The Qudit ZH-Calculus: Generalised Toffoli+Hadamard and Universality

TL;DR

This paper introduces the qudit ZH-calculus, generalizes qubit rules to qudits, and proves its universality for matrices over specific rings, extending the connection between ZH-diagrams and universal quantum circuits to higher dimensions.

Contribution

It generalizes the phase-free ZH-calculus to qudits, proves its universality for prime dimensions, and extends the connection between ZH-diagrams and universal quantum circuits beyond qubits.

Findings

Phase-free qudit ZH-calculus is universal for matrices over Z[e^{2 ext{pi}i/d}] for prime d.

Two-qudit |0>-controlled X gates can construct all classical reversible qudit logic circuits in odd dimensions.

Circuits of |0>-controlled X and Hadamard gates are approximately universal for qudit quantum computing.

Abstract

We introduce the qudit ZH-calculus and show how to generalise all the phase-free qubit rules to qudits. We prove that for prime dimensions d, the phase-free qudit ZH-calculus is universal for matrices over the ring Z[e^2(pi)i/d]. For qubits, there is a strong connection between phase-free ZH-diagrams and Toffoli+Hadamard circuits, a computationally universal fragment of quantum circuits. We generalise this connection to qudits, by finding that the two-qudit |0>-controlled X gate can be used to construct all classical reversible qudit logic circuits in any odd qudit dimension, which for qubits requires the three-qubit Toffoli gate. We prove that our construction is asymptotically optimal up to a logarithmic term. Twenty years after the celebrated result by Shi proving universality of Toffoli+Hadamard for qubits, we prove that circuits of |0>-controlled X and Hadamard gates are approximately universal for qudit quantum computing for any odd prime d, and moreover that phase-free ZH-diagrams correspond precisely to such circuits allowing post-selections.