With support from current literature, we discuss its potential impact on requirements engineering. We identified four

main trends. A feasibility study focusing on quantum aspects is required before or after requirements analysis. The

(*) Corresponding Author.

challenges and difficulties of developing complex software systems. The initial concepts in this discipline began to be

discussed in the late 1960s and early 1970s [1], coinciding with a high degree of maturity in hardware technology, the

proliferation of programming languages, and the software ecosystem stratifying into layers. Since then, SE has evolved

to consolidate processes for the different challenges in developing complex and high-quality software, always on the

development that SE cannot ignore [2].

incorporating the implications of this new technological milestone [2, 5].

It is not foreseen that quantum computers will replace classical computers entirely [6, 7]. Instead, quantum and classical

software will operate together [8]. At the moment, quantum software requires classical programs acting as drivers,

from which they obtain inputs and other parameters that are sometimes used to build quantum software dynamically.

These drivers run quantum software on specific quantum computers in the cloud. Classical software is also devoted to

receiving the execution answers from quantum software and interpreting them to provide an output to the end-users.

This interface between both paradigms, in which a classical program requires a quantum service, reinforces the idea of

defining Quantum Software Engineering (QSE) as a classical SE encompassing the peculiarities of quantum software

development.

currently in the literature.

life cycle, laying the foundation for all subsequent design and development activities. It ensures stakeholder alignment,

enhances cost and time efficiency, mitigates risks, assures quality, and facilitates regulatory compliance. Proper

the development process. This is especially critical in quantum-classical software development, where hybrid systems

and various technologies are involved, necessitating precise and comprehensive requirements to integrate diverse

functionalities seamlessly.

There are three main philosophies of software development: classic, agile, and hybrid. Every strategy organizes the

development process and deals with the requirements differently. In a classic life cycle, extensive documentation is

(algorithms that transform input to output data). In addition to functional requirements, some requirements describe

how the algorithms must be implemented regarding speed (performance), readability (legibility), resource usage, etc.

These requirements are called non-functional requirements and condition the implementation of the functional

requirements. Since quantum computing introduces radically different concepts and paradigms from classical systems,

non-functional requirements have become especially relevant.

technical, operational, and economic feasibility specifically focused on quantum aspects is made. This initial phase,

preceding requirement specification, addresses key questions: Is quantum hardware capable of supporting the required

Volume metric, which includes the number of qubits, error rates, and qubit connectivity, is an important part. In

addition, the Physical Machine Description (PMD), which describes the quantum hardware technology, completes the

specification of quantum requirements. Similarly, Hernández González et al. [16], while trying to approach quantum

software development from an agile perspective, suggest adding an initial Inception phase to an agile methodology

such as Scrum (which then becomes a hybrid methodology). In the Inception phase, all project participants become

familiar with the project characteristics and technological requirements.

The above discussion includes several aspects and activities that current research considers in different phases and/or

with different denominations, groupings, or intensities. Such is the case of the work by Weder et al. [9]. They propose

a phase after the requirements analysis, called quantum-classical splitting, within a general model of QDLC that

establish solid foundations for effective Quantum Software Requirements Engineering (QSRE). However, how to

accomplish this remains unclear. The initial studies in this field exhibit varied content, ranging from those suggesting

a starting point for discussion to those proposing solutions to specific QSRE issues. Among those pursuing a solution,

one suggestion involves dividing requirements for quantum software into two well-defined sets: domain requirements

and quantum requirements [17]. The former requires knowledge of the problem domain similar to traditional

development practices, while the latter necessitates familiarity with Quantum Computing (QC). This latter case poses

a significant challenge for Software Engineers, as there is generally a lack of domain-specific knowledge in quantum

mechanics [18]. In another surveyed study, a review of the impact of quantum computing on all phases of a classic

waterfall SDLC is conducted, where the brief Requirements phase suggests that in the process of specifying

requirements, it is crucial to define user acceptability early on and establish critical performance requirements to

determine if the benefits (still not fully defined) of quantum technology are sufficient over classical computing [19].

and the NISQ (Noisy Intermediate-Scale Quantum) Analyzer [22] that analyzes and selects an appropriate algorithm

implementation and a suitable quantum computer for a chosen quantum algorithm and specific input data.

Classifying requirements is a common proposal for several of the studied works. They propose identifying the

functional and non-functional requirements, as in classical RE, and classifying them into classical, quantum, or hybrid

categories. In the case of hybrid requirements, further disaggregation is necessary to accurately determine whether they

in [23] can provide additional assistance in decision-making.

tools like the one described in [25] may be useful for estimating the necessary quantum hardware applicable to various

quantum technologies and strategies for fault tolerant encoding. The Quantum Operational Feasibility involves

assessing the extent to which the required software applies to solving the business problem and meeting user

requirements. In this regard, it is necessary to address the availability of appropriate algorithms, assessing whether the

less complex quantum part will result in a significant speedup and whether the more complex one will not entail an

unreasonable cost increase. Furthermore, to complete this task, analyzing the availability of skills in quantum

computing expertise within the development team is necessary. And finally, Economic Feasibility evaluates if the

resulting software will represent a useful gain for the client.

When a requirement has been classified as “quantum” and “feasible,” it is advisable to record in its specification

programming languages and compilers; Quantum Integrator Plugins, which are interfaces between the programming

language and the simulator or quantum hardware; Logical Quantum Circuit Synthesizer, an optimizer of quantum

circuits aimed at improving their efficiency and reducing error probabilities; Classical Validator, the classical software

for verifying the solution provided by a quantum algorithm; and Classical Software Requirement Module, a classical

software tool for systematically organizing all requirements in a Software Requirement Specification (SRS) document.

The Specifications section considers both quantum and classical hardware requirements for the project. Thus, it is

necessary to specify at least the Qubit count, which is the number of qubits required by the quantum computer; the QV,

a single-number metric that measures the largest random circuit of equal width and depth that the computer successfully

implements; the Physical Machine Description (PMD) of the underlying quantum hardware technology, the physical

arrangement of qubits, error rates of quantum operations, qubit connectivity, and other aspects related to the physical

implementation of the quantum system; and the Classical Hardware Processor, which is the classical hardware

where a design model can be achieved using a widely used tool in the software engineering community. A consistent

classical-quantum requirement and design model is an important starting point for verifying and validating

requirements.

One converging aspect in the literature is the necessity to classify requirements into functional and non-functional

categories, followed by further sub-classification into quantum, classical, or hybrid. This initial task is common across

Another activity that should be included early in the software development process, even before requirements analysis,

recent start of a transition towards Fault-Tolerant Quantum Computing (FTQC) [27] invites us to consider both QSRE

and the entire QSE with that perspective in mind. Therefore, the discussion in this field remains wide open for ongoing

impact assessment and the search for more suitable alternatives for QSRE tailored to the achieved hardware.

Although the feasibility study mentioned earlier includes the economic aspect, a more comprehensive investigation is

required to evaluate the relative benefits of the speedup provided by the quantum solution compared to the increased

associated costs. The involvement of a collaborative team of quantum experts and the utilization of quantum facilities,

among other factors, may contribute to raising the project budget.

Finally, regarding the methodological approach used for the QRE process, the analyzed research does not thoroughly

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1. Conception and design of the study

2. Data acquisition

3. Data analysis

4. Discussion of the results

5. Writing of the manuscript

6. Approval of the last version of the manuscript

Hernández Gobertti.