physics//thermodynamics//reversible computing

Reversible computing is computation in which every step can be undone because no information is erased, and it is the only known way to compute below the floor set by Landauer's principle. Charles Bennett showed in 1973 that any computation can be made reversible by keeping a record of its intermediate results and then running the steps backwards to clean them up.

Reversible computing is computation in which every step can be undone because no information is erased, and it is the only known way to compute below the floor set by Landauer's principle. Charles Bennett showed in 1973 that any computation can be made reversible by keeping a record of its intermediate results and then running the steps backwards to clean them up.

Ordinary logic gates destroy information: an AND gate that outputs 0 does not say which input was 0. Reversible gates, such as the Toffoli gate, have as many outputs as inputs, so the inputs can always be recovered.

The price is paid in other currencies: more gates and memory to hold what would have been erased, and slower, adiabatic switching so that the circuit itself does not dissipate the savings.

It remains laboratory work. Its practical meaning today is to show that the thermodynamic limit on computing is weaker than Landauer's number suggests, since that number only applies to erasure.