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arXiv · 2509.16880

The exact relation between the entanglement entropies of the $XY$ and quantum Ising chains with free and fixed boundary conditions

Tianhao He·Xintian Wu
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Abstract

The entanglement entropies of $XY$ chains and quantum Ising chains (QICs) with fixed boundary conditions are studied here. Three kinds of boundary conditions (BCs) are considered: fixed up--up or down--down (the spins at both ends are aligned in the same direction), fixed up--down or down--up (the spins at the two ends are aligned in opposite directions), and fixed--free (the spin at one end is aligned, and the other end is free). It is shown that i) the entanglement entropy of an $XY$ chain with a fixed--free BC is the sum of those of QICs with a fixed--free BC and with a free--free BC; ii) the entanglement entropy of an $XY$ chain with a fixed up--up boundary condition is the sum of those of QICs with a fixed up--up BC and with a free--free BC; and iii) the entanglement entropy of an $XY$ chain with a fixed up--down BC is the sum of that of a QIC with a fixed up--up BC and that of the first excited state of a QIC with a free--free BC.

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Tianhao He, Xintian Wu. 2025-09-21. The exact relation between the entanglement entropies of the $XY$ and quantum Ising chains with free and fixed boundary conditions. https://arxiv.org/abs/2509.16880

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