The successful adoption of quantum computing in the real world will depend not only on the necessary scientific/technical breakthroughs in the field of quantum hardware, but also on the pace of development of the natural companion for successful computing technologies: software. Of crucial importance in this process will be the capabilities we are able to build in the short term to produce software solutions and systems that realise quantum business benefits and, sooner rather than later, monetise their practical value to industry. To be successful in this regard, it will be essential to develop quantum software for industry with the design, architecture, extensibility, scalability, high performance and security guarantees that industry needs and demands. Based on our extensive experience in the development of software for industry, we know that the natural integration of all these elements, each with its own specific weight as part of the whole, is only feasible if the development of quantum software is done from a professional perspective.

Motivated by the relevance of this issue, on 14 April, World Quantum Day, we published the short article Some reflections on professional quantum/classical software development[1]. Having made that sort of general introduction to the subject, we thought it appropriate to address, in more detail, some of the different aspects of the broad, complex, and changing “world” of professional quantum software development for industry. To this end, in this instalment we will focus on the lifecycle of hybrid quantum/classical software systems.

The development of industry-ready quantum software systems is hybrid because, for these systems to be useful, they must necessarily integrate and operate dynamically and transparently for users with classical IT, as is the case with very many other services in the classical software domain. The classic IT of each company or organisation is specific, different, often custom developed and/or implemented to suit the needs and objectives of its owners. Therefore, among other issues, designing and implementing quantum/classical hybrid systems useful for the real world is often quite far from the limits and scopes that are usually set for proof-of-concept programming, prototyping or laboratory/academic experiments.

Successfully tackling challenges of this magnitude requires much more than conceiving and developing the indispensable and fabulous quantum algorithms, it is also necessary to do so in such a way that these algorithms are a harmonious part of a software system capable of integrating with classical industry technologies, robustly withstanding the passage of time and able to absorb the changing business needs and the other solutions and technologies with which they interact, be part of a complex and evolving set of IT services, be projects based on good software engineering practices, and have the right tools to make them possible in an “industrial” and efficient way (in the real world this means being cost competitive and offering the best time to market).

When hybrid quantum/classical software solutions are developed in the framework described above we are in the realm of what we consider professional development and application of Quantum Software Engineering principles[2] that are directly linked to the proven Software Engineering best practice of managing hybrid quantum/classical software systems throughout their lifecycle:

·       embracing the coexistence of classical and quantum computing

·       support the management of quantum software development projects

·       consider the evolution of quantum software

·       aim to deliver quantum software with zero desirable defects

·       promote the reusability of quantum software

·       cover software governance and management

The quantum/classical hybrid software lifecycle

The first complexity we must address when developing industry-ready hybrid quantum/classical software systems is that these projects are composed of two parts, each with their respective development teams, their own characteristics, complexities, and corresponding software lifecycle:

·       Classical software lifecycle

·       Quantum software lifecycle

As it is not difficult to guess, each of these cycles has its specificities, functional, technical and integration requirements, its technological environments, but both share a great common goal to contribute to the creation of a harmonious quantum/classical hybrid software system that can be used by business users and organisations in their daily activities.

Each of these life cycles has its own phases which, although they share the same name, differ substantially in the scope and content of the work to be done in each of them, namely: from the conception and analysis activities to the design, implementation and testing of the system, as well as its different evolutions.

How can we integrate both cycles in such a way that we work with a single hybrid software development cycle? Our solution to this question has been to develop a model based on the ICSM[3] model that is able to address the most common challenges faced by hybrid software development projects[4]:

·       Multi-owner and multi-mission systems of systems (SoS).

·       Emergence and “human intensive” with many non-pre-specifiable requirements, non-pre-specifiable budgets and schedules leading to the need to manage uncertainty and risk.

·       Rapid pace of change requiring incremental developments and adaptability to avoid technology obsolescence and changes in mission priorities.

Moreover, because this spiral model is based on four fundamentals that we consider essential for hybrid software development projects:

·       stakeholder value orientation

·       incremental commitment and accountability

·       concurrent multidisciplinary engineering

·       evidence- and risk-based decisions

These fundamentals, combined with appropriate models of team management, collaboration, product engineering, project planning and management, configuration and maintenance, and quality, result in a framework that facilitates the successful management of hybrid quantum/classical software development projects.

Given that the management of the classic software lifecycle is widely known, we will not delve into the details of its management, although this does not mean that we do not consider it to be very important to ensure a good hybridization of classic and quantum software[5]. In this sense, we will only highlight that in order to make a good hybridization with quantum services, the classical solution in charge of the connection with quantum services, as with any other professionally developed classical software, the development team must design, build, test, deploy and maintain the software solution throughout its useful life. The only caveat is that as long as there are no error-free quantum vendor proposals, the development team should seriously consider how to deal with this risk, as well as the risk associated with the availability of different technology proposals.

We know that one of the characteristics of quantum software development teams globally today is the multidisciplinary composition of the teams, so it is very common that among those who perform tasks as developers there are many technicians who, by profession, are not programmers and/or computer engineers, so not only are they unaware of the good practices of quantum software engineering, but they are also unaware of the importance of developing quantum software. We also know that, even for computer engineers, there is not much knowledge about the emerging discipline of quantum software engineering and, therefore, about the specifics of the quantum software lifecycle.

With all of them in mind, we have designed and developed QuantumPath®, a full-stack platform for the professional development of quantum software and the control of its lifecycle: from the creation of the quantum algorithm, through its development, testing and application, to its deployment, execution, and reuse. A platform that makes it possible to work with Software Engineering best practices, even without knowing them. And considering the risk associated with the adoption of the aforementioned technology, a platform that, thanks to its agnosticism by design, will reduce this level of risk to a controlled point that will not critically affect the end user.

Hybrid quantum/classical software lifecycle management with QuantumPath®

QuantumPath® is an ecosystem of tools, services and processes that simplify the development of quantum algorithms embedded in hybrid information systems, a platform capable of managing quantum algorithms and quantum technology agnostic solutions (gate-based, quantum annealing) assisted by general purpose tools.

QuantumPath® provides developers with a wide range of objects for the organization of quantum software projects, including project management (team, roles, permissions, security, etc.), as well as management of the solution itself, its assets, etc.

As shown in Figure 1, QuantumPath® not only supports a complete lifecycle for creating and maintaining professional quantum software solutions (from quantum algorithm creation through development, testing and deployment, to deployment and reuse), but also offers a complete ALM stack that accelerates the design, construction, and deployment of quantum applications.