Hadamard-$Π$: Equational Quantum Programming

Abstract: Quantum computing offers advantages over classical computation, yet the precise features that set the two apart remain unclear. In the standard quantum circuit model, adding a 1-qubit basis-changing gate -- commonly chosen to be the Hadamard gate -- to a universal set of classical reversible gates yields computationally universal quantum computation. However, the computational behaviours enabled by this addition are not fully characterised. We give such a characterisation by introducing a small quantum programming language extending the universal classical reversible programming language $\Pi$ with a single primitive corresponding to the Hadamard gate. The language comes equipped with a sound and complete categorical semantics that is specified by a purely equational theory, enabling reasoning about the equivalence of quantum programs in a way that can be automated. Completeness is shown by means of a novel finite presentation, and corresponding synthesis algorithm, for the groups of orthogonal matrices with entries in the ring $\mathbb{Z}[\tfrac{1}{\sqrt{2}}]$.