Data processing for the sandwiched R,nyi divergence: a condition for equality

The alpha-sandwiched R,nyi divergence satisfies the data processing inequality, i.e. monotonicity under quantum operations, for . In this article, we derive a necessary and sufficient algebraic condition for equality in the data processing inequality for the -sandwiched R,nyi divergence for all . For the range , our result provides the only condition for equality obtained thus far. To prove our result, we first consider the special case of partial trace and derive a condition for equality based on the original proof of the data processing inequality by Frank and Lieb (J Math Phys 54(12):122201, 2013) using a strict convexity/concavity argument. We then generalize to arbitrary quantum operations via the Stinespring Representation Theorem. As applications of our condition for equality in the data processing inequality, we deduce conditions for equality in various entropic inequalities. We formulate a R,nyi version of the Araki-Lieb inequality and analyze the case of equality, generalizing a result by Carlen and Lieb (Lett Math Phys 101(1):1-11, 2012) about equality in the original Araki-Lieb inequality. Furthermore, we prove a general lower bound on a R,nyi version of the entanglement of formation and observe that it is attained by states saturating the R,nyi version of the Araki-Lieb inequality. Finally, we prove that the known upper bound on the entanglement fidelity in terms of the usual fidelity is saturated only by pure states.