# The Quantum Web: Interconnecting Quantum Processors for Larger-Scale Machines

## Article #4 of Engineering the Quantum Future Series: Interconnected quantum processors link multiple quantum units to enhance system-wide performance, enabling breakthroughs in computing speed and efficiency across diverse scientific fields.

A close-up view of a quantum computer chip

*This is the fourth article in a six-part series featuring articles on "**Engineering the Quantum Future**". The series explains the revolutionary advancements in quantum computing and their implications for various industries. Each article discusses a specific aspect of this transformative technology, from the fundamental concepts of quantum computing to the practical applications and challenges. This series is sponsored by Mouser Electronics. Through the sponsorship, Mouser Electronics promotes innovation and the exchange of knowledge, aiming to harness the revolutionary capabilities of quantum computing for a smarter and safer technological future.*

Quantum computing is rapidly advancing beyond the limitations of traditional processing, venturing into the space of interconnected quantum processors. This development is pivotal for creating larger-scale machines that integrate multiple processors, exponentially enhancing computational abilities and operational efficiency. Such interconnected systems promise to address complex challenges across various fields by performing intricate calculations at unprecedented speeds, setting the stage for breakthroughs in science and technology that were once beyond reach.

This article explores the cutting-edge field of quantum interconnectivity, detailing how linking quantum processors can amplify computing power and accelerate complex calculations. It dives into the groundbreaking advancements and challenges shaping this rapidly evolving technology.

## Quantum Connectivity: Building the Next Generation of Quantum Processors

From accelerating drug discovery to advancing artificial intelligence and cryptography, quantum computing is expected to positively impact the lives of billions of people worldwide. Forget days, months, and years—quantum computers can rapidly complete complicated tasks in milliseconds. For example, in July of 2023, Google's quantum system outperformed the world's fastest supercomputer when it instantly—and accurately—processed a series of complex calculations that would have taken its rival over 47 years to complete!^{[1]}

Although it will take some time for quantum computing systems to become as ubiquitous as the laptops, smartphones, and tablets we use every day, Sebastian Weidt, chief executive of Brighton-based start-up Universal Quantum, foresees an exciting future in which quantum computers with thousands of quantum bits (qubits) "deliver value to society in a way that classical computers never will be able to."^{[2]}

According to McKinsey, 5,000 quantum computers will likely be operational by 2030, with wider availability of specialized hardware and software expected in 2035.^{[3]} However, system designers can potentially speed this incremental timeline by interconnecting quantum processors to build large-scale, fault-tolerant machines powered by thousands of qubits.

What are the advantages and challenges of interconnecting quantum processors, you might ask. Let’s take a closer look at current interconnectivity techniques and explore how quantum systems could be designed in the future.

## Why Interconnect Quantum Processors?

Classical—or conventional Von Neumann computing—utilizes bits stored as either zeros or ones. Unlike classical bits, which can be in one of two states (0 or 1), qubits exist simultaneously in a superposition of both states. This allows quantum computers to process many possibilities at once—a technique known as quantum parallelism. Moreover, qubits can be entangled, meaning the state of one qubit depends on the state of another to support highly coordinated computational processes.

Although quantum systems are continuously evolving, most quantum processors currently pack only a limited number of qubits. Interconnecting multiple processors in a single quantum system significantly increases qubit counts while supporting access to crucial computational resources such as on-chip memory. Put simply, this approach allows quantum systems to calculate complex equations more quickly and efficiently for a broader range of practical, real-world applications.

Interconnecting quantum processors also enables quantum engineers to increase system-wide fault tolerance and allocate additional qubits to support resource-intensive error correction schemes. The latter typically involves encoding a bit of quantum information into an ensemble of qubits that function as a single logical qubit.

## Challenges in Quantum Interconnection

Interconnecting quantum processors—and the qubits they contain—presents a distinct set of challenges for system designers that differ from those in classical computing. Some of the challenges include:

**Decoherence:**Qubits can lose their quantum state due to interactions with other (system) qubits or the surrounding environment. While beneficial in many ways, interconnecting quantum processors and their qubits increases the chances of decoherence, which can be mitigated with advanced error correction schemes. Researchers at Google Quantum AI recently created a surface code scheme that effectively scales to support an error rate of approximately one in a million.^{[4]}**Loss of fidelity:**As quantum information is transferred between processors and qubits, there's a potential loss of fidelity, meaning the accuracy of information can degrade. Decreasing the critical current, Josephson capacitance, and gate capacitance are just some of the techniques quantum engineers use to maintain fidelity.^{[5]}**Timing and synchronization:**Quantum operations require precise timing for reliable and consistent calculations. Although delivering perfect synchronization across interconnected processors is difficult, researchers are improving clock synchronization reliability with different quantum entanglement techniques.^{[6]}

## Current Solutions and Techniques

Despite the challenges, system designers are successfully interconnecting processors and transferring data between quantum processors using various solutions and techniques.

For example, photonic technology—including readily available optical switches, splitters, and fiber optic cables—enables quantum data to move seamlessly between processors. By converting the quantum state of a qubit to a photon, transferring the photon to another location, and then converting it back to a qubit state, quantum information can be transmitted between processors without a physical connection.

System designers also use quantum repeaters to help maintain data fidelity between multiple processors. Similar in principle to classical repeaters that boost signal strength in traditional networks, quantum repeaters rely on advanced quantum entanglement techniques. According to the Argonne National Laboratory, entanglement swapping is currently the most effective method of transferring quantum information over long and lossy channels without losing or corrupting fragile quantum states.^{[7]}

Looking beyond quantum entanglement, a simpler technique called "UQConnect" uses electric field links to move data rapidly and precisely between quantum processors. Researchers at the University of Sussex say UQ Connect, which allows chips to slot together like a jigsaw puzzle to create a large-scale quantum computer, delivers a stable connection rate of 2424/s.^{[8]}

Lastly, microwave channels and interconnects are helping system designers link quantum processors and enable the transfer of various quantum states between qubits. However, more work needs to be done in this area, as propagating quantum microwave technology is around two decades behind quantum optics at visible and infrared wavelengths.

## The Future of Quantum Interconnection

As MIT's Bharath Kannan recently noted, the successful implementation of quantum interconnects marks a crucial step toward modular iterations of larger-scale machines built from smaller individual components.^{[9]} To be sure, ensuring seamless communication between smaller subsystems will enable a more modular architecture for quantum processors, which Kannan describes as a "simpler way" of scaling to support larger system sizes.

In the future, the need for large-scale quantum machines will only continue to grow, driving innovative new approaches to improve quantum interconnectivity significantly. For example, next-generation quantum systems could feature hybrid architectures that combine different types of qubits across multiple processors to achieve optimal performance. Concurrently, the number of qubits per processor is likely to scale from dozens and hundreds to tens of thousands.

Although there are inherent challenges in maintaining fragile quantum states when transferring data between interconnected processors, recent technological advancements and new techniques are paving the way for large-scale quantum machines. The next decade promises exciting developments in this domain, bringing the semiconductor industry closer to designing powerful, interconnected quantum computers capable of solving many real-world problems beyond the reach of classical machines.

## Amphenol SV Microwave Non-Magnetic RF Connectors & Adapters: Revolutionizing Connectivity in Sensitive Environments

Amphenol SV Microwave Non-Magnetic RF Connectors and Adapters are engineered to deliver high performance in environments where magnetic interference is a concern. These connectors and adapters use non-magnetic materials and plating to ensure minimal magnetic susceptibility and no electric field distortion. Available in both threaded (SMA series) and blind-mate (SMPM series) configurations, they support frequencies up to 65GHz. They are particularly suited for critical applications in medical, aerospace, and quantum computing sectors where maintaining signal integrity is crucial.

## Conclusion

Interconnected quantum processors involve linking multiple quantum computing units to enhance collective performance. This approach can surpass current computational limits across a number of sectors, such as medicine and artificial intelligence. Despite facing significant challenges like quantum decoherence and synchronization issues, the potential for faster, more efficient computing could fundamentally extend our understanding of various scientific disciplines.

*This article was initially published in " METHODS: Engineering the Quantum Future," an e-magazine by Mouser Electronics. It has been substantially edited by the Wevolver team and Ravi Y Rao. It's the fourth article in the Engineering the Quantum Future Series. Upcoming articles will introduce readers to more trends and technologies transforming quantum computing.*

*The introductory article explores the current state of quantum computing, highlighting its challenges and potential*

*The first article dives into the foundational concepts of quantum computing, covering its theoretical basis, technological evolution, and potential applications*

*The second article takes a look at some transformative breakthroughs in quantum computing*

*The third article examines the role of quantum computing in enhancing energy efficiency*

*The fourth article explains quantum interconnectivity, detailing how linking quantum processors can amplify computing power*

## References

[1] James Titcomb. “Supercomputer Makes Calculations in Blink of an Eye That Take Rivals 47 Years.” The Telegraph, July 2, 2023. https://www.telegraph.co.uk/business/2023/07/02/google-quantum-computer-breakthrough-instant-calculations/.

[2] Universal Quantum. What we do [Internet]. Universal Quantum; Available from: https://universalquantum.com/what-we-do

[3] “What Is Quantum Computing?” McKinsey & Company, May 1, 2023. https://www.mckinsey.com/featured-insights/mckinsey-explainers/what-is-quantum-computing

[4] Physics World. Breakthrough in quantum error correction could lead to large-scale quantum computers [Internet]. Physics World; 2023 Mar 20. Available from: https://physicsworld.com/a/breakthrough-in-quantum-error-correction-could-lead-to-large-scale-quantum-computers/

[5] Liang X-T, Xiong Y-J. The Loss of fidelity due to quantum leakage for Josephson charge qubits [Internet]. arXiv; 2004 July 6. Available from: https://arxiv.org/abs/quant-ph/0407027

[6] Shi J, Shen S. A clock synchronization method based on quantum entanglement. Sci Rep. 2022 Jun 17;12(10185). Available from: https://www.nature.com/articles/s41598-022-14087-z

[7] Kevin Jackson. Quantum repeaters and their role in information technology, December 13, 2022. https://www.anl.gov/article/quantum-repeaters-and-their-role-in-information-technology

[8] M. Akhtar, F. Bonus, F. R. Lebrun-Gallagher, N. I. Johnson, M. Siegele-Brown, S. Hong, S. J. Hile, et al. "A High-Fidelity Quantum Matter-Link between Ion-Trap Microchip Modules." Nature Communications 14, 531 (2023). https://doi.org/10.1038/s41467-022-35285-3.

[9] Bharath Kannan, Aziza Almanakly, Youngkyu Sung, Agustin Di Paolo, David A. Rower, Jochen Braumüller, Alexander Melville, et al. "On-Demand Directional Microwave Photon Emission Using Waveguide Quantum Electrodynamics." Nature Physics 19 (2023): 394–400, https://doi.org/10.1038/s41567-022-01869-5.