  Quantum computers (QCs) aim to disrupt the status-quo of computing --
replacing traditional systems and platforms that are driven by digital circuits
and modular software -- with hardware and software that operates on the
principle of quantum mechanics. QCs that rely on quantum mechanics can exploit
quantum circuits (i.e., quantum bits for manipulating quantum gates) to achieve
"quantum computational supremacy" over traditional, i.e., digital computing
systems. Currently, the issues that impede mass-scale adoption of quantum
systems are rooted in the fact that building, maintaining, and/or programming
QCs is a complex and radically distinct engineering paradigm when compared to
challenges of classical computing and software engineering. Quantum service
orientation is seen as a solution that synergises the research on service
computing and quantum software engineering (QSE) to allow developers and users
to build and utilise quantum software services based on pay-per-shot utility
computing model. The pay-per-shot model represents a single execution of
instruction on quantum processing unit and it allows vendors (e.g., Amazon
Braket) to offer their QC platforms, simulators, software services etc. to
enterprises and individuals who do not need to own or maintain quantum systems.
This research contributes by 1) developing a reference architecture for
enabling quantum computing as a service, 2) implementing microservices with the
quantum-classic split pattern as an architectural use-case, and 3) evaluating
the reference architecture based on feedback by 22 practitioners. In the QSE
context, the research focuses on unifying architectural methods and
service-orientation patterns to promote reuse knowledge and best practices to
tackle emerging and futuristic challenges of architecting and implementing
Quantum Computing as a Service (QCaaS).
