Characterizing Bugs and Quality Attributes in Quantum Software: A Large-Scale Empirical Study

Quantum Software Engineering (QSE) is essential for ensuring the reliability and maintainability of hybrid quantum-classical systems, yet empirical evidence on how bugs emerge and affect quality in real-world quantum projects remains limited. This study presents the first ecosystem-scale longitudinal analysis of software defects across 123 open source quantum repositories from 2012 to 2024, spanning eight functional categories, including full-stack libraries, simulators, annealing, algorithms, compilers, assembly, cryptography, and experimental computing. Using a mixed method approach combining repository mining, static code analysis, issue metadata extraction, and a validated rule-based classification framework, we analyze 32,296 verified bug reports. Results show that full-stack libraries and compilers are the most defect-prone categories due to circuit, gate, and transpilation-related issues, while simulators are mainly affected by measurement and noise modeling errors. Classical bugs primarily impact usability and interoperability, whereas quantum-specific bugs disproportionately degrade performance, maintainability, and reliability. Longitudinal analysis indicates ecosystem maturation, with defect densities peaking between 2017 and 2021 and declining thereafter. High-severity defects cluster in cryptography, experimental computing, and compiler toolchains. Repositories employing automated testing detect more defects and resolve issues faster. A negative binomial regression further shows that automated testing is associated with an approximate 60 percent reduction in expected defect incidence. Overall, this work provides the first large-scale data-driven characterization of quantum software defects and offers empirical guidance for improving testing, documentation, and maintainability practices in QSE.