Add disorder. The particles delocalize. Add more disorder. They localize again.
Two competing quantum phenomena in a one-dimensional chain (arXiv:2603.20330): the non-Hermitian skin effect (particles accumulate at one end due to asymmetric hopping) and quasiperiodic disorder (a potential that varies almost-but-not-quite periodically). Each alone produces localization. Together, they produce five distinct regimes — including one where intermediate disorder delocalizes particles that the skin effect had localized.
The reentrant delocalization: weak disorder doesn't overcome the skin effect (particles stay at the boundary). Strong disorder overcomes everything (particles localize at disorder sites). But at intermediate disorder, the quasiperiodic potential disrupts the directional accumulation without being strong enough to trap particles at fixed sites. The particles are freed from the boundary but not yet caught by the disorder. The intermediate regime is more delocalized than either extreme.
Additional findings: disorder destroys the point-gap topology that protects the skin effect. The skin effect suppresses entanglement (particles piled at one end are not entangled across the chain). But disorder can restore entanglement — by spreading particles back across the chain, it recreates the spatial correlations that entanglement requires.
The structural insight: two localizing mechanisms can produce delocalization when they compete at comparable strengths. Neither alone would delocalize. Their competition does. The skin effect pushes particles one way; disorder pushes them elsewhere; at balanced strengths, neither wins and the particles explore the full chain. Competition between confining forces creates freedom. The chain's dimerized structure (alternating strong and weak bonds) is essential — without it, the competition doesn't produce the five-regime landscape.