Qudit versions of the qubit "pi-over-eight" gate

TL;DR

This paper generalizes the qubit pi-over-eight gate to prime-dimensional qudits, analyzes their algebraic structure, and explores their potential for robust, fault-tolerant quantum computation through magic-state distillation.

Contribution

It derives explicit formulas for qudit pi-over-eight gates, examines their group structure, and investigates their robustness and utility in universal quantum computing.

Findings

Qudit pi-over-eight gates are maximally robust to certain noise types.

The group structure of these gates varies with qudit dimension.

Evidence suggests these gates can enable universal quantum computation when combined with Clifford gates.

Abstract

When visualised as an operation on the Bloch sphere, the qubit "pi-over-eight" gate corresponds to one-eighth of a complete rotation about the vertical axis. This simple gate often plays an important role in quantum information theory, typically in situations for which Pauli and Clifford gates are insufficient. Most notably, when it supplements the set of Clifford gates then universal quantum computation can be achieved. The "pi-over-eight" gate is the simplest example of an operation from the third level of the Clifford hierarchy (i.e., it maps Pauli operations to Clifford operations under conjugation). Here we derive explicit expressions for all qudit (d-level, where d is prime) versions of this gate and analyze the resulting group structure that is generated by these diagonal gates. This group structure differs depending on whether the dimensionality of the qudit is two, three or greater than three. We then discuss the geometrical relationship of these gates (and associated states) with respect to Clifford gates and stabilizer states. We present evidence that these gates are maximally robust to depolarizing and phase damping noise, in complete analogy with the qubit case. Motivated by this and other similarities we conjecture that these gates could be useful for the task of qudit magic-state distillation and, by extension, fault-tolerant quantum computing. Very recent, independent work by Campbell, Anwar and Browne confirms the correctness of this intuition, and we build upon their work to characterize noise regimes for which noisy implementations of these gates can (or provably cannot) supplement Clifford gates to enable universal quantum computation.