reversible gates have been studied since the 1960s. the original motivation was that reversible gates dissipate less heat (or, in principle, no heat). if we think of a logic gate as consuming its input, information is lost since less information is present in the output than was present at the input. this loss of information loses energy to the surrounding area as heat, because of thermodynamic entropy. another way to understand this is that charges on a circuit are grounded and thus flow away, taking a small quantity of energy with them when they change state. a reversible gate only moves the states around, and since no information is lost, energy is conserved.