Optimal Universal Bounds for Quantum Divergences

We identify a universal structural principle underlying the smoothing of classical divergences: the optimizer of the smoothing problem is a clipped probability vector, independently of the specific divergence. This yields a divergence-independent characterization of all smoothed classical divergences and reveals a common geometric structure behind seemingly different quantities. Building on this structural insight, we derive optimal universal bounds for smoothed quantum divergences, including quantum R'enyi divergences of arbitrary order and the hypothesis testing divergence. Our inequalities relate divergences of different orders through bounds of the form $D_\beta^{\varepsilon} \le D_\alpha + \mathrm{correction}$ and $D_\beta^{\varepsilon} \ge D_\alpha + \mathrm{correction}$, and we prove that the correction terms are optimal among all universal, state-independent inequalities of this type. Consequently, our results strictly improve previously known bounds whenever those were suboptimal, and in cases where earlier bounds coincide with ours, our analysis establishes their optimality. In particular, we obtain optimal universal bounds for the hypothesis testing divergence.