Strange metals and entanglement witnesses

Quantum entanglement provides a powerful lens for probing and understanding extremely collective quantum states of matter. Here we show that experimentally measurable multipartite entanglement elucidates the nature of strange metals [1,2]. Our study focuses on Kondo systems, which exemplify strange metallicity in the quantum critical regime through the physics of Kondo destruction [3]. The experimentally observed singular behavior of charge and spin responses suggests a charge-spin entangled quantum critical fluid [3], motivating the quantification of quantum entanglement. We analyze the quantum Fisher information of local spin operators [1], and reveal amplified multipartite entanglement in the quantum critical fluid. We further elucidate our finding by contrasting it with the case of Fermi liquids, where we prove that the quantum Fisher information of local operators remains trivial and only that of nonlocal operators can witness the underlying entanglement [2]. Our theoretical predictions on the spin quantum Fisher information are supported by experiments on quantum critical heavy fermion metals [1,4]. I will explain how our findings on the spin quantum Fisher information provide a direct and quantitative characterization of the loss of quasiparticles in strange metals through the principle of quantum monogamy [1,5].

[1] Y. Fang, M. Mahankali, Y. Wang, L. Chen, H. Hu, S. Paschen, and Q. Si, Amplified multipartite entanglement witnessed in a quantum critical metal. Nature Communications16, 2498 (2025)

[2] Y. Wang*, Y., Fang*, F. Xie*, and Q. Si, Local and non-local entanglement witnesses of Fermi Liquid. arXiv:2502.13958.

[3] H. Hu, L. Chen and Q. Si, Quantum critical metals and loss of quasiparticles. Nat. Phys. 20, 1863 (2024); Q. Si et al, Locally-critical quantum phase transitions in strongly correlated metals'', Nature 413, 804 (2001).

[4] F. Mazza, S. Biswas, X. Yan, A. Prokofiev, P. Steffens, Q. Si, F.F. Assaad, and S. Paschen, Quantum Fisher information in a strange metal. arXiv:2403.12779.

[5] Y. Fang, invited talk at Workshop on Hidden Orders and Quantum Entanglement, Oct 6-8, 2025, Rice University.