  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.
