Strongly Universal Reversible Gate Sets

Abstract

It is well-known that the Toffoli gate and the negation gate together yield a universal gate set, in the sense that every permutation of $\{0,1\}^n$ can be implemented as a composition of these gates. Since every bit operation that does not use all of the bits performs an even permutation, we need to use at least one auxiliary bit to perform every permutation, and it is known that one bit is indeed enough. Without auxiliary bits, all even permutations can be implemented. We generalize these results to non-binary logic: For any finite set A, a finite gate set can generate all even permutations of $A^n$ for all n, without any auxiliary symbols. This directly implies the previously published result that a finite gate set can generate all permutations of $A^n$ when the cardinality of A is odd, and that one auxiliary symbol is necessary and sufficient to obtain all permutations when the cardinality of A is even. We also consider the conservative case, that is, those permutations of $A^n$ that preserve the weight of the input word. The weight is the vector that records how many times each symbol occurs in the word. It turns out that no finite conservative gate set can, for all n, implement all conservative even permutations of $A^n$ without auxiliary bits. But we provide a finite gate set that can implement all those conservative permutations that are even within each weight class of $A^n$.

The authors would like to acknowledge the contribution of the COST Action IC1405. This work was partially funded by Austrian national research agency FWF research grants P24077 and P24285, and by FONDECYT research grant 3150552.