### Introduction

Quantum computing has emerged as a groundbreaking field that promises to revolutionize the way we process information and solve complex problems. Among the exciting advancements in this realm, modular quantum computers have taken center stage. These modular systems offer the potential to scale up and unlock the possibilities of large-scale programmable quantum computing and the quantum internet. In this article, we delve into the fascinating world of modular quantum computers and their role in paving the way towards a quantum future.

### Current State of Quantum Computer Development

Quantum computers shall bring power of massive parallel processing, equivalent of supercomputer to a single chip. They can consider different possible solutions to a problem simultaneously, quickly converge on the correct solution without check each possibility individually. This dramatically speed up certain calculations, such as number factoring.

They can process huge datasets in a fraction of a second that would have previously taken days and weeks. This speeds up Big Data analysis, searching very large, unstructured, unsorted data sets discovering patterns or anomalies extremely quickly.

They could discover new efficient catalytic processes, creating efficient solid-state batteries, climate change mitigation by removing carbon dioxide from our atmosphere. In agriculture they can quickly detect weed and analyze possible catalyst combinations to come up with effective fertilizers.

The power of quantum computers depends on the number of qubits and their quality measured by coherence, and gate fidelity. Qubit is very fragile, can be disrupted by things like tiny changes in temperature or very slight vibrations. Coherence measures the time during which quantum information is preserved.

We are now in era of Noisy intermediate-scale quantum (NISQ) in which quantum computers are composed of hundreds of noisy qubits that are not error-corrected. They Physical qubits are realized using superconducting Josephson junction qubits and the trapped-ion qubits. Other promising Qubits are Semiconductor based qubits; Topological qubits; and Photonic qubits.

IBM’s introduced 433-quantum bit (qubit) processor in Nov 2022, the IBM Osprey, that has the largest qubit count of any IBM quantum processor, more than tripling the 127 qubits on the IBM Eagle processor unveiled in 2021. This processor has the potential to run complex quantum computations well beyond the computational capability of any classical computer.

Calculations using these noisy qubits can introduce errors and make long computations impossible. However, these computers still can demonstrate the advantages of quantum computing and various algorithms are being developed in disciplines such as machine learning, quantum chemistry and optimization.

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However, the most important challenge in development of scalable quantum computer with hundreds and thousands of bits. This will allow it to do something beyond the ken of a classical computer, such as simulate molecular structures in chemistry and materials science, or tackle certain problems in cryptography or machine learning. However, constructing a large-scale quantum processor is challenging because of the errors and noise that are inherent in real-world quantum systems.

One approach to addressing this challenge is to utilize modularity—a strategy used frequently in nature and engineering to build complex systems robustly. Such an approach manages complexity and uncertainty by assembling small, specialized components into a larger architecture. These considerations have motivated the development of a quantum modular architecture, in which separate quantum systems are connected into a quantum network via communication channels. Networks would open the door to several applications, including solving computations that are too large to be handled by a single quantum computer and establishing unbreakably secure communications using quantum cryptography.

### Understanding Modular Quantum Computers

Modular quantum computers are a paradigm shift from traditional quantum systems. They consist of interconnected modules, each housing a certain number of qubits and the necessary control and measurement infrastructure. These modules can be interconnected to create larger computing resources, thereby addressing the limitations faced by standalone quantum computers.

The ability to connect qubits across modules is critical for building scalable quantum computers. Recent work has focused on developing new methods for inter-module connectivity, such as using microwave resonators or photonics-based links.

One of the challenges with modular quantum computers is that the modules must be precisely aligned and calibrated to work together. This requires advanced engineering and control systems to ensure that the modules are properly synchronized, which can be difficult and time-consuming.

Despite these challenges, modular quantum computers have significant potential for advancing quantum computing research and applications.

### Advantages of Modular quantum computers

The advantages of modular design are manifold. Firstly, modularity allows for scalability, enabling the addition of more modules as needed. This scalability is crucial for building large-scale quantum computers capable of tackling complex problems beyond the reach of classical computing. Additionally, modular design enhances fault tolerance, as a failure in one module does not necessarily affect the functionality of the entire system. Furthermore, maintenance and upgrades become more manageable with modular systems, as individual modules can be repaired or replaced without impacting the overall system.

One advantage of modular quantum computers is that they are more fault-tolerant than other types of quantum computers. This is because if one module fails, the rest of the system can continue to operate, allowing for the detection and correction of errors. This makes modular quantum computers more reliable and robust, which is critical for large-scale quantum computing.

Another advantage of modular quantum computers is that they are more flexible in terms of the number of qubits they can accommodate. Traditional quantum computers are limited by the number of physical qubits they can accommodate, which is determined by the size of the quantum processor. With modular quantum computers, additional modules can be added to increase the number of qubits, making them more scalable and versatile.

They offer greater flexibility and scalability, as well as improved fault-tolerance, which is critical for large-scale quantum computing. As quantum technology continues to advance, it is likely that modular quantum computers will play an important role in the development of practical quantum computing applications.

### Large-Scale Programmable Quantum Computing

One of the key challenges in quantum computing is the limited number of qubits and the associated fragility of quantum states. Modular quantum computers offer a solution to this challenge by enabling the creation of larger qubit arrays through module interconnectivity. By combining multiple modules, researchers can exponentially increase the number of qubits and effectively scale up computational power.

The interconnectivity of modules also facilitates resource sharing, allowing multiple modules to work together on a single computation. This collaboration not only enhances computational capabilities but also enables the execution of more complex algorithms. Moreover, programming modular quantum computers becomes more manageable, as the interconnected modules can be accessed and controlled collectively or independently, offering flexibility and adaptability for different applications.

### Quantum Internet: Connecting the Quantum World

The concept of a quantum internet has gained significant attention in recent years. It aims to enable secure, high-speed communication using quantum principles. Modular quantum computers play a crucial role in the development of the quantum internet by acting as quantum network nodes.

These modular nodes serve as intermediaries for quantum communication, facilitating the transfer of quantum information between different points in the network. By leveraging the interconnected modules, researchers can create a network that spans across large distances, enabling quantum communication and teleportation. This quantum internet holds the promise of enhancing encryption protocols, secure data transfer, and quantum-enhanced distributed computing.

### Modular Quantum Computer technology advancements

Modular quantum computing has witnessed significant advancements in recent years, paving the way for large-scale programmable quantum computing and the realization of the quantum internet. Researchers have made notable breakthroughs in teleporting quantum operations in real-time, demonstrating the feasibility of scalable and deterministic operations. By utilizing error-correctable coding, the process has achieved a reliability of 79%, marking a milestone in error-correctable qubits for quantum information processing.

Efforts to develop fully modular large-scale quantum computers are underway, with researchers exploring the networking of individual quantum computing modules. This networking approach enables the transfer of quantum information between systems that are encoded differently, thereby enhancing the feasibility of implementing modular quantum computers with a large number of qubits.

IBM, for instance, is making strides in modular quantum computing with its Heron processor, which boasts high-quality qubits and the ability to connect directly to other Heron processors. IBM is also expected to debut its Heron processor in 2023, which will have just 133 qubits. This shift towards modular quantum computers built from multiple processors is expected to significantly enhance scalability. IBM envisions a future where distributed, large-scale quantum computers with millions of connected qubits can run useful, error-corrected quantum algorithms.

PsiQuantum, another company in the quantum computing arena, is working on a silicon-based modular chip. Once the last missing component—an extremely fast, low-loss optical switch—is fully demonstrated, PsiQuantum plans to assemble the silicon chips into a building-scale, high-performance computer-like system, ushering in a new era of modular quantum computing.

The development of coherent linkage between modules is crucial for scaling quantum computing. Quantum communications, involving the transfer of coherent qubits over long distances, will play a key role in enabling coherent links between modular quantum computing systems. Technologies such as fiber-optic networks with quantum repeaters or satellite networks are being explored to establish coherent links across various distances.

In summary, advancements in modular quantum computing have showcased the teleportation of quantum operations in real-time, the networking of individual modules, and the development of high-quality qubit processors that can connect directly. These breakthroughs pave the way for scalable, large-scale quantum computing and the establishment of coherent links between modular systems. The future of quantum computing holds immense potential, as researchers continue to work towards developing robust and reliable modular quantum computers and realizing the transformative power of the quantum internet.

### Entanglement lights the way to scalable quantum computers: Nov 2019

Researchers in the UK have developed a technique for entangling ions of strontium that allows for more accurate and higher-rate entanglement than previous methods. This advancement opens the door to scalable quantum computers built from multiple ion traps interconnected via photonic interconnects.

Trapped ions offer low-noise qubits with long coherence times, making them suitable for quantum computations. However, accommodating a large number of qubits becomes challenging due to the wiring and laser beam requirements. To overcome this limitation, the researchers sought to connect ion qubits in different traps.

The team, led by Christopher Ballance at Oxford University, improved the rate and fidelity of entanglement by collecting more photons emitted by the ions and minimizing emission imperfections. They achieved this by using short laser pulses split in two, directing each half to an ion, and collecting the emitted photons. The entanglement of ions was achieved through a beam splitter and measured using Bell-state measurement.

The experiment yielded impressive results, generating an average of 182 entangled ion pairs per second with a fidelity of 94%. This is a significant improvement over previous achievements, with a rate of only five entangled pairs per second in 2014 and 0.001 per second in 2007.

The researchers believe that with further improvements, such as using reflective surfaces closer to the ions to collect more light, the entanglement rate could be increased by up to 100 times. This advancement in off-chip quantum communication between ions is crucial for scaling quantum computers.

The goal is to reach a point where the interconnectivity between ions is not the limiting factor, allowing for the efficient scaling of quantum computing systems. These developments bring us closer to building large-scale quantum computers interconnected via photonic interconnects, paving the way for the realization of powerful quantum computational capabilities.

### Using Fiber Optics, ORNL Team Demonstrates Universal Quantum Computing: Dec 2018

Researchers at Oak Ridge National Laboratory (ORNL) have demonstrated a frequency-based approach to quantum computing. The researchers performed two distinct, independent operations simultaneously on two qubits encoded on photons of different frequencies. Qubits are the smallest unit of quantum information.

Quantum scientists working with frequency-encoded qubits have been able to perform a single operation on two qubits in parallel, but never two distinct operations, said the ORNL team. According to the researchers, coherent quantum frequency operations are challenging because it is difficult to mix frequencies arbitrarily and with low noise. “To realize universal quantum computing, you need to be able to do different operations on different qubits at the same time, and that’s what we’ve done here,” Pavel Lougovski, a research scientist, said.

For their experiment, the team used two entangled photons contained in a single strand of fiber optic cable. Because the photons were traveling through the same device, stability and control over the photons were maintained. “When the photons are taking different paths in the equipment, they experience different phase changes, and that leads to instability,” said Brian Williams, a researcher on the team.

The researchers implemented distinct quantum gates in parallel on two entangled frequency-bin qubits in the optical fiber. The team’s quantum frequency processor allowed it to manipulate the frequency of photons to bring about superposition, the state that allows quantum computers to perform operations concurrently. Through this quantum operation the researchers were able to control the spectral overlap between adjacent spectral bins, observe frequency-bin interference, and demonstrate 97 percent interference visibility (i.e., a measure of how alike two photons are). These results indicate that the photons’ quantum states were virtually identical. By integrating this tunability with frequency parallelization, the researchers were able to synthesize independent gates on entangled qubits.

The researchers applied Bayesian inference — a statistical method associated with machine learning — to confirm that the operations on the quantum processor were done with high fidelity and with absolute control. “A lot of researchers are talking about quantum information processing with photons, and even using frequency,” researcher Joseph Lukens said. “But no one had thought about sending multiple photons through the same fiber optic strand, in the same space, and operating on them differently.”

Lukens said the team’s results show that “we can control qubits’ quantum states, change their correlations, and modify them using standard telecommunications technology in ways that are applicable to advancing quantum computing.” Once the building blocks of quantum computers are in place, he said, “We can start connecting quantum devices to build the quantum internet, which is the next exciting step.”

The team believes that leveraging the existing fiber optic network infrastructure — which cost billions of dollars — is practical. Its realization of closed, user-defined gates on frequency-bin qubits in parallel could be used to develop fiber-compatible quantum information processing and quantum networks.

### First ever blueprint unveiled to construct a large scale quantum computer: Feb 2017

In February 2017, an international team of scientists, led by a researcher from the University of Sussex, unveiled the first practical blueprint for constructing a large-scale quantum computer. Quantum computers are considered the most powerful computers on Earth due to their ability to perform complex calculations at unprecedented speeds.

The team’s work introduced a groundbreaking invention that enables the transmission of actual quantum bits (qubits) between individual quantum computing modules, allowing for a fully modular large-scale machine. This advancement opens the door to achieving nearly arbitrary computational processing powers.

Previous proposals for connecting individual computer modules relied on fiber optic connections. However, the new blueprint introduces connections created by electric fields, enabling the transportation of charged atoms (ions) from one module to another. This novel approach provides connection speeds that are 100,000 times faster than the current state-of-the-art fiber link technology.

The international team behind the blueprint comprises scientists from the University of Sussex, Google, Aarhus University, RIKEN, and Siegen University. Prof Winfried Hensinger, head of the Ion Quantum Technology Group at the University of Sussex, has been at the forefront of this research. He emphasized that the blueprint not only proves the feasibility of constructing an actual quantum computer, but it also provides a practical construction plan for building a large-scale machine.

The development of a large-scale quantum computer aligns with the UK Government’s plan to advance quantum technologies for industrial applications. The blueprint incorporates a recent invention by the Sussex team, which replaces billions of laser beams typically required for quantum computing operations within a large-scale system. Instead, the application of voltages to a microchip simplifies the computational processes.

This blueprint represents a significant milestone in the journey towards realizing large-scale quantum computers. It sets the stage for further advancements and paves the way for the industrial exploitation of quantum technologies, bringing us closer to harnessing the immense computational power of quantum computers for practical applications.

### Researchers Develop Data Bus for Quantum Computer

Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation, however, because quantum systems are very sensitive to environmental noise. Although systems can be protected from noise in principle, researchers have been able to build only small prototypes of quantum computers experimentally.

One way to reduce the error rate is by encoding quantum information not in one single quantum particle but in several quantum objects. These logical quantum bits or qubits are more robust against noise. In the last few years, theoretical physicists have developed a whole range of error correction codes and optimized them for specific tasks.

Physicists Hendrik Poulsen Nautrup and Hans Briegel from the Institute of Theoretical Physics of the University of Innsbruck and Nicolai Friis, now at the Institute of Quantum Optics and Quantum Information in Vienna, have found a technique to transfer quantum information between systems that are encoded differently.

### Interface between processor and memory

Similar to classical computers, future quantum computers might be built with different components. Scientists have already built small-scale quantum processors and memories experimentally, and they have used different protocols to encode logical qubits: For example, for quantum processors they use so-called color codes and for quantum memories surface codes.

“For the two systems to interact with each other quantum mechanically, we have to connect them,” says PhD student Hendrik Poulsen Nautrup. “We have developed a protocol that allows us to merge quantum systems that are encoded differently.” The scientists suggest to locally modify specific elements of the encoded quantum bits.

This process is also called lattice surgery, which is used to couple systems such as quantum processors and memories. Once the systems are temporarily “sewed” together, quantum information can be teleported from the processor to the memory and vice versa. “Similar to a data bus in a conventional computer, scientists can use this technique to connect the components of a quantum computer,” explains Poulsen Nautrup.

This new scheme is another step towards building a universal quantum computer and research for experimental realization is under way. The research was conducted within the framework of the doctoral program Atoms, Light, and Molecules offered at the University of Innsbruck and was funded by the Austrian Science Fund and the Templeton World Charity Foundation.

### Challenges and Future Outlook

While modular quantum computers offer immense potential, several challenges must be addressed to unleash their full power. Overcoming technical hurdles such as maintaining coherence, minimizing noise, and improving the qubit connectivity within modules remains a significant focus of research. Additionally, developing efficient control mechanisms and programming languages that can harness the capabilities of modular systems is crucial.

Looking ahead, the future of modular quantum computing and the quantum internet appears promising. Researchers and industry leaders are actively working towards scaling up modular systems and commercializing quantum technologies. As advancements continue, the potential applications of large-scale programmable quantum computers and the quantum internet in areas such as optimization, drug discovery, cryptography, and simulations are becoming increasingly evident.

### Conclusion

Modular quantum computers are revolutionizing the world of quantum computing, offering scalability, fault tolerance, and improved maintenance capabilities. By interconnecting modules, these systems unlock the potential for large-scale programmable quantum computing, enabling the solution of complex problems that are beyond the reach of classical computers. Furthermore, modular quantum computers serve as the building blocks for the quantum internet, facilitating secure and high-speed quantum communication. While challenges remain, continued research and development in this field are paving the way towards a future where modular quantum computers and the quantum internet become integral components of our technological landscape.

### References and Resources also include:

https://www.technologyreview.com/2023/01/06/1066317/whats-next-for-quantum-computing/