Quantum Fourier Transforms and the Complexity of Link Invariants for Quantum Doubles of Finite Groups

TL;DR

This paper explores the computational complexity of link invariants derived from quantum doubles of finite groups, develops quantum Fourier transform algorithms for these structures, and analyzes their implications for quantum computation and complexity classes.

Contribution

It introduces efficient quantum Fourier transform circuits for D(G), analyzes the complexity of related link invariants, and advances understanding of simulating anyonic quantum computation in group-based models.

Findings

Efficient quantum Fourier transform circuits for D(G) and D(S_n).

Certain D(G) invariants are BPP-hard, SBP-hard, and #P-hard to approximate or evaluate.

Partial progress on simulating anyonic computation using fluxon irreps and Clebsch-Gordan transforms.

Abstract

Knot and link invariants naturally arise from any braided Hopf algebra. We consider the computational complexity of the invariants arising from an elementary family of finite-dimensional Hopf algebras: quantum doubles of finite groups (denoted D(G), for a group G). Regarding algorithms for these invariants, we develop quantum circuits for the quantum Fourier transform over D(G); in general, we show that when one can uniformly and efficiently carry out the quantum Fourier transform over the centralizers Z(g) of the elements of G, one can efficiently carry out the quantum Fourier transform over D(G). We apply these results to the symmetric groups to yield efficient circuits for the quantum Fourier transform over D(S_n). With such a Fourier transform, it is straightforward to obtain additive approximation algorithms for the related link invariant. Additionally, we show that certain D(G) invariants (such as D(A_n) invariants) are BPP-hard to additively approximate, SBP-hard to multiplicatively approximate, and #P-hard to exactly evaluate. Finally, we make partial progress on the question of simulating anyonic computation in groups uniformly as a function of the group size. In this direction, we provide efficient quantum circuits for the Clebsch-Gordan transform over D(G) for "fluxon" irreps, i.e., irreps of D(G) characterized by a conjugacy class of G. For general irreps, i.e., those which are associated with a conjugacy class of G and an irrep of a centralizer, we present an efficient implementation under certain conditions such as when there is an efficient Clebsch-Gordan transform over the centralizers. We remark that this also provides a simulation of certain anyonic models of quantum computation, even in circumstances where the group may have size exponential in the size of the circuit.