The necessary and sufficient conditions when global and local fidelities are equal

Abstract: In the field of quantum information theory, the concept of quantum fidelity is employed to quantify the similarity between two quantum states. It has been observed that the fidelity between two states describing a bipartite quantum system $A \otimes B$ is always less than or equal to the quantum fidelity between the states in subsystem $A$ alone. While this fidelity inequality is well understood, determining the conditions under which the inequality becomes an equality remains an open question. In this paper, we present the necessary and sufficient conditions for the equality of fidelities between a bipartite system $A \otimes B$ and subsystem $A$, considering pure quantum states. Moreover, we provide explicit representations of quantum states that satisfy the fidelity equality, based on our derived results.