近日，越南荣市大学的Khoi-Nguyen Huynh-Vu与新加坡国立大学的Valerio Scarani等人合作并取得一项新进展。经过不懈努力，他们通过总角动量测量证明自旋系综中真正的多体纠缠。相关研究成果已于2024年4月1日在国际知名学术期刊《物理评论A》上发表。

该研究团队引入了自旋系综的纠缠见证，它仅使用总角动量的测量来检测真正的多体纠缠。大多数其他基于角动量的自旋系综目击者所遗漏的态，包括Greenberger-Horne-Zeilinger态和Dicke态的某些叠加态，可以被目击者有效地检测到。该方案包括估计在平面上沿等间距方向的总角动量为正的概率。或者，在假定总自旋经历均匀进动的情况下，研究人员可以在不同的时间沿着一个方向进行测量。

当观察到的分数超越了可分离界限时，研究人员就能确切地检测到真正的多体纠缠。经过深入研究，他们得到了自旋系综的可分离界限的精确解析表达式，适用于j1⊗j2⊗…⊗jN的情形，且总自旋为半整数。对于其他情况，他们也进行了数值计算并报道了相应结果。最后，他们推测了总自旋未知时的可分离界限的表达式，数值结果很好地支持了这一表达式。

附：英文原文

Title: Certification of genuine multipartite entanglement in spin ensembles with measurements of total angular momentum

Author: Khoi-Nguyen Huynh-Vu, Lin Htoo Zaw, Valerio Scarani

Issue&Volume: 2024/04/01

Abstract: We introduce entanglement witnesses for spin ensembles which detect genuine multipartite entanglement using only measurements of the total angular momentum. States that are missed by most other angular-momentum-based witnesses for spin ensembles, which include Greenberger-Horne-Zeilinger states and certain superpositions of Dicke states, can be effectively detected by our witness. The protocol involves estimating the probability that the total angular momentum is positive along equally spaced directions on a plane. Alternatively, one could measure along a single direction at different times, under the assumption that the total spins undergo a uniform precession. Genuine multipartite entanglement is detected when the observed score exceeds a separable bound. Exact analytical expressions for the separable bound are obtained for spin ensembles$j_1\otimes j_2\otimes ?\otimes j_N$ such that the total spin is a half-integer and numerical results are reported for the other cases. Finally, we conjecture an expression for the separable bound when the total spin is not known, which is well supported by the numerical results.

DOI: 10.1103/PhysRevA.109.042402

Source: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.109.042402