A journey through Flatland: What does the antiflatness of a spectrum teach us?

TL;DR

This paper introduces antiflatness as a new spectral fluctuation measure in quantum entanglement, develops a novel partial ordering, and explores its implications for state convertibility and spectral extremality.

Contribution

It defines antiflat majorization, unifies antiflatness measures, and links capacity of entanglement with quantum Fisher information, advancing understanding of quantum spectral fluctuations.

Findings

Antiflatness captures spectral fluctuations beyond average measures.

Capacity of Entanglement relates to the second derivative of KL divergence.

Maximal antiflatness states form a Pareto frontier with jump spectra.

Abstract

We explore the concept of antiflatness to characterize the structural fluctuations within the entanglement spectrum of a quantum state (i.e., the spectrum of its reduced density operator). As a measure of the interplay between entanglement and magic, two fundamental quantum resources, antiflatness provides second-order information about quantum correlations that standard average measures fail to capture. Recognizing that standard majorization theory fundamentally orders states by purity and is structurally blind to spectral fluctuations, we introduce a novel partial ordering known as antiflat majorization, based on the R\'enyi entropy spread. We define Flatness-Preserving Operations (FPOs), establishing new necessary conditions for state convertibility. Furthermore, we unify different measures of antiflatness-such as Capacity of Entanglement, Linear R\'enyi spread, and Logarithmic antiflatness-using the frameworks of escort distributions and Bregman divergences. We prove that the Capacity of Entanglement can be expressed as a second derivative of the Kullback-Leibler divergence along the escort trajectory, connecting it with the Quantum Fisher Information. Finally, we demonstrate that absolute maximal antiflatness is not achieved by a single universal state, but rather by a continuous Pareto frontier of extremal states with jump spectra, and we analyze the typicality of these spectral fluctuations using Haar, Bures-Hall and t-doped Clifford random state ensembles.