A strictly positive lower bound on the thermodynamic cost of running a Boolean circuit

Abstract

All digital computers implement input-output functions using logic gates connected into circuits. Different circuits computing the same function may nevertheless incur different resource costs, and circuit complexity theory studies these costs through measures such as the number of gates and the length of the longest path from input to output. Energetic cost is another important resource, however, that is typically not included among these measures. To address this, we use mismatch cost (MMC): a nonnegative contribution to the entropy production of a process that can be characterized largely independently of the detailed physical implementation of that process, providing a natural way to analyze the thermodynamic cost of Boolean circuits at an abstract, computational level. We derive an expression for the MMC of circuits composed of Boolean gates, relate it to standard complexity measures such as circuit size and depth, and use it to define mismatch cost complexity as a measure of thermodynamic resource cost. For Boolean circuits computing a non-constant Boolean function, this expression also implies a strictly positive MMC for every input distribution, and therefore a strictly positive lower bound on total entropy production. We characterize when MMC scales linearly with circuit size and when it does not, and compare the MMC of different circuit families that compute the same Boolean function. Together, these results lay the foundation for treating mismatch cost as a resource within circuit complexity theory.