Probing quantum entanglement from magnetic-sublevels populations: beyond spin squeezing inequalities

Guillem Müller-Rigat1, Maciej Lewenstein1,2, and Irénée Frérot3

1ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
2ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
3Univ Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France

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Spin squeezing inequalities (SSI) represent a major tool to probe quantum entanglement among a collection of few-level atoms, and are based on collective spin measurements and their fluctuations. Yet, for atomic ensembles of spin-$j$ atoms and ultracold spinor gases, many experiments can image the populations in all Zeeman sublevels $s=-j, -j+1, \dots, j$, potentially revealing finer features of quantum entanglement not captured by SSI. Here we present a systematic approach which exploits Zeeman-sublevel population measurements in order to construct novel entanglement criteria, and illustrate our approach on ground states of spin-1 and spin-2 Bose-Einstein condensates. Beyond these specific examples, our approach allows one to infer, in a systematic manner, the optimal permutationally-invariant entanglement witness for any given set of collective measurements in an ensemble of $d$-level quantum systems.

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Cited by

[1] Irénée Frérot, Matteo Fadel, and Maciej Lewenstein, "Probing quantum correlations in many-body systems: a review of scalable methods", Reports on Progress in Physics 86 11, 114001 (2023).

[2] Joana Fraxanet, Tymoteusz Salamon, and Maciej Lewenstein, Lecture Notes in Physics 1000, 85 (2023) ISBN:978-3-031-32468-0.

[3] Anubhav Kumar Srivastava, Guillem Müller-Rigat, Maciej Lewenstein, and Grzegorz Rajchel-Mieldzioć, "Introduction to quantum entanglement in many-body systems", arXiv:2402.09523, (2024).

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