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Physical Review Letters

Intrinsic Generation of Angular Momenta and Entanglement in Fission

B. Li, D. D. Zhang, D. Vretenar, Tamara Nikšić, P. W. Zhao, Jie Meng

Published May 21, 2026• 0 citations
Publisher

Abstract

Nuclear time-dependent density functional theory is used to investigate spin generation and entanglement of fission fragments in spontaneous fission of ^{252}Cf, incorporating both axial and nonaxial deformations. Axially symmetric fission trajectories enforce strict constraints: counterrotation (twisting mode) along the fission axis and equiprobable bending and wriggling modes perpendicular to it. Nonaxial modes broaden the distributions of fission fragment spin projection on the fission axis, and allow for axial (tilting) collective rotations, which are forbidden on axially symmetric trajectories. Mutual information analysis reveals that axial-symmetry breaking reduces spin-spin correlations along the fission axis of symmetric cases, while perpendicular correlations remain more resilient. The effect of triaxial degrees of freedom on the opening angle distribution between the spins of the fission fragments is analyzed.

DOI
10.1103/tyht-frnf

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Nuclear time-dependent density functional theory is used to investigate spin generation and entanglement of fission fragments in spontaneous fission of ^{252}Cf, incorporating both axial and nonaxial deformations. Axially symmetric fission trajectories enforce strict constraints: counterrotation (twisting mode) along the fission axis and equiprobable bending and wriggling modes perpendicular to it. Nonaxial modes broaden the distributions of fission fragment spin projection on the fission axis, and allow for axial (tilting) collective rotations, which are forbidden on axially symmetric trajectories. Mutual information analysis reveals that axial-symmetry breaking reduces spin-spin correlations along the fission axis of symmetric cases, while perpendicular correlations remain more resilient. The effect of triaxial degrees of freedom on the opening angle distribution between the spins of the fission fragments is analyzed.
Nuclear time-dependent density functional theory is used to investigate spin generation and entanglement of fission fragments in spontaneous fission of 252^{252}252Cf, incorporating both axial and non-axial deformations. Axially symmetric fission trajectories enforce strict constraints: counter rotation (twisting mode) along the fission axis and equiprobable bending/wriggling modes perpendicular to it. Non-axial modes broaden the distributions of fission fragment spin projection on the fission axis, and allow for axial (tilting) collective rotations, which are forbidden on axially symmetric trajectories. Mutual information analysis reveals that axial-symmetry breaking reduces spin-spin correlations along the fission axis of symmetric cases, while perpendicular correlations remain more resilient. The effect of triaxial degrees of freedom on the opening angle distribution between the spins of the fission fragments is analyzed.
The generation of angular momentum (intrinsic spin) in fission fragments has recently attracted renewed attention. While several microscopic approaches reproduce the spin distribution qualitatively using projection techniques, the physical origin of the fragments' angular momentum in density functional theory remains unclear. In this work, we investigate the mechanisms responsible for the spin distribution of fission fragments within a microscopic TDDFT framework. We compare spin distributions obtained from projection operators with those predicted by a simple expression derived from the uncertainty relation between angle and angular momentum, where angular fluctuations are estimated using a Monte Carlo sampling of nucleon positions. We find that a large portion of the spin distribution obtained from projection methods can be explained by the uncertainty principle. Our results thus show that, within microscopic approaches, the spin of fission fragments originates primarily from quantum uncertainty associated with their orientation angle with respect to the fission axis, mainly due to quadrupole deformation and, to a lesser extent, octupole deformation.
Nuclear fission provides a unique manifestation of spatially nonlocal many-body entanglement. We compute the bipartite spatial entanglement entropy exactly along dynamical fission trajectories, by leveraging the fermionic Gaussian state formulation. Across seven representative fissioning channels, the final entanglement entropy correlates strongly with the intrinsic particle number variance of the fragment, yet exhibits no simple dependence on scission geometries. Most notably the entanglement is significantly suppressed when fragments are magic nuclei, revealing a shell anti-entanglement effect. This work establishes entanglement entropy as a novel lens that extends the conventional conception of nuclear fission.
A comparative analysis of experiments devoted to studying, in the fission process, ( n,fn,fn,f ) angular correlations, on one hand, and ( n,nn,nn,n ) and ( n,nfn,nfn,nf ) angular correlations, on the other hand, is performed on the basis of the model proposed earlier for describing muon conversion in fragments originating from the prompt fission of 238{}^{238}238 U nuclei that is induced by negatively charged muons. It is shown that each of these experiments may provide information that does not depend on the results of the other experiments and which is complementary to them. Historically, the situation resembles the Einstein–Podolsky–Rosen (EPR) paradox. An experimental verification of the theoretical relation between the alignment and polarization of fragment spins in the CORA experiment is of crucial importance.
In the first part, calculated lifetimes for spontaneous fission from the ground and from the isomer state are presented. The agreement reached with experiments correponds to the crude approximations done, but is better than expected. The calculations are based on the shell correction approach for the potential energy and on the cranking model for the collective mass. In the second part we discuss the behaviour of the so-called least action trajectories: there are two of them, one for symmetric and one for asymmetric shapes. The systematic behaviour of the dynamic barriers and of the peak-to-peak ratio of the fission fragment mass distribution and their correlation with the experimental in formation support this point of view.