The CAS key lab of quantum information makes an important progress in the study of quantum entanglement networks: Li Chuan-Feng, Chen Geng and their colleagues have realized the first experimental study of self-testing of Bell state measurements, by linking it to the non-locality of entangled states. The research results were published on March 8th in Physical Review Letters.
Quantum entanglement is an important resource for quantum communication and quantum computing. In the process of constructing quantum entangled networks, it is not only necessary to prepare high-quality quantum entangled states, but also high-quality entanglement swapping between nodes, in order to entangle each node. The entanglement swapping needs to be implemented through Bell-based measurements. For example, at the beginning, there are one entangled pair A1 and A2 on Alice side and the other entangled pair B1 and B2 on the Bob side, but these two pairs are not entangled. Alice and Bob can respectively send A2 and B2 (usually photons) to an intermediate node for Bell state measurements. As a result, at the cost of consuming two particles A2 and B2, the two remaining particles A1 and B1 become entangled. The device-independent quality test of the entangled states and the Bell state measurements are called self-testing, which is proposed to guarantee the security of the constructed quantum entangled network when the used devices are not trustable. Previously, Li Chuanfeng and Chen Geng et al. have achieved self-testing of entangled states [Phys. Rev. Lett. 121, 240402 (2018)] and high-dimensional entangled states [npj Quant. Info. 5, 4 (2019)]. However, for Bell state measurements, the scientific community has not found a suitable method to measure its quality, so it cannot quantitatively characterize its accuracy, and cannot achieve its self-testing.
The experimental setup to realize the self-testing of Bell state measurements.
Li Chuan-Feng and Chen Geng et al. realized experimentally robust self-testing of Bell state measurements in optical systems by correlating them with the non-locality of entangled states. The experiment not only includes the self-testing of the two-body entanglement generated by the entanglement swapping, but also the self-testing of the independence of the two pairs of entangled states before the entanglement swapping. This makes the experimental results very robust and the estimated fidelity of the Bell state measurements reaches 0.87. This is the first principle verification experiment in the world for the self-testing of Bell state measurements. It solves the key problem of self-testing of quantum entangled networks and guarantees the security of quantum networks.
The first author of the article is Zhang Wen-Hao, a doctoral student of CAS key lab of quantum information. The work was funded by the Ministry of Science and Technology, the National Natural Science Foundation of China, and CAS Center For Excellence in Quantum information and quantum physics.
(School of Physical Sciences)