## What are the Major Challenges in Building a Practical Quantum Computer?

Quantum computing promises to revolutionize many aspects of modern technology, from cryptography to machine learning, chemistry to materials science. However, despite decades of research and significant breakthroughs, building a large-scale, practical quantum computer remains one of the most significant technological challenges of the 21st century.

In this article, we will explore the major challenges that are currently standing in the way of practical quantum computing, ranging from the inherent fragility of qubits to the difficulties of scaling up quantum systems. These challenges must be overcome for quantum computers to transition from the experimental lab to widespread, real-world use.

Building a Practical Quantum Computer |

## From Theory to Reality: The Journey of Building a Practical Quantum Computer

Quantum computing leverages quantum mechanics to perform complex calculations at unprecedented speeds using qubits, which can exist in multiple states simultaneously.

Building a practical quantum computer involves transforming theoretical principles into tangible technology. This journey begins with understanding quantum mechanics, the foundation of quantum computing, which leverages qubits to perform complex calculations at unprecedented speeds.

The initial phase involves extensive research into creating stable qubits, which are highly susceptible to environmental disturbances.

Scientists employ advanced materials and cryogenic techniques to maintain qubit coherence, ensuring they remain in their quantum state long enough to perform computations.

The next phase focuses on error correction and scalability. Quantum error correction is essential because qubits are prone to errors due to decoherence and quantum noise.

Researchers develop sophisticated algorithms to detect and correct these errors, ensuring reliable computation. Additionally, integrating quantum processors with classical computing systems enhances their functionality, allowing for practical applications.

As these technological advancements converge, the vision of a practical quantum computer becomes more tangible, promising revolutionary impacts in fields such as cryptography, drug discovery, and artificial intelligence. This journey from theory to reality is not just about building a new type of computer; it’s about unlocking new possibilities for the future.

## The Road to Practical Quantum Computers: Major Challenges and Solutions

Building a practical quantum computer is a complex and multifaceted challenge. Here are some key hurdles to overcome.

### 1. Qubit Stability and Coherence Time

At the heart of any quantum computer are qubits, the fundamental units of quantum information. However, one of the most critical challenges in quantum computing is ensuring qubit stability, which refers to the ability of qubits to maintain their quantum state over time. This property is essential because quantum computations rely on maintaining superposition and entanglement between qubits, which are both fragile quantum states.

Qubit stability is closely related to a phenomenon known as quantum coherence. Coherence time is the length of time that a qubit can maintain its superposition state before it collapses due to interaction with its environment, a process called decoherence.

Decoherence can be caused by a wide range of environmental factors, such as temperature fluctuations, electromagnetic radiation, or material imperfections. Once decoherence occurs, the quantum state is lost, rendering the computation meaningless.

Improving the coherence time of qubits is a key area of research, but even the best qubits available today can only maintain coherence for a few microseconds to milliseconds, depending on the qubit technology. This is far too short for most meaningful quantum computations, which require complex sequences of quantum gates (the operations that manipulate qubits) to be performed without significant errors.

Furthermore, different qubit implementations (e.g., superconducting qubits, trapped ions, or topological qubits) face different challenges in terms of stability, and none have yet achieved the level of coherence needed for large-scale quantum computing.

### 2. Error Rates and Quantum Error Correction

Because qubits are so delicate and prone to decoherence, quantum computers are far more error-prone than classical computers.

In classical computers, errors can often be easily corrected using simple error-detection and error-correction methods. However, due to the nature of quantum mechanics, error correction in quantum computers is vastly more complicated.

Quantum error correction (QEC) involves encoding quantum information in such a way that it can be protected against errors, including both bit-flip (changing from 0 to 1 or vice versa) and phase-flip (changing the relative phase between qubit states) errors.

The most famous QEC codes, such as the surface code, require many physical qubits to represent a single logical qubit. For example, implementing the surface code typically requires on the order of 1000 physical qubits to correct errors on just one logical qubit.

This overhead presents a significant challenge to building large-scale quantum computers. If we need hundreds or thousands of physical qubits to protect just one logical qubit, the total number of qubits required to perform meaningful quantum computations becomes astronomically high.

Therefore, achieving high fidelity (low-error) quantum gates and reducing the overall error rate is crucial to scaling up quantum computers. Significant progress has been made in error correction, but it remains one of the most significant hurdles.

### 3. Scalability of Quantum Systems

The path from small, few-qubit quantum systems to large-scale quantum computers with millions of qubits is another significant challenge.

Most current quantum processors have only a few dozen qubits, with some of the most advanced systems approaching 100 qubits. However, to outperform classical computers in solving practical problems (such as breaking modern cryptographic systems or simulating complex molecules), we will need quantum computers with thousands, if not millions, of qubits.

There are several obstacles to scaling up quantum systems:

- Physical Space: As the number of qubits increases, so does the physical space needed to house them, especially if we consider the cooling requirements for certain types of qubits (such as superconducting qubits, which need to operate at temperatures close to absolute zero). Building a quantum processor that can support a large number of qubits without suffering from excessive noise and interference is a major engineering challenge.

- Connectivity: In quantum computing, qubits often need to be entangled or interact with each other to perform computations. As the number of qubits increases, ensuring that all qubits can be effectively connected and that interactions can occur without introducing significant errors becomes increasingly difficult. Achieving scalable and efficient qubit connectivity is a key hurdle.

- Control Systems: As quantum systems grow in size, the control systems required to operate the qubits become exponentially more complex. This includes everything from microwave control systems for superconducting qubits to laser systems for ion-trap qubits. Ensuring that control signals remain precise and error-free as the number of qubits increases is a non-trivial challenge.

### 4. Quantum Hardware and Materials Science

Developing the physical hardware for quantum computers requires innovations not just in computer science and engineering but also in materials science.

The choice of materials and fabrication techniques can have a significant impact on the performance of qubits, as well as their coherence times and error rates.

For example, in superconducting qubits, tiny imperfections in the materials used to make the qubit can cause unwanted noise and decoherence.

Similarly, trapped ion qubits require precise control of the ions' electromagnetic environment, and even small impurities in the trapping apparatus can cause significant errors.

As quantum systems become more complex, finding materials that are both highly stable and suitable for large-scale fabrication is essential.

Furthermore, the long-term durability of quantum hardware is still an open question. Quantum computers must operate in highly controlled environments, such as ultra-low temperatures or ultra-high vacuum, and the ability to maintain these conditions for extended periods remains an ongoing engineering challenge.

### 5. Software, Algorithms and Optimization

The software layer of quantum computing is still in its infancy compared to classical computing. Even though many quantum algorithms have been proposed and theoretically shown to outperform classical ones (such as Shor’s algorithm for factoring large numbers or Grover’s algorithm for database search), the development of practical quantum software faces several challenges:

- Algorithm Efficiency: Not all quantum algorithms can be directly applied to real-world problems, and many are not yet optimized for practical use. Researchers are still discovering which classes of problems can be efficiently solved by quantum computers, and developing the corresponding quantum algorithms is a highly active area of research.

- Compiling and Optimizing Quantum Circuits: Translating high-level quantum algorithms into low-level quantum circuits (the equivalent of machine code in classical computers) is a complex task. Quantum compilers need to minimize errors and gate operations while ensuring that the algorithm runs efficiently on noisy quantum hardware. Developing compilers and optimizers that can achieve this is an ongoing challenge.

### 6. Quantum Decoherence and Environmental Noise

Quantum systems are extremely sensitive to their environment, and even minute disturbances, such as electromagnetic radiation or temperature fluctuations, can cause decoherence and errors in quantum computations. Shielding qubits from these external influences is crucial but difficult.

Various types of quantum systems are affected by different environmental factors. For example, superconducting qubits need to be cooled to near absolute zero to minimize thermal noise, while trapped ion qubits must be kept in ultra-high vacuum chambers to avoid interactions with gas molecules.

In both cases, even small imperfections in the shielding or cooling apparatus can introduce errors into the system.

Developing better shielding techniques and improving the robustness of quantum systems against environmental noise is a key area of focus.

### 7. Cost and Resource Requirements

Building quantum computers is extremely expensive due to the highly specialized equipment required to maintain qubits and minimize errors.

Superconducting quantum computers, for example, require sophisticated cryogenic systems to keep qubits at temperatures close to absolute zero, while trapped ion systems need precision laser arrays and vacuum chambers.

Additionally, the research and development costs for quantum computing are high, involving interdisciplinary teams of physicists, engineers, and computer scientists.

While there has been significant investment from governments and private companies, the cost of building and maintaining large-scale quantum computers is still prohibitive for most organizations.

Reducing the cost and improving the accessibility of quantum hardware will be essential for quantum computers to become widely adopted.

### 8. Theoretical Limitations and Quantum Supremacy

While quantum computing holds immense promise, there are still many open theoretical questions regarding the true power of quantum computers.

One milestone that has garnered significant attention is achieving quantum supremacy, which refers to the ability of a quantum computer to solve a problem that is infeasible for any classical computer to solve within a reasonable timeframe.

The term "quantum supremacy, coined by John Preskill in 2012, refers to the point at which a quantum computer can perform a calculation that is beyond the reach of the most powerful classical computers.

In essence, it signifies that quantum computers have crossed a threshold where they can solve problems that would take classical computers an infeasibly long time to compute.

#### Google's Quantum Supremacy Claim: The Sycamore Processor

In October 2019, Google announced that it had achieved quantum supremacy with its 53-qubit quantum processor, called Sycamore. The task they chose for this demonstration was to generate random numbers through a process called random circuit sampling—a task that would require a classical supercomputer an impractical amount of time to complete.

According to Google, Sycamore managed to perform this task in 200 seconds, whereas they estimated that the world’s fastest classical supercomputer, IBM’s Summit, would take 10,000 years to complete the same task.

This achievement garnered significant media attention as a monumental step forward for quantum computing. However, it came with several important caveats:

Specialized Problem: The problem Google chose for its demonstration—random circuit sampling—is a very specific type of computational problem that does not have direct practical applications. It was chosen precisely because it is extremely hard for classical computers but relatively easier for quantum systems.

In contrast, most real-world problems, such as simulating chemical reactions or factoring large numbers, have different computational structures that are not as naturally suited to quantum systems (at least with current quantum algorithms).

Debates Over Practical Supremacy: Shortly after Google's announcement, IBM challenged the supremacy claim, arguing that with optimized classical algorithms and a more efficient approach, their Summit supercomputer could perform the same task in a few days, rather than the 10,000 years estimated by Google. This sparked a debate over what exactly constitutes quantum supremacy.

Does it require that quantum computers outperform classical computers by an unbridgeable margin, or is it enough for them to demonstrate a significant speedup, even if classical methods can eventually catch up?

Beyond Supremacy: Quantum Advantage: Many researchers emphasize that quantum supremacy, while a critical milestone, is not the final goal of quantum computing.

The more meaningful achievement would be quantum advantage, which refers to quantum computers not only outperforming classical ones on specific benchmarks but also solving useful, real-world problems faster than classical computers.

Quantum supremacy, as demonstrated by Google, was more of a proof of concept than a demonstration of practical usefulness.

Quantum advantage, on the other hand, will require solving tasks like optimizing logistics, simulating chemical processes for drug discovery, or enhancing machine learning algorithms—tasks that would provide tangible benefits across industries.

Noisy Quantum Systems: One of the biggest challenges in achieving quantum supremacy is that current quantum systems are still relatively noisy.

Noise refers to unwanted interactions between qubits and their environment, which can introduce errors in computations. Google's Sycamore processor, like other quantum processors, operates in what’s known as the Noisy Intermediate-Scale Quantum (NISQ) era.

NISQ processors have between 50 and a few hundred qubits and can perform short quantum computations, but they are still far from error-corrected, large-scale quantum systems.

The computations that Google's Sycamore performed were highly sensitive to noise, and ensuring that the processor could maintain a sufficiently low error rate for the entire computation was a major engineering feat.

## Future Directions Beyond Quantum Supremacy

While Google's achievement was an impressive demonstration of the potential of quantum computing, there are several challenges that remain before quantum computers can outperform classical computers on tasks of practical importance.

### Scaling Up to Fault-Tolerant Quantum Computing

One of the biggest obstacles to achieving quantum advantage lies in scaling quantum computers while simultaneously reducing error rates. Today’s NISQ devices are far from fault-tolerant, meaning they can only run shallow quantum circuits (a limited number of operations) before noise degrades the computation.

Fault-tolerant quantum computers will require quantum error correction (QEC) to mitigate the impact of noise and errors. QEC works by encoding logical qubits into multiple physical qubits, such that errors can be detected and corrected without disrupting the computation. However, implementing QEC is extremely resource-intensive. For example, the surface code—a leading QEC method—requires on the order of 1000 physical qubits to protect just one logical qubit from errors. This means that a practical, error-corrected quantum computer would likely need millions of physical qubits to perform meaningful computations.

Achieving this level of scale will require breakthroughs in both hardware (to build systems with large numbers of high-quality qubits) and software (to develop more efficient quantum error correction codes and optimize quantum algorithms for noisy hardware).

### Noise Mitigation and Error Reduction

In the NISQ era, researchers are exploring various noise mitigation techniques to make the most of today’s noisy quantum computers. These include:

- Error Mitigation: Techniques like zero-noise extrapolation and quantum subspace expansion attempt to reduce the impact of noise without requiring full-blown quantum error correction. While these methods don’t eliminate errors entirely, they can improve the fidelity of quantum computations, making it possible to perform longer and more complex calculations than would otherwise be possible on noisy hardware.

- Quantum Error Correction (QEC): While error mitigation techniques are useful in the short term, QEC will ultimately be necessary for large-scale quantum computations. Significant progress has been made in developing error-correcting codes that can protect quantum information from both bit-flip and phase-flip errors, but implementing these codes on real hardware is still a major challenge.

- Improved Hardware: Advances in materials science and quantum hardware design can also help reduce noise at the source. For example, researchers are working on developing more stable qubits with longer coherence times, as well as improving the precision of control systems for manipulating qubits.

### Hybrid Classical-Quantum Computing

In the near future, quantum computers are likely to be used in conjunction with classical computers, rather than replacing them entirely. Hybrid classical-quantum algorithms divide tasks between the two types of processors, with quantum systems handling the parts of the computation that can benefit most from quantum speedups, while classical computers manage the rest.

For example, in the variational quantum eigensolver (VQE) algorithm—a leading method for simulating molecular energies—quantum computers are used to compute the energies of trial wavefunctions, while a classical optimization routine adjusts the parameters of the wavefunction. By leveraging the strengths of both classical and quantum systems, hybrid algorithms allow us to make progress in solving practical problems with today’s noisy quantum devices.

### The Future is Now: Practical Quantum Computing Applications

For quantum computers to be useful in everyday applications, they need to solve problems that have real-world implications. Some of the areas where quantum computing is expected to make the most impact include:

- Cryptography: Shor’s algorithm, developed in 1994, promises to break widely used cryptographic systems like RSA by efficiently factoring large numbers—a task that classical computers struggle with. However, this requires large, error-corrected quantum computers, which are still years or even decades away.

- Chemistry and Drug Discovery: Quantum computers are particularly well-suited for simulating quantum mechanical systems, such as molecules and chemical reactions. This could revolutionize fields like materials science and pharmaceuticals, allowing researchers to discover new materials and drugs far more efficiently than with classical simulations.

- Optimization Problems: Many industries, from logistics to finance, deal with complex optimization problems that require finding the best solution from a vast number of possibilities. Quantum computers may offer significant speedups in solving such problems using algorithms like the quantum approximate optimization algorithm (QAOA).

- Machine Learning: Quantum machine learning is an emerging field that seeks to apply quantum algorithms to tasks like data classification, clustering, and pattern recognition. While classical machine learning has made huge strides in recent years, quantum systems could potentially offer faster and more efficient algorithms for analyzing large datasets.

The transition from theoretical quantum algorithms to practical, real-world applications, however, is still a work in progress. Many quantum algorithms that offer a speedup over classical ones require fault-tolerant quantum computers, which are currently beyond reach.

Read Here: Applications of Quantum Computing Across Various Industries

## Conclusion: The Road Ahead for Practical Quantum Computing

While the achievement of quantum supremacy by Google was a landmark moment for the field, the road to practical quantum computing remains long and challenging. Significant progress is still needed in reducing error rates, scaling up quantum systems, and developing quantum software that can tackle real-world problems.

As research continues, we can expect quantum computers to gradually improve, transitioning from specialized research tools to practical devices capable of solving problems that classical computers cannot. The full potential of quantum computing is still on the horizon, but each breakthrough brings us closer to a future where quantum computers transform industries and drive new technological revolutions.

### Key References:

#### 1. Books

- Nielsen, M. A., & Chuang, I. L. (2000). Quantum Computation and Quantum Information. Cambridge University Press. ISBN: 978-1107002173.
- Rieffel, E., & Polak, W. (2011). Quantum Computing: A Gentle Introduction. MIT Press. ISBN: 978-0262015066.
- Vedral, V. (2006). Introduction to Quantum Information Science. Oxford University Press. ISBN: 978-0198563333.

#### 2. Research Papers

- Fowler, A. G., Mariantoni, M., Martinis, J. M., & Cleland, A. N. (2012). "The Surface Code: A Blueprint for Fault-Tolerant Quantum Computation". Physical Review A, 86(3), 032324.
- Arute, F., Arya, K., Babbush, R., Bacon, D., Bardin, J. C., Barends, R., et al. (2019). "Quantum Supremacy Using a Programmable Superconducting Processor". Nature, 574(7779), 505-510.
- Gottesman, D. (2009). "Quantum Error Correction for Beginners". arXiv, arXiv:0904.2557.

#### 3. Review Articles

- Preskill, J. (2018). "Quantum Computing in the NISQ Era and Beyond". Quantum, 2, 79.
- Blais, A., Grimsmo, A. L., Girvin, S. M., & Wallraff, A. (2021). "Circuit Quantum Electrodynamics". Reviews of Modern Physics, 93(2), 025005.
- Pirandola, S., Andersen, U. L., Banchi, L., Berta, M., Bunandar, D., Colbeck, R., et al. (2020). "Advances in Quantum Cryptography". Advances in Optics and Photonics, 12(4), 1012-1236.

#### 4. Conferences and Workshops

- QIP2024: Quantum Information Processing (QIP) Conference
- IEEE International Conference on Quantum Computing and Engineering (QCE)
- The American Physical Society (APS) March Meeting 2024 - Event - Quantum Computing

#### 5. Government Reports and Roadmaps

- National Quantum Initiative. (2018).The National Quantum Initiative Act, signed into law in 2018, outlines the U.S. government’s strategy to support quantum research and development.
- European Quantum Flagship. (2020). Quantum Flagship Program. The Quantum Flagship is a 10-year, €1 billion research initiative funded by the European Union.