Authentication of physical entities

An authentication device comprising a plurality of flakes of a substantially two-dimensional material, each flake having with an operative area configured to emit, by nonresonant photoluminescence, electromagnetic radiation with a property that varies with position in the operative area, as a result of a band gap of the substantially two-dimensional material varying spatially across the operative area of each flake, and the band gap of the substantially two-dimensional material varying between flakes of the substantially two-dimensional material, such that each flake of the plurality of flakes has a different variation in the property, wherein each flake of the substantially two-dimensional material is sized such that motion of electrons into, and out of, a two dimensional plane of the material is governed by quantum mechanical effects, wherein the optical property is one of polarization or wavelength, and wherein the operative area of each flake is sized to permit optical detection of the property across the operative area.