a) A hypothetical nanopore sensor (green) with two reading heads, R1 and R2, which could in principle extract more sequence information from a DNA strand (red) than a device with a single reading head. b) To illustrate the idea, we assume that the four bases of DNA at reading head R1 produce 4 distinct current levels (widely dispersed as shown). Each of the levels is split into 4 additional levels (with a lesser dispersion, for the purpose of illustration) by the second reading head R2, yielding 16 current levels in total and providing redundant information about the DNA sequence.