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What Quantum Computers Cannot Do: An Overview

Dr. Alexander Tuzhilin, Dean, Computer Science

Published: March 29, 2024

Published: March 29, 2024

a graphical depiction of quantum computers

Quantum computing has touched off a paradigm shift in computing by using quantum mechanics principles to achieve what classical computers cannot. While both kinds of computers are problem-solving tools, the way they process data is distinctively different.

Classical computing can be likened to library research. Say you are looking for an answer in a library book, so you check each book sequentially. In classical computing, these two states – “yes, this book has the answer” or “no, this book doesn’t have the answer” – are represented by a 0 or a 1. 

Continuing the library analogy, quantum computing enables you to examine all of the books in the library simultaneously. Making this operation possible are quantum bits or qubits. Unlike the binary 0-or-1 bits of classical computing, qubits exploit a quantum phenomenon known as superposition, enabling them to represent both states simultaneously. This ability allows quantum computers to process complex calculations at speeds unattainable by their classical counterparts, providing a new option for situations where classical systems falter. 

For instance, a quantum computer’s potential to decipher complex cryptographic codes could redefine data security. Moreover, their ability to simulate quantum phenomena makes them an indispensable tool in fields like materials science, chemistry, and fundamental physics. They can also optimize business processes, from logistics to supply chain management, identifying the most efficient solutions with a speed and precision that classical systems cannot match.

The future prospects of quantum computing are compelling. According to an article in Nature, quantum computers “could accelerate drug discovery, crack encryption, speed up decision-making in financial transactions, improve machine learning, develop revolutionary materials, and even address climate change.”

Currently, however, quantum computers haven’t reached their potential, leading some people to believe they don’t exist. “It’s a common misconception that the quantum computer doesn’t exist yet,” said Technical University of Denmark (DTU) Associate Professor Sven Karlsson, who works with the technology. “It does already exist, so it’s not something we need to wait for. However, the current quantum computers aren’t yet all that large, which limits how complicated the calculations can be.”

To build larger, more complex computers, scientists need to overcome the limitations of quantum computing, which include infrastructure, scalability, error correction, software development, and the availability of a trained workforce.

At the core of these challenges is the issue of maintaining quantum coherence. The qubits that form the backbone of quantum computing are very sensitive to their surroundings. Any external interference, from thermal fluctuations to electromagnetic waves, can cause qubits to lose their quantum state, a problem known as decoherence. Besides needing to be in a stable environment, the computers must also operate in cold climates to counteract the heat they generate. It requires a significant investment in infrastructure to house a functioning quantum computer.

Error correction presents another substantial hurdle. In classical computing, small errors can be corrected with relative ease, but in the quantum realm, the error correction mechanisms must operate in a way that does not collapse the delicate quantum state. Developing algorithms that can efficiently correct errors without undermining the quantum computation process is a complex task that researchers are actively working to solve.

On the software side, the development of quantum algorithms is still in its infancy. This task is similar to writing a new language or creating a new form of mathematics; it requires not only expertise but also creativity and a deep understanding of the problems at hand.

Furthermore, quantum computing demands a highly specialized workforce. The interdisciplinary nature of the field requires expertise in quantum mechanics, computer science, mathematics, and engineering, usually at the graduate level. The most directly relevant degree would be a Ph.D. in Quantum Information Science, which is an interdisciplinary field encompassing aspects of physics, computer science, mathematics, and engineering. Building a sufficiently large and skilled workforce is essential to drive innovation and maintain the momentum of quantum computing research and development.

As the capabilities of quantum computers become more well-known in the public realm, it is important to correct several misconceptions about their capabilities. For instance, contrary to some reports, quantum computers cannot store infinite data. While qubits can hold more information than binary bits because of their ability to exist in multiple states simultaneously, there is still a finite limit to the number of qubits and the data they can represent. Each qubit added to a system increases the computational power exponentially, yet it also requires an exponential increase in error correction and stability, which current technology has not yet mastered.

In terms of security, quantum computers possess the theoretical ability to solve certain cryptographic problems exponentially faster than classical computers, which could render current encryption methods vulnerable. This has led to the belief that quantum computers are ‘unhackable.’ However, this is a misunderstanding. Quantum computers are not inherently unhackable; they simply operate on a different computational framework that can challenge the algorithms we currently rely on for security.  

The assertion that quantum computers are “unhackable” primarily revolves around quantum cryptography, rather than the quantum computers themselves. Quantum cryptography uses the principles of quantum mechanics to make communication channels impervious to eavesdropping. This makes the technology highly secure.

There are many companies and governments at work on leveraging quantum computing. The United States and China are racing to master the science while also working to protect sensitive data. IBM is developing a quantum processor, and multiple companies are working on software, scalability, and error correction. Meanwhile, a team of researchers at the City College of New York is building an unhackable quantum internet.

Quantum computing stands on the precipice of becoming a transformative technology, one that may redefine problem-solving in fields as diverse as cryptography, materials science, and optimization. Currently, we are witnessing the nascent stages of this evolution, where the focus is on creating quantum systems that are stable and reliable enough for practical applications. 

As researchers and engineers refine the technology to overcome decoherence and temperature control issues, we expect quantum computers to achieve ‘quantum advantage’ for specific tasks where they can outperform the best classical supercomputers. This milestone will likely usher in a new era of computing, with pilot programs and early adopters pioneering the integration of quantum computing into sectors such as pharmaceuticals, energy, and finance.

Looking to the long-term horizon, the potential achievements of quantum computing could be profound, reshaping our technological landscape in ways we are only beginning to imagine. With the ability to simulate complex molecular structures, quantum computers could revolutionize drug discovery and material design, potentially leading to cures for diseases and the creation of new, eco-friendly materials. 

In the realm of artificial intelligence, quantum computers’ speed and parallel processing capabilities might unlock new levels of machine learning and data analysis. The development of quantum networks could even lead to a quantum internet, offering unprecedented security and data transmission capabilities. 

While these advancements may seem like the domain of science fiction, they are the very real possibilities that quantum computing holds for our future, promising to expand the boundaries of human knowledge and capability.