Quantum Rényi and f-Divergences from Integral Representations

Smooth Csiszár f-divergences can be expressed as integrals over so-called hockey stick divergences. This motivates a natural quantum generalization in terms of quantum Hockey stick divergences, which we explore here. Using this recipe, the Kullback–Leibler divergence generalises to the Umegaki relative entropy, in the integral form recently found by Frenkel. We find that the Rényi divergences defined via our new quantum f-divergences are not additive in general, but that their regularisations surprisingly yield the Petz Rényi divergence for \(\alpha < 1\) $\alpha <1$ and the sandwiched Rényi divergence for \(\alpha > 1\) $\alpha >1$ , unifying these two important families of quantum Rényi divergences. Moreover, we find that the contraction coefficients for the new quantum f divergences collapse for all f that are operator convex, mimicking the classical behaviour and resolving some long-standing conjectures by Lesniewski and Ruskai. We derive various inequalities, including new reverse Pinsker inequalites with applications in differential privacy and explore various other applications of the new divergences.