The atomic nucleus is not just a bag of protons and neutrons. It is a quantum system where particles are bound by invisible threads of entanglement. When two protons or neutrons interact, their states become intertwined so that describing one without the other is impossible.
Scientists took measures of entanglement (entropy, mutual information, and relative entropy) and applied them to neon, magnesium, and silicon nuclei. These nuclei lie near the so-called island of inversion, where the usual order of energy levels flips. Calculations showed that in ground states, entanglement between protons and neutrons is weak, but between identical particles it is strong. But when the nucleus gets excited, proton-neutron correlations become just as strong as those between identical particles.
This is like a dance floor: in a calm state, pairs dance with themselves, but when the music speeds up, everyone starts swapping partners. Such a picture helps explain why some neutron-rich nuclei behave differently than the simple shell model predicts. Entanglement turns out to be a sensitive tool that catches subtle changes in nuclear structure.
The island of inversion is a region of nuclei where the magic number 20 ceases to be magic: energy levels rearrange, and the nucleus becomes unexpectedly stable or deformed.
In Greg Egan's novel "Schild's Ladder," quantum entanglement is used for instantaneous communication between distant points in space, echoing the idea of nonlocal correlations in the nucleus.