Avner Bensoussan

Testability—the degree to which faults can be revealed and diagnosed—has long been fundamental in software engineering but remains largely unexplored for quantum software. This work presents the first formalisation of testability for quantum programs, adapting classical concepts to quantum computation. We introduce Quantum Squeeziness, an information-theoretic metric inspired by classical squeeziness, to quantify how faults are masked and prevented from propagating to observable outcomes. We treat it as an inverse measure of testability: higher squeeziness implies stronger fault masking and reduced detectability. We propose an efficient method to compute Quantum Squeeziness and validate it through large-scale quantum mutation analysis. Across more than 50k mutants, we observe a strong negative correlation between squeeziness and mutation score, showing that higher squeeziness predicts lower testability. We further introduce a technique that constrains where and how perturbations are injected, reducing cost while preserving accuracy. The computation is 15% faster than a standard error-propagation baseline, while the optimised variant, FastSqueeziness, reduces mean runtime from 3200 s to 189 s (16.9× speedup) with 95% accuracy. By establishing a rigorous foundation of testability for quantum software, this work opens new directions at the intersection of software engineering, quantum information theory, and testing. Beyond testing and debugging, it also supports applications in quantum error mitigation and correction, enabling principled assessment and improvement of program robustness.

With the popularity of Hybrid Quantum-Classical architectures, particularly noisy intermediate-scale quantum (NISQ) architectures in the gate-based quantum computing paradigm, comes the need for quality assurance methods tailored to their specific faults. In this study, we propose a taxonomy of faults in gate-based Hybrid Quantum-Classical architectures accompanied by a dataset of real faults in the identified categories. To achieve this, we empirically analysed open-source repositories for fixed faults. We analysed over 5000 closed issues on GitHub and pre-selected 529 of them based on rigorously defined inclusion criteria. We selected 133 faults that we labelled around symptoms and the origin of the faults. We cross-validated the classification and labels assigned to every fault between two of the authors. As a result, we introduced a taxonomy of real faults in gate-based Hybrid Quantum-Classical architectures. Subsequently, we validated the taxonomy through interviews conducted with eleven developers. The taxonomy was dynamically updated throughout the cross-validation and interview processes. The final version was validated and discussed through surveys conducted with an independent group of domain experts to ensure its relevance and to gain further insights.

Contemporary quantum computers are inherently noisy, posing significant challenges for the development and testing of quantum software. Simplified or outdated noise assumptions can lead to incorrect assessments of program correctness, obscure debugging, and hinder cross-platform portability, creating a critical quantum software development gap. Providing accurate, practical noise characterisation is challenging as traditional reconstruction methods scale exponentially and rapidly become outdated. In this vision paper, we address this gap via a novel classical shadow tomography-based pipeline, SIMSHADOW, enabling efficient, continuously updatable noise fingerprinting from empirical observations, suitable for integration into quantum software development workflows, including testing and validation. We prototyped the pipeline to investigate fingerprints’ ability to capture structured, interpretable noise and cross-platform discrepancies affecting quantum programs’ behaviour to support realistic testing and debugging in future tools. Our evaluation with Qiskit and Cirq under widely used hardware-informed profiles, IBM Boston and Quantinuum H2, shows fingerprints exhibit channel-specific structure and yield interpretable heatmaps. We observed systematic cross-platform discrepancies under matched noise configurations, quantified by large Frobenius distances at a fraction of full tomography cost. On 69 MQTBENCH programs, larger fingerprint differences correlate with output distributions divergences, highlighting threats for testing and cross-platform debugging tasks.