Entanglement, as a peculiar quantum phenomenon, is so important and we need carefully study on its features. No instrument can directly measure the quantum entanglement of a system at present. Both theory and experiment, following the well-known Bell inequality, reveal the existence of the entanglement phenomenon in quantum mechanics, which is incompatible with classical correlations. Investigating the entanglement of a system helps us to understand the complex phenomenon of nonlocality. These features make quantum entanglement essential to quantum information science.
To examine the characteristics of quantum entanglement, optical-field manipulation group led by Prof. Hui-Tian Wang from National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, carried out the research of entanglement and nonlocality in a two-site coupled-cavity system, in which each cavity contains a Λ-type three-level atom. Related research results are published in Photonics Research, Volume 5, No. 3, 2017 (Z. Zhang, et al., Entanglement and nonlocality in a coupled-cavity system).
Coupled-cavity system, allowing the manipulation of states of quantum system, is a kind of perfect candidate for quantum entanglement, in which a Λ-type three-level atom is embedded in each of the two cavities. The intersect states, the pure atomic state, and all photon states play different roles according to different parameters, but they are connected with the entropy of the system based on the von Neumann entropy method. Bipartite entanglement entropy and nonlocality were examined, which definitely indicated the existence of entanglement in the ground state. As the detuning changes, however, the entanglement between the photons in different cavities may become so small that the classical effect plays an important or even a main role in the experiment.
Prof. Hui-Tian Wang believes that this work should have important reference significance for understanding and exploring the multipartite entanglement and nonlocality in multiple coupled-cavity system in the future and enhancing the capacity in the quantum manipulation.
Further work will focus on extending the present method to the multiple coupled-cavity system embedded with the Λ-type three-level atoms, and the two and multiple coupled-cavity systems embedded with the other three-level atoms.
为研究量子纠缠的特征，南京大学固体微结构物理国家重点实验室和物理学院王慧田教授所领导的光场调控团队开展了每个腔仅嵌入一个 Λ 型三能级原子的两腔耦合系统的量子纠缠与非定域性问题的研究。相关研究结果发表于Photonics Research 2017年第5卷第3期上(Z. Zhang, et al., Entanglement and nonlocality in a coupled-cavity system）。
耦合腔系统，因允许对量子系统状态的操控，而成为研究量子纠缠的一类最佳候选对象。在该项研究中，耦合腔系统的每个腔仅嵌入一个 Λ 型三能级原子。虽然交叉态、纯原子态和全光子态依据参数的不同而扮演不同的角色，但是基于冯诺依曼熵方法它们均与系统熵相关。两体纠缠熵和非定域性的研究结果无疑证实了基态纠缠的存在。然而，随着失谐的变化，不同腔中的光子纠缠可能变得很小，以至于经典效应在实验中扮演一个重要甚至主要的角色。