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Distributed Quantum Computing: From Single QPU to High-Performance Quantum Systems


Core Concepts
Distributed quantum computing is an emerging approach that aims to vastly increase computational power by interconnecting multiple quantum processing units (QPUs) through quantum networks and communications.
Abstract
This paper provides a comprehensive survey of the current state of the art in distributed quantum computing (DQC). It explores the foundational principles, landscape of achievements, challenges, and promising directions for further research in this field. The key highlights and insights are: Distributed quantum computing is an attractive approach to address the limitations of standalone quantum systems, such as decoherence, dissipation, and crosstalk, by interconnecting multiple QPUs through quantum networks. The review presents a layered model for DQC, covering the physical layer for quantum communications (e.g., quantum entanglement, teleportation, and transducers), the network layer for interconnecting QPUs, the development layer for distributing and executing quantum applications, and the application layer for distributed quantum algorithms. At the physical layer, the paper discusses the quantum mechanical tools available for transmitting quantum information, including quantum entanglement, teleportation, entanglement swapping, and quantum gate teleportation. It also reviews the various quantum devices, such as transducers, memories, repeaters, and switches, that enable the implementation of these quantum communication protocols. The network layer explores proposals for quantum network architectures, protocols, and protocol stacks that enable the creation and distribution of entanglement between QPUs, both in local and wide-area networks. The development layer discusses solutions for partitioning, distributing, compiling, and mapping quantum applications to run on distributed quantum systems. The application layer presents different proposals for distributed quantum algorithms, such as the distributed versions of the Grover and Shor algorithms. The review concludes by summarizing the current state of the art and highlighting open research directions in the field of distributed quantum computing.
Stats
As researchers continue to push the boundaries of quantum technologies to unprecedented levels, distributed quantum computing raises as an obvious path to explore with the aim of boosting the computational power of current quantum systems. Problems of both fundamental origin – decoherence, dissipation, and crosstalk – and practical origin – processor topology, cabling, connectors, and control electronics – hinder the fabrication of ultra-large Quantum Processing Units (QPUs). A distributed infrastructure with several quantum processors that contain a limited number of qubits could overcome this difficulty. There is almost a consensus among both the academic community and companies that the practical realization of large-scale quantum processors should adopt a distributed approach based on clusters of small, modular quantum chips within a network infrastructure, with classical and/or quantum communications.
Quotes
"As researchers continue to push the boundaries of quantum technologies to unprecedented levels, distributed quantum computing raises as an obvious path to explore with the aim of boosting the computational power of current quantum systems." "Problems of both fundamental origin – decoherence, dissipation, and crosstalk – and practical origin – processor topology, cabling, connectors, and control electronics – hinder the fabrication of ultra-large Quantum Processing Units (QPUs)." "There is almost a consensus among both the academic community and companies that the practical realization of large-scale quantum processors should adopt a distributed approach based on clusters of small, modular quantum chips within a network infrastructure, with classical and/or quantum communications."

Deeper Inquiries

How can the development of quantum network protocols and architectures be accelerated to enable seamless integration of distributed quantum computing systems?

The development of quantum network protocols and architectures can be accelerated through collaborative efforts between researchers, industry experts, and policymakers. One approach is to establish standardized protocols and interfaces for quantum communication, similar to classical networking standards like TCP/IP. This would streamline the integration of different quantum technologies and facilitate interoperability between quantum devices from various manufacturers. Additionally, investing in research and development to improve the efficiency and reliability of quantum communication channels, such as quantum repeaters and quantum memories, can enhance the scalability of quantum networks. Furthermore, creating testbeds and simulation environments for testing and validating quantum network protocols can help identify and address potential challenges before real-world deployment.

What are the potential trade-offs between the complexity of distributed quantum systems and their computational advantages compared to standalone quantum processors?

Distributed quantum systems offer several computational advantages, such as increased qubit connectivity, fault tolerance, and scalability. However, these advantages come with trade-offs in terms of complexity. One major trade-off is the increased overhead in managing entanglement resources and coordinating operations between multiple qubits in distributed systems. This complexity can lead to higher latency and resource consumption compared to standalone quantum processors. Additionally, distributed systems may face challenges in maintaining entanglement over long distances, which can introduce errors and reduce the overall computational efficiency. Balancing the benefits of increased computational power with the complexities of managing distributed quantum systems is crucial for optimizing performance and achieving practical applications.

How can the field of distributed quantum computing leverage advancements in classical high-performance computing to achieve breakthroughs in practical applications?

The field of distributed quantum computing can leverage advancements in classical high-performance computing (HPC) in several ways to achieve breakthroughs in practical applications. One approach is to integrate quantum processors with classical HPC infrastructure to create hybrid quantum-classical supercomputers. This hybrid approach can harness the strengths of both quantum and classical computing to solve complex problems more efficiently. Additionally, classical HPC techniques can be used to optimize quantum algorithms, improve error correction protocols, and enhance the overall performance of distributed quantum systems. Collaborative research projects between quantum computing experts and HPC specialists can drive innovation and accelerate the development of practical applications in areas such as cryptography, optimization, and material science. By leveraging the complementary strengths of classical and quantum computing, breakthroughs in practical applications can be achieved more effectively.
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