Entropic Continuity Bounds & Eventually Entanglement-Breaking Channels

TL;DR

This thesis develops a unified approach to continuity bounds for quantum entropy functions using majorization and trace distance, and analyzes entanglement-breaking in repeated quantum interactions with implications for thermodynamics.

Contribution

It introduces a novel technique linking majorization and trace distance to establish continuity bounds, and characterizes entanglement-breaking in repeated interaction systems with adiabatic limits.

Findings

New proof of Audenaert-Fannes entropy continuity bound

Novel bounds for α-Rényi entropy with α > 1

Characterization of entanglement-breaking in repeated interactions

Abstract

In the first part of this thesis, we present a general technique for establishing local and uniform continuity bounds for Schur concave functions. Our technique uses a particular relationship between majorization and the trace distance between quantum states. Namely, the majorization pre-order attains a minimum over $\epsilon$-balls in this distance. By tracing the path of the majorization-minimizer as a function of the distance $\epsilon$, we obtain the path of "majorization flow". This yields a new proof of the Audenaert-Fannes continuity bound for the von Neumann entropy in a universal framework which extends to the other functions, including the $\alpha$-R\'enyi entropy, for which we obtain novel bounds in the case $\alpha > 1$. We apply this technique to other Schur concave functions, such as the number of connected components of a certain random graph model, and the number of distinct realizations of a random variable. In the second part, we consider repeated interaction systems, in which a system of interest interacts with a sequence of probes one at a time. We characterize which repeated interaction systems break any initially-present entanglement between the system and an untouched reference after finitely many steps. Additionally, when the probes and their interactions with the system are slowly-varying (i.e. adiabatic), we analyze the saturation of Landauer's bound, an inequality between the entropy change of the system and the energy change of the probes, in the limit in which the number of steps tends to infinity and both the difference between consecutive probes and the difference between their interactions vanishes. This analysis proceeds at a fine-grained level by means of a two-time measurement protocol, in which the energy of the probes is measured before and after each interaction.