Reversible Computing

At the lowest level, reversible (quantum) computing (circuitry) can be done by maintaining the inputs from each computation. Thus there along with 2 inputs for an AND gate, there is an unknown input. There are 3 outputs as well, the 2 inputs, and the result from anding the two inputs. This allows computations to be reversed and checked as well. There is no loss of information, and unknowns progress into knowns. The only thing that you can't recover is the unknowns.