Tight Contraction Rates for Primitive Channels under Quantum $f$-Divergences

TL;DR

This paper investigates contraction rates of quantum channels using $f$-divergences, establishing bounds and conditions for tightness, and applies these to various quantum divergences to improve existing results.

Contribution

It proves a local reverse Pinsker inequality for quantum $f$-divergences and links contraction rates to SDPI constants, enhancing understanding of quantum channel behavior.

Findings

Quantum $f$-divergences satisfy a local reverse Pinsker inequality.

Asymptotic contraction rate is upper bounded by the SDPI constant of $oldsymbol{ ext{chi}^2}$-divergence.

Conditions for bounds to be tight are established using quantum detailed balance.

Abstract

Data-processing inequalities capture the phenomenon that two probability distributions can only become less distinguishable under any common post-processing. For more fine-grained inequalities, one turns to strong data-processing inequality (SDPI) constants, which give the strongest inequalities for a given channel and reference state for a fixed measure of distinguishability. These quantities have been used to quantify the rate at which time-homogeneous Markov chains contract towards a fixed point both in the classical and quantum setting. In this work, we establish that quantum $f$-divergences satisfy a local reverse Pinsker inequality, which implies the asymptotic contraction rate of a primitive channel to its stationary state is upper bounded by the SDPI constant of any non-commutative $\chi^2$-divergence. Using quantum-detailed balance, we establish a sufficient condition for these bounds to be tight. Finally, we apply these results to Petz, Matsumoto, and Hirche-Tomamichel $f$-divergences, establishing new and strengthening previously known results.