Negative quasiprobability trajectories for Bell-diagonal states under local decoherence

The fluctuation theorem (FT) relates microscopic trajectory distributions to macroscopic averages. Using quasiprobability trajectories, this framework has recently been extended to quantum information dynamics. Despite sharing the form of conventional thermodynamic FTs, information FTs involve quasiprobabilities whose negativity and statistical properties remain poorly understood. We analytically determine the properties of negative distributions based on the two-qubit Bell-diagonal states evolving under local dephasing, depolarizing, and amplitude-damping channels. We show that the occurrence of negative quasiprobabilities is not determined by the entanglement or Bell nonlocality of the initial state. Instead, a diagonal-interference decomposition of the transition quasiprobability provides a sufficient condition for negativity. More importantly, we prove that quasiprobability distributions containing negative weights can violate Horv'ath's necessary and sufficient criterion for the Jensen-Steffensen inequality to hold for every continuous convex function, even though the specific exponential Jensen-like relation enforced by the FT and the quantum data-processing inequality remains valid. This trajectory-level contrast with classical stochastic descriptions reveals a distinctive feature of quantum information dynamics that is invisible when only the corresponding average-level information inequalities are considered.