Reversible computing with mechanical links and pivots

Summary

And now we are ready to construct a NAND gate, which is a “universal gate” (i.e., you can implement any truth table using only NAND gates). We first use two “wires” for each input. If A is TRUE, then the first wire is pushed forward. If A is FALSE, then the second wire is pushed forward. Likewise, if B is TRUE then the third wire is pushed forward, and the fourth wire is pushed forward if B is FALSE. We expect to always have exactly two of these wires push forward for any computation. Following these input wires, we see that each individual wire engages exactly two locks, preventing the thrust of the clock from flowing through that pathway. Since there are four pathways, we must make sure that only one pathway is unlocked at any time. Then, when the clock arrives, that pathway will receive the forward thrust. If you’re interested in learning more, one of the authors gave a 20 minute overview of this paper at the CCC Workshop on Reversible Computing: Ralph Merkle: Molecular Mechanical Computing.