If we use 2 lasers with selectable light intensities as input, with each level of intensity corresponding to decimal number. Like level 0 intensity - decimal 0 (MIN) level 1 intensity - decimal 1 level 2 intensity - decimal 2 ... level 9 intensity - decimal 9 (MAX) and directly shine both of them over photo detector, there intensities will add up to produce sum. thus we can measure the current from photo detector to see what number is it. we can also use laser of other frequencies to implement parallelism. I know this solution is simple but I don't get it why wont it work either.