Quantum computing:
accelerating progress through co-ordinated action

Accelerating progress through co-ordinated action

Quantum computing is entering a crucial phase. Hardware platforms have transitioned from  experimental prototypes to programmable systems that can be integrated with existing infrastructure  for high-performance computing. With the support of both public and private investment, hardware  developers have a clear mission to scale up their systems while also improving the accuracy and  reliability of qubit operations. Some companies are predicting that they will reach the million-qubit  milestone within the next few years and has set out a credible pathway to achieve fault-tolerant  quantum computing by the end of the decade. 

So far, however, there has been much less focus on the software tools and algorithms that will be  needed to produce useful outcomes from such large-scale quantum computers. Quantum software  challenges have been identified as being just as daunting if not more than the hardware challenges. Important progress has been made in developing quantum algorithms, but major breakthroughs are  still needed to expand the portfolio of useful algorithms and to achieve the dramatic speed-ups that  have been predicted for quantum computing. There is also a mismatch to a certain extent between  the problems that are expected to be useful in industrial setting and computational problems that are  well understood from the perspective of the theory of quantum algorithms. Likewise, quantum  compilers and control systems have been developed for small-scale hardware platforms, but there is  no clear picture of the software environment that will enable high-level programs to run efficiently on many-qubit quantum systems.  

In this transition towards practical utility and adoption, software tools and techniques must be  developed in tandem with evolving hardware platforms to deliver quantum systems that can be deployed and operated at scale. Software solutions will also play a pivotal role in driving the adoption  of quantum computing, building trust in the capabilities of the technology and creating applications  that enable end users in academia and industry to solve problems that are beyond the capabilities of  classical supercomputers. Without a strategic focus on the development of quantum software and  algorithms, there is a risk that unsolved challenges in the software stack will slow progress at a time  when society, industry and government are demanding useful quantum applications.  

The Quantum Software Alliance (QSA) has been formed to highlight the critical role that quantum  software and applications must play as quantum computing enters this new era of practical utility and  advantage. Bringing together expertise from different domains and research institutes around the  world, the QSA aims to provide a coherent vision that will guide the global research agenda as well as  policy and funding decisions. While the QSA can act as a neutral convener for such co-ordinated  action, dedicated focus and support from policymakers and funding agencies will also be critical for  long-term success. 

Delivering the promise of quantum computing

Even the most powerful quantum computer will be unable to deliver a practical advantage without  effective quantum software and algorithms. By taking a strategic approach to the development of  quantum software, we can deliver tangible benefits that will accelerate the pathway towards scalable  quantum applications that deliver real value to society and the economy. 

Benefit 1: Build trust and confidence in the capabilities of emergent quantum technologies 

• Establish standard performance metrics and benchmarking protocols for hardware platforms
• Encourage early adoption by providing an easy and objective comparison between the  performance of different platforms and qubit modalities 
• Verify claims of quantum advantage by establishing a clear framework for assessing the  performance and resource requirements of quantum solutions

Benefit 2: Generate near-term value from early-stage quantum hardware

• Enable seamless integration between quantum hardware and existing supercomputing infrastructure to provide a quantum acceleration for specific computational tasks
• Identify applications where practical quantum advantage can be achieved by matching current algorithms to the physical characteristics of different qubit modalities
• Explore the synergy between quantum computing and artificial intelligence to develop more powerful AI models that consume less energy

Benefit 3: Accelerate progress towards fault-tolerant quantum computing

• Develop robust techniques to correct for errors in small and noisy quantum systems
• Improve control systems to enable reliable operation in a standard computing environment
• Exploit quantum software to identify and refine strategies for scaling up hardware systems, while also adapting the software design to support the evolving needs of the hardware

Benefit 4: Realize the promise of quantum computing to deliver dramatic performance gains

• Identify and develop applications that will enable quantum computing to transform outcomes for substantial social and economic development
• Develop software solutions that make the most efficient use of the resources in many-qubit quantum systems
• Build tools and interfaces that enable end users in industry and academia to develop high value applications featuring practical utility without a detailed understanding of the hardware architecture

Making applications work for industry and society

As quantum computing enters a new phase of practical utility, focus has shifted to understanding the real-world applications that will benefit most from this new computing paradigm. It may be too early to identify a single “killer app”, but quantum computing is expected to deliver transformative outcomes in scientific fields and industries that are constrained by the current capabilities of classical computers, or that rely on understanding and manipulating the nature of quantum systems.

Quantum researchers are already developing algorithms that have the potential to drive positive change for society and economic growth (Table 1). Near-term applications discovery has focused on solving useful problems that yield a practical advantage on small and noisy devices, and has also offered valuable insights that will inform continued development as the technology scales.

As yet, however, only a small number of quantum algorithms have outperformed classical methods, and even fewer have been relevant to real-world applications. More work is needed to identify and develop foundational quantum algorithms that solve a particular type of problem, which would then deliver performance benefits across multiple sectors and scientific domains. Such foundational work will also help to identify the criteria that will enable quantum algorithms to deliver the dramatic speed ups that are expected from quantum computing.

As part of this applications discovery, software tools are also needed to assess whether a quantum solution delivers a genuine advantage over classical computing. This requires standard frameworks for verification and benchmarking, as well as accurate and transparent estimates of all the resources needed to execute a quantum algorithm.

Software systems for scalable quantum computing

While the potential impact of emerging applications tends to capture most attention, improved tools and techniques in the quantum software stack will also be critical to enable quantum computers to execute complex programs while also making most efficient use of the physical quantum resources.

Error correction is essential to obtain reliable results from noisy physical systems, particularly the fragile quantum states used in quantum computing. Error correction is inherently a software problem, albeit one that closely related to the hardware, and improved methodologies are needed to prevent errors from accumulating during long computations, achieve more efficient use of quantum resources, and to scale codes to many-qubit systems. Faster schemes for detecting and correcting errors in decoders will also be vital for real-time quantum computing.

Quantum compilers translate high-level algorithms into code that runs on specific devices; they schedule tasks, reduce computational overheads, manage error correction, and test performance and efficiency. The aim at higher levels of the stack is to create standard solutions that can operate on any hardware, while closer to the hardware layer the subroutines need to be more sensitive to the physical characteristics of the qubits.

Systems-level design makes quantum processors usable at scale, enabling fault-tolerant operation, classical co-processing for control, and integration with high-performance computing. There is still no clear picture of the optimal software environment for a large-scale quantum computer, but the classical model of a layered software stack is unlikely to deliver the best performance. Instead, new frameworks and architectures will be needed to enable high-level programmes to interact more directly with the physical hardware. For integration with high-performance computing, new programming models are needed to embed quantum accelerators within classical codes.

Verification and testing are critical elements of any computing system, and the use of objective and consistent benchmarking will help to assess performance, inform design improvements, and understand the real-world impact of quantum computing. Benchmarking on the hardware level is rather well developed, but known methods are not in the same way able to certify large-scale quantum devices, demanding the development of new tools and techniques that can be integrated into every element of the software stack.

Distributed quantum computing will enable operation at scale by connecting multiple quantum processors together, requiring entangled quantum information to be transmitted over classical communications links. Networking algorithms are needed for real-time routing and scheduling, while additional software is needed to perform authentication tasks. Longer term, maintaining the security of information in any computer system will require new models that enable the migration to quantum safe standards. Quantum communication has the potential to offer improved and novel security guarantees for privacy preserving computing.