Improved quantum circuits for division

TL;DR

This paper presents optimized fault-tolerant quantum circuits for integer division that significantly reduce gate counts and depth, improving efficiency for quantum algorithms involving arithmetic operations.

Contribution

The authors introduce new quantum division circuits using a primitive called COMP-N-SUB, achieving substantial reductions in T-count, CNOT-count, and T-depth compared to prior methods.

Findings

Up to 76.08% reduction in T-count

Up to 68.35% reduction in CNOT-count

Asymptotic T-depth improved to O(n log n)

Abstract

Arithmetic operations are an important component of many quantum algorithms. As such, coming up with optimized quantum circuits for these operations leads to more efficient implementations of the corresponding algorithms. In this paper, we develop new fault-tolerant quantum circuits for various integer division algorithms (both reversible and non-reversible). These circuits, when implemented in the Clifford+T gate set, achieve an up to 76.08\% and 68.35\% reduction in T-count and CNOT-count, respectively, compared to previous circuit constructions. Some of our circuits also improve the asymptotic T-depth from $O(n^2)$ to $O(n \log n),$ where $n$ is the bit-length of the dividend. The qubit counts are also lower than in previous works. We achieve this by expressing the division algorithms in terms of a primitive we call COMP-N-SUB, that compares two integers and conditionally subtracts them. We show that this primitive can be implemented at a cost, in terms of both Clifford and non-Clifford gates, that is comparable to one addition. This is in contrast to performing comparison and conditional subtraction separately, whose cost would be comparable to a controlled addition plus a regular addition.