Quantum compiler passes get better automated testing with retromorphic methods
Retromorphic Testing of Quantum Compiler Passes
Software Engineering
Summary
Checking if quantum programs have been correctly changed by software is hard because quantum circuits can be very complex and tricky to predict. The authors studied how current tests work in several popular quantum programming tools and found most rely on small, simple example programs. To improve testing, they created a new way to automatically check if the changes preserve the program’s meaning using a technique inspired by quantum algorithms. They built this into a tool called RetroQ and found real bugs in existing compilers that were missed before, showing this method can make quantum programming more reliable.
What this means in practice
- •For quantum software developers: Find and fix errors in compiler transformation steps to improve reliability of quantum programming frameworks.
- •For hardware design engineers: Ensure that hardware-specific compiler optimizations do not change the intended quantum circuit behavior before deployment.
Authors
Mushahid Khan, Olivia Di Matteo
Abstract
Quantum compilers play a critical role in transforming high-level quantum programs into optimized, hardware-compatible circuits. However, verifying the correctness of compiler passes remains challenging, as determining the expected output of large, deeply entangled quantum circuits is computationally intractable. This challenge is further amplified when compiler passes modify already complex circuit structures, making manual validation of transformed circuits impractical. In this work, we perform a systematic analysis of unit tests for quantum compiler passes in four quantum programming frameworks (PennyLane, Qiskit, Cirq, and pytket). Our findings indicate validation is dominated by program-content and program-metric assertions, and test circuits are generally small and shallow. Motivated by these observations, we introduce a testing methodology for automated validation of quantum compiler passes based on retromorphic testing and principles from the Hadamard test. This methodology analyzes a compiler pass, test circuit, and expected pass behavior to verify semantic preservation and intended structural modifications. We implement our methods in a framework, RetroQ, and apply it to compiler passes in PennyLane and Qiskit. Experimental evaluation reproduced several existing bugs as well as uncovered previously undetected defects, such as flawed symbolic parameter handling, incorrect commutation logic, failure to recognize self-adjointness of gates, and runtime crashes. These findings highlight the need for compiler-pass-specific testing methodologies to improve the reliability of the evolving quantum software stack.