space to the receiver Bob. It has important uses in areas such as positioning and navigation. A simple quantum orientation scheme is that Alice sends particles with spin to Bob, and the direction of the particle's spin is the direction to send.

As early as 1999, Nicolas Gisin, a professor at the University of Geneva, Switzerland, discovered that there are novel phenomena when using two spin particles for quantum orientation. Alice uses two spin particles to encode one direction. There can be two encoding methods, namely spin parallel encoding and anti-parallel encoding. It is found that when Bob uses classic local measurement methods to extract information, the information transmission efficiency of anti-parallel coding and parallel coding is the same. When Bob uses the quantum measurement method, anti-parallel coding is more efficient. There is no entanglement of quantum states in Alice's two encoding methods, so this anomaly comes from the existence of quantum entanglement in the quantum measurement of Bob's decoding direction information. Since quantum entanglement measurement is difficult to achieve, there has been no reliable experimental verification of this quantum orientation scheme for two decades.

Quantum entanglement can exist in both quantum states and quantum measurements. Quantum entangled states are widely known and can be used in processes such as quantum communication and quantum computing. Experimental research on quantum entanglement measurement has just begun. Xiang Guoyong and others have developed quantum entanglement measurement technology based on photon quantum walking in recent years. This technology has the advantages of high fidelity and no need for post selection. They adopted this technique to improve the precision of quantum state measurement [Nature Communications 9,1414 (2018)] and reduce the reaction force of quantum measurement in quantum thermodynamics [Science Advances 5,3 (2019)]. Recently they applied the technology to research on quantum orientation.

Xiang Guoyong and others cleverly used the polarization and path of a single photon to implement two spin bits, and the half-wave plate could be used to achieve parallel and anti-parallel encoding of spins. Then with the help of the certainty of photon quantum walking, the optimal entanglement measurement of quantum states under parallel coding and anti-parallel coding is realized. Experimental results confirm that quantum entanglement measurement has higher information extraction efficiency than local measurement in the quantum orientation task, and the average fidelity of anti-parallel coding is improved by 3.9% compared to the average fidelity of parallel coding.

This work experimentally reveals a non-classical phenomenon caused by entanglement in quantum measurement, and also provides a set of methods for implementing deterministic entanglement measurement in optical subsystems. This research is beneficial to the development of quantum entanglement and quantum measurement research, and has potential applications in quantum information processing.

Tang Junfeng, Ph.D. candidate in the Key Laboratory of Quantum Information of the Chinese Academy of Sciences, and Hou Zhibo, associate researcher, are co-first authors of the paper. The research was supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences and the Ministry of Education.