Data processing for the sandwiched Rényi divergence: a condition for equality

Abstract

The $\alpha$-sandwiched Rényi divergence satisfies the data processing inequality, i.e. monotonicity under quantum operations, for $\alpha \ge 1/2$. In this article, we derive a necessary and sufficient algebraic condition for equality in the data processing inequality for the $\alpha$-sandwiched Rényi divergence for all $\alpha \ge 1/2$. For the range $\alpha \in [1/2,1)$, 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 (2013), p. 122201] 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 Rényi version of the Araki-Lieb inequality and analyze the case of equality, generalizing a result by Carlen and Lieb [Lett. Math. Phys. 101.1 (2012), pp. 1-11] about equality in the original Araki-Lieb inequality. Furthermore, we prove a general lower bound on a Rényi version of the entanglement of formation, and observe that it is attained by states saturating the Rényi 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.

17 pages, no figures, comments welcome! v4: published version in Letters in Mathematical Physics

References in corpus (3)

• Structure of states which satisfy strong subadditivity of quantum entropy with equality
• Different quantum f-divergences and the reversibility of quantum operations
• Preservation of a quantum Renyi relative entropy implies existence of a recovery map

Cited by in corpus (12)

• Different quantum f-divergences and the reversibility of quantum operations
• Entanglement, quantum randomness, and complexity beyond scrambling
• Coherence and entanglement measures based on Rényi relative entropies
• Recoverability for optimized quantum $f$-divergences
• Recoverability of quantum channels via hypothesis testing
• Equality conditions of Data Processing Inequality for $\alpha$-$z$ Rényi relative entropies
• Generalizing measurement-induced phase transitions to information exchange symmetry breaking
• Relating relative Rényi entropies and Wigner-Yanase-Dyson skew information to generalized multiple quantum coherences
• Approximate Petz recovery from the geometry of density operators
• A simple coin for a $2d$ entangled walk
• Revisiting the equality conditions of the data processing inequality for the sandwiched Rényi divergence
• Geometric conditions for saturating the data processing inequality