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Double interval entanglement in quasiparticle excited states

We investigate double-interval entanglement measures, specifically reflected entropy, mutual information, and logarithmic negativity, in quasiparticle excited states for classical, bosonic, and fermionic systems. We develop an algorithm that efficiently calculates these measures from density matrices expressed in a non-orthonormal basis, enabling straightforward numerical implementation. We find a universal additivity property that emerges at large momentum differences, where the entanglement measures for states with distinct quasiparticle sets equal the sum of their individual contributions. The classical limit arises as a special case of this additivity, with both bosonic and fermionic results converging to classical behavior when all momentum differences are large.