Affine Equivalence in the Clifford Hierarchy

TL;DR

This paper investigates the structure of permutations within the Clifford Hierarchy, classifies 4-qubit permutations, and introduces cycle structures, revealing new insights into the algebraic properties of quantum gates.

Contribution

It provides a complete classification of 4-qubit permutations in the Clifford Hierarchy and proves all 4-qubit third-level gates are semi-Clifford, introducing cycle structures for permutations.

Findings

All 4-qubit permutations in the Clifford Hierarchy are classified.

All third-level 4-qubit gates are semi-Clifford.

Cycle structures are classified up to affine equivalence, independent of qubit number.

Abstract

In this paper we prove a collection of results on the structure of permutations in the Clifford Hierarchy. First, we leverage results from the cryptography literature on affine equivalence classes of 4-bit permutations which we use to find all 4-qubit permutations in the Clifford Hierarchy. We then use the classification of 4-qubit permutations and previous results on the structure of diagonal gates in the Clifford Hierarchy to prove that all 4-qubit gates in the third level of the Clifford Hierarchy are semi-Clifford. Finally, we introduce the formalism of cycle structures to permutations in the Clifford Hierarchy and prove a general structure theorem about them. We also classify many small cycle structures up to affine equivalence. Interestingly, this classification is independent of the number of qubits.