Reversible circuit realizations of Boolean functions

Reversible circuits are a concrete model of reversible computation with applications in areas such as quantum computing and analysis of cryptographic block cyphers. In 1980, Toffoli showed how to realize a Boolean function by a reversible circuit, however the resulting complexity of such circuits has remained an open problem. We investigate the reversible circuit complexity of families of Boolean functions and derive conditions that characterize whether a polynomial realization is possible. First, we derive sufficient conditions on families of Boolean functions that guarantee a polynomial-size reversible circuit realization. Namely, we show that if a Boolean function can be embedded into an even parity permutation that has a polynomial-size cycle representation, then the Boolean function can be realized by a polynomial-size reversible circuit. Furthermore, we provide a construction for the realization. Second, we provide concrete realizations for several families of Boolean functions, such as the adder, incrementor, and threshold functions, which do not necessarily satisfy the preceding condition, but still have polynomial-size realizations; this is important because such realizations will necessarily form the building blocks of quantum computers.