Reversible Computation
By: Ammar Abushukur Energy
Energy is king, it rules us all, and it determines what things we can afford to do and what we cannot. Computation is not exempt from energy limitations, which is the general form of money. Scientists are constantly working on making computers more and more efficient, and every joule matters. However, what if I told you that there is a minimum amount of energy that computers will always dissipate regardless of what scientists do?
Landauer's Limit In 1961, physicist Rolf Landauer, working at IBM, applied the laws of thermodynamics to computers and information theory. He argued that for every irreversible operation done, you must dissipate a minimum amount of energy, called Landauer's limit. This loss was not due to inefficiencies in the design of computers. But rather a limitation of the mode of computation we use, namely irreversible computation.
Irreversible computation Our computers are built using irreversible gates, but what does that mean? Take the AND gate; it takes in two inputs and produces one output, representing the logical ANDing of the two inputs. Here is the truth table of the AND gate:
Now, if we have an output of 0 from an AND gate, can you tell me what the input was? There are three possibilities, so we can’t deterministically go from outputs to inputs. Thus making the model irreversible, and the same applies to OR gates. There is a loss of information from inputs to outputs dissipating energy in the process, but what if we don’t have to use irreversible gates to build our computers?
Reversible computation Reversible computation is described as a computational model in which the process is time-reversible; it allows you to go from inputs to outputs and the other way back. Landauer's limit does not apply to reversible computation; reversible computers are limitless and can be as efficient as possible. Many reversible gates have been devised. We will discuss two of them.