QbC: Quantum Correctness by Construction

TL;DR

QbC introduces a novel approach to quantum program development by constructing programs directly from specifications, ensuring correctness through formal refinement rules, and aiding in the design of quantum algorithms.

Contribution

It presents Quantum Correctness by Construction (QbC), a method for building correct quantum programs from specifications using formal refinement rules.

Findings

QbC effectively constructs quantum programs from specifications.

The approach highlights key design choices in quantum algorithm development.

QbC supports the systematic design and classification of quantum software.

Abstract

Thanks to the rapid progress and growing complexity of quantum algorithms, correctness of quantum programs has become a major concern. Pioneering research over the past years has proposed various approaches to formally verify quantum programs using proof systems such as quantum Hoare logic. All these prior approaches are post-hoc: one first implements a program and only then verifies its correctness. Here we propose Quantum Correctness by Construction (QbC): an approach to constructing quantum programs from their specification in a way that ensures correctness. We use pre- and postconditions to specify program properties, and propose sound and complete refinement rules for constructing programs in a quantum while language from their specification. We validate QbC by constructing quantum programs for idiomatic problems and patterns. We find that the approach naturally suggests how to derive program details, highlighting key design choices along the way. As such, we believe that QbC can play a role in supporting the design and taxonomization of quantum algorithms and software.