Space-efficient quantum multiplication of polynomials for binary finite fields with sub-quadratic Toffoli gate count

TL;DR

This paper presents a space-efficient quantum algorithm for multiplying binary polynomials over finite fields, significantly reducing Toffoli gate count using space-efficient Karatsuba methods, with practical CNOT gate counts also improved.

Contribution

It introduces a novel quantum multiplication algorithm that reduces Toffoli gate complexity using space-efficient Karatsuba techniques without requiring ancillary qubits.

Findings

Requires $O(n^{ ext{log}_2(3)})$ Toffoli gates, less than quadratic.

CNOT gate count is higher theoretically but better in practice.

No ancillary qubits needed, assuming error-free quantum computation.

Abstract

Multiplication is an essential step in a lot of calculations. In this paper we look at multiplication of 2 binary polynomials of degree at most $n-1$, modulo an irreducible polynomial of degree $n$ with $2n$ input and $n$ output qubits, without ancillary qubits, assuming no errors. With straightforward schoolbook methods this would result in a quadratic number of Toffoli gates and a linear number of CNOT gates. This paper introduces a new algorithm that uses the same space, but by utilizing space-efficient variants of Karatsuba multiplication methods it requires only $O(n^{\log_2(3)})$ Toffoli gates at the cost of a higher CNOT gate count: theoretically up to $O(n^2)$ but in examples the CNOT gate count looks a lot better.