A Practical Quantum Hoare Logic with Classical Variables, I

TL;DR

This paper introduces a Hoare-style logic tailored for quantum programs with classical variables, enhancing expressivity, clarity, and simplicity to facilitate verification and integration with classical logic.

Contribution

It presents a novel quantum Hoare logic that incorporates classical variables, quantum arrays, parameterized gates, and an intuitive proof system compatible with classical verification tools.

Findings

Enhanced expressivity for quantum programs with classical variables

More intuitive correctness specifications combining classical and quantum predicates

Simplified proof system that integrates with classical Hoare logic

Abstract

In this paper, we present a Hoare-style logic for reasoning about quantum programs with classical variables. Our approach offers several improvements over previous work: (1) Enhanced expressivity of the programming language: Our logic applies to quantum programs with classical variables that incorporate quantum arrays and parameterised quantum gates, which have not been addressed in previous research on quantum Hoare logic, either with or without classical variables. (2) Intuitive correctness specifications: In our logic, preconditions and postconditions for quantum programs with classical variables are specified as a pair consisting of a classical first-order logical formula and a quantum predicate formula (possibly parameterised by classical variables). These specifications offer greater clarity and align more closely with the programmer's intuitive understanding of quantum and classical interactions. (3) Simplified proof system: By introducing a novel idea in formulating a proof rule for reasoning about quantum measurements, along with (2), we develop a proof system for quantum programs that requires only minimal modifications to classical Hoare logic. Furthermore, this proof system can be effectively and conveniently combined with classical first-order logic to verify quantum programs with classical variables. As a result, the learning curve for quantum program verification techniques is significantly reduced for those already familiar with classical program verification techniques, and existing tools for verifying classical programs can be more easily adapted for quantum program verification.