Functional Size Measurement of Hybrid Classical Quantum Software Systems

Accurate sizing of software systems is essential for effective project management and resource allocation. Hybrid classical-quantum software systems consist of a quantum component, which executes quantum algorithms and incorporates error correction mechanisms (overhead)-a critical aspect in the Noisy Intermediate-Scale Quantum (NISQ) era-and a classical component, which controls quantum operations and manages error propagation. This paper explores functional sizing for hybrid classical-quantum software. It highlights the challenges arising from the interaction between the classical and quantum components, particularly with respect to the abstraction layers and granularity levels. A key principle emphasized is that functional sizing should not extend across multiple abstraction layers within a single measurement scope. While quantum software typically operates at a low-level implementation-specific layer, classical software generally functions at a higher level of abstraction. This divergence emphasizes the need to clarify how functional size measurement should be applied using set theory to account for requirements specified at different abstraction layers in hybrid computing environments.