Coupling effect of nearest-neighbor interacting qubit chains to a single qubit system

This study establishes a theorem that provides sufficient criteria for identifying terms in any time-independent Hamiltonian that have no influence on local dynamics, local observables, or any local phenomena, without any approximation. The usefulness of this theorem is demonstrated by predicting the behavior of three systems. The first system consists of multiple qubit chains with Ising interactions. The second system is formed by a single qubit chain, differentiated by the substitution of Dzyalonshinskii-Moriya (DM) interaction for the last Ising interaction. The predictions were verified by analytically deriving the reduced dynamics of both systems. A third system was also considered, namely, a short qubit chain with a transverse magnetic field on the intermediary environment qubit. This transverse magnetic field signals that the commutation relation required by our theorem no longer holds. The third system’s local dynamics were also derived analytically, demonstrating that all Hamiltonian constituents contributed to the reduced dynamics. The physical consequences of our theorem’s inapplicability were also explored using this third system by analyzing the entanglement dynamics between the qubits that do not directly interact. The results showed that the entanglement is caused by an $\mathit{\text{emergent}}\mathit{\text{coupling}}$ between the two non-directly interacting subsystems. This $\mathit{\text{emergent}}\mathit{\text{coupling}}$ arises from the non-commutativity of the intermediate interactions connecting the two subsystems. When searching for these $\mathit{\text{emergent}}\mathit{\text{couplings}}$, our theorem can eliminate intermediate interactions that will not produce them.