Quantum technology has led to increasingly sophisticated and complex quantum devices. Assessing their reliability (quantum reliability) is an important issue. Although reliability theory for classical devices has been well developed in industry and technology, a suitable metric on quantum reliability and its loss has not been systematically investigated. Since reliability-loss depends on the process, quantum fidelity does not always fully depict it. This study provides a metric of quantum reliability by shifting the focus from state-distinguishing to trajectory-distinguishing. In contrast to the conventional notion of classical reliability, which is evaluated using probabilistic measurements of binary logical variables, quantum reliability is grounded in the quantum probability amplitude or wave function. This research provides a universal framework for reliability theory encompassing both classical and quantum devices. It offers a new perspective on quantum engineering by elucidating how intensely the real quantum process a device undergoes influences its performance.

5 pages, 3 figures. Comments welcome!