Population-coherence routes to purity in Page-type models of black-hole evaporation

TL;DR

This paper introduces a population-coherence framework to analyze purity routes in black-hole evaporation, revealing that late-time purity recovery is coherence-driven under realistic assumptions.

Contribution

It develops a formalism for decomposing density-matrix purity into population and coherence contributions, applying it to black-hole evaporation models to identify dominant routes to purity.

Findings

Late-time purity is coherence-dominated in evaporation models.

Population remains nearly uniform while coherence accounts for purity recovery.

The framework distinguishes different internal structures with identical spectra and purity.

Abstract

We revisit the black-hole information problem from the viewpoint of a population-coherence decomposition of density-matrix purity. Building on a previously developed formalism for ndimensional density matrices, we characterize each state by a normalized global purity index and two complementary indices, which quantify the contributions of level populations and coherences. This yields a simple quadratic relation and a geometric representation in a population-coherence plane, where different routes to purity can be distinguished. In the two-level case we construct explicit families of states with identical spectra and global purity but opposite internal structure, realizing population-dominated and coherence-dominated routes. We then apply this framework to a standard Page-type evaporation model without an explicit Hamiltonian, in which a black hole and its Hawking radiation form a bipartite pure state with varying Hilbert-space dimensions. Using known results for typical reduced states in large dimensions, we analyze the behavior of population and coherence components of purity along the evaporation process. Under the physically motivated requirement that, in this energy-free setting, the radiation populations remain nearly uniform in the chosen basis, we show that the late-time recovery of purity must be coherence-dominated: the global purity of the radiation approaches unity while the population index stays small and the coherence index carries essentially all the purity.