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Quantum-mechanically enhanced water flow in sub-nanometer carbon nanotubes

Water-flow in carbon nanotubes (CNT's) starkly contradicts classical fluid mechanics, with permeabilities that can exceed no-slip Haagen-Poiseuille predictions by two to five orders of magnitude. Semi-classical molecular dynamics accounts for enhanced flow-rates, that are attributed to curvature-dependent lattice mismatch. However, the steeper permeability-enhancement observed experimentally at $\sim$nm-size radii remains poorly understood, and suggests emergence of puzzling non-classical mechanisms. Here we address water-CNT friction from a quantum-mechanical perspective, in terms of water-energy loss upon phonon excitation. We find that combined weak water-phonon coupling and selection rules hinder water-CNT scattering, providing effective protection to water superflow, whereas comparison with a semiclassical theory evidences a friction increase that can exceed the quantum-mechanical prediction by more than two orders of magnitude. Quasi-frictionless flow up to sub-nm CNT's opens new pathways towards minimally-invasive trans-membrane cellular injections, single-water fluidics and efficient water filtration.