An apparatus including a first electrode; a second electrode including graphene; and a dielectric between the first electrode and the second electrode; input circuitry configured to change a charge state of the dielectric by causing electric charges to be trapped in the dielectric; and output circuitry configured to detect a value dependent upon a quantum capacitance of the graphene of the second electrode, wherein the quantum capacitance of the graphene is dependent upon the charge state of the dielectric.