What Is Quantum Computing?

Introduction 

A century ago, a group of scientists in Europe began developing a theory to explain how the world’s smallest particles interact. Their discoveries underpinned a new branch of physics and laid the foundation for the modern science of quantum computing, which could herald transformative changes in a wide array of fields, including medicine, high-tech vehicles, and espionage.

At a basic level, quantum computers solve some complex math problems much faster than any existing computer can do so. Because of this ability, experts say that quantum computing could lead to code-breaking advancements that shatter current models of math-dependent encryption, raising national security concerns around the world. The technology could also spur economic development by accelerating research in a range of industries. Most computer scientists agree that quantum computers will continue to develop at a rapid pace, but the question of when—or if—they will have practical utility remains a subject of heated debate.

What is quantum computing?

Quantum computers exploit the principles of quantum mechanics—entanglement and superposition among them—to model scenarios with a massive number of probabilities. Earlier generations of physicists, including Albert Einstein, saw the theoretical underpinnings of the field as purely academic, but a growing body of research is demonstrating that in highly specialized conditions, scientists can use these quantum principles for practical effect.

Classical computing encodes information in long sequences of zeros and ones, with each zero or one known as a “bit.” Encoding information in this way places limits on computing power, as computers can only process so many zeros and ones. Quantum bits, or qubits, sidestep these limits through superposition; qubits can exist simultaneously as a zero and a one. If effectively harnessed, quantum computers that encode information in qubits could perform calculations—this is how computers process information—far faster than their classical counterparts.

What are the potential practical applications of quantum technology?

Experts see quantum’s potential impact in three main arenas:

Sensing. Quantum devices can more precisely track miniscule changes in physical variables, such as gravity, motion, pressure, time, and Earth’s electromagnetic field. This technology could theoretically be applied to make any measurement-taking device more accurate, including thermometers, radar systems, or magnetic resonance imaging (MRI) machines.

Communications. Governments and companies invest massive amounts of time and money on encrypting sensitive information. Quantum technologies could both supercharge decryption and facilitate new forms of cryptography. Quantum communications exploits superposition to facilitate a form of cryptography known as quantum key distribution, which could make the concept of hacker-proof cybersecurity into a reality. 

Computing. Quantum computers could rapidly increase the speed with which it is possible to solve problems that involve quantum mechanics. These include pharmaceutical development, battery design, and semiconductor manufacturing.

Because of its widespread applicability, some scientists think quantum computing will be the most transformative of all quantum technologies. In 2023, researchers at Google said their quantum computer could solve an equation in seconds that would take a classical supercomputer forty-seven years. Still, some computer scientists are skeptical, arguing that quantum computing has limited practical utility. 

How did quantum computing develop?

Quantum dynamics have been theorized for centuries, but historians generally agree that the field entered a new era in the 1920s, with a wave of breakthroughs in quantum mechanics from physicists including Niels Bohr, Werner Heisenberg, and Erwin Schrödinger. Another step forward came in 1981, when computer scientist Richard Feynman proposed applying those principles to computers.

Quantum computing offered a glimpse of its future code-breaking potential just more than a decade later. In 1994, Massachusetts Institute of Technology (MIT) scientist Peter Shor designed an algorithm that demonstrated that quantum techniques could crack classical encryption methods used to secure state secrets far faster than a conventional computer. Governments were soon pouring billions into research, spurring a race to develop quantum primacy. In 2011, U.S.-based defense manufacturer Lockheed Martin bought the first commercially available quantum computer from the Canadian technology company D-Wave; since then, the U.S. government and private American firms have together spent more than $6 billion on the sector. 

Why does it matter for national security? 

If quantum computing makes standard encryption obsolete, governments will have to contend with significant military and economic disruptions. This possibility has come to be known as “Q-day,” and some sources in the industry say it could come as soon as 2025. However, many computer scientists say it could take another decade. For its part, the U.S. intelligence community does not expect quantum computers to be cryptographically relevant until the early 2030s. But to national security experts, including CFR Military Fellow Marine Colonel Wilfred Rivera, rapid technological advancement in artificial intelligence (AI) and other industries is a sign that breakthroughs could happen far sooner than policymakers anticipate. Some observers note that governments are unlikely to announce Q-day so as not to spoil a code-breaking advantage.

In a Q-day scenario, hackers would more easily be able to break through defensive barriers to access critical systems, such as energy grids and nuclear reactors. To protect against this threat, the World Economic Forum estimated in 2023 that the world will need to upgrade or replace more than twenty billion devices to be made secure. U.S. officials recognize these risks; in 2022, President Joe Biden issued a national security memo warning that code-breaking quantum computers “could jeopardize civilian and military communications, undermine supervisory and control systems for critical infrastructure, and defeat security protocols for most Internet-based financial transactions.” 

Governments are now collecting massive amounts of their adversaries’ encrypted data with the goal of deciphering it after quantum computing matures. The United States and China—the two leaders in quantum technology—have accused each other of taking this “harvest now, decrypt later” approach. In December 2022, the U.S. Congress found that adversaries could “steal sensitive encrypted data today using classical computers, and wait until sufficiently powerful quantum systems are available to decrypt it.” Almost a year later, China’s Ministry of State Security accused the U.S. National Security Agency (NSA) of “systematic” attacks to steal Chinese data. Meanwhile, some governments are pursuing quantum advancements in military hardware, such as advanced radar systems.

Still, skeptics argue that Q-day is no guarantee, maintaining that it is too soon to expect quantum computers to become capable of anything beyond abstract calculation. They argue that quantum computing’s prowess remains strictly theoretical [PDF], and some say that it will never be capable of practical tasks such as decryption. “There is a great deal of money to be made in speculating on the future of quantum cryptography and quantum computing, and a great deal of contract dollars available to people who tell a convincing story about why it’s dangerous,” says CFR Senior Fellow Tarah Wheeler, who calls a Q-day scenario “science fiction.”

How is the United States preparing for quantum?

U.S. efforts to accelerate research and development (R&D) in quantum technologies have picked up in the past decade, often with bipartisan support. In 2018, U.S. President Donald Trump signed the National Quantum Initiative Act (NQIA) to increase funding for the technology, citing its importance to U.S. “economic and national security.” Over the next three fiscal years, federal spending on quantum R&D doubled.

Federal agencies have also moved to develop standards around quantum computing. In 2022, the National Institute of Standards and Technology (NIST) selected four post-quantum cryptography (PQC) algorithms that it hopes could guard against quantum-boosted decryption, and released three in August 2024. 

But further developing these standards could face funding challenges. The spending authorized by the NQIA expired in September 2023, even as many experts argue that more investment is necessary to maintain a competitive advantage. Reauthorization has stalled in Congress, with $3.6 billion in appropriations toward quantum R&D hanging in the balance.

Are other countries pursuing quantum technology?

International interest in quantum computing lags behind AI and other emerging technologies. While more than twenty countries have national quantum initiatives, few have the capacity to domestically produce quantum computers. That’s in part because governments are reluctant to share quantum technology; Canada, France, the Netherlands, Spain, and the United Kingdom each restrict exports of quantum computers. Globally, governments and companies have invested $42 billion in quantum computing to date—just one-third of the overall investment in AI in 2022 alone. 

Many experts agree that the United States is the overall leader in quantum technology but note that China is not far behind. Some analysts say China outpaces the United States in theoretically unhackable quantum communications, while Washington leads Beijing in quantum sensing and computing. Biden has sought to maintain that lead by going after China’s quantum computing sector; he has overseen a wide range of export controls aimed at curtailing China’s access to the advanced computing chips necessary for quantum computers, and signed an executive order explicitly restricting private sector investment in Chinese quantum computing.

Still, China has announced more public investment in quantum technology than any other country, with an estimated total surpassing $15 billion. Some leading Chinese quantum developers have cast doubt over that widely reported figure, but Communist Party officials have emphasized the technology’s importance. In 2020, Chinese President Xi Jinping stressed quantum’s “strategic value” in a speech to party leaders; Beijing is currently building a $10 billion national quantum lab.

Are there other potential benefits?

Major management consultancies project that businesses could reap profits from quantum technologies as soon as 2025. Quantum could add $2 trillion in value over the next decade, according to a report by the consulting firm McKinsey, with benefits concentrated in four industries: chemicals, finance, life sciences, and transportation. For instance, in 2023 the car manufacturer Mercedes-Benz announced a partnership with tech giant International Business Machines (IBM) to use quantum computing to develop more efficient batteries. Other examples include Goldman Sachs and other financial firms expecting that quantum will allow them to more accurately bet on various markets and pharmaceutical companies hoping to use the technology to speed up the development of new treatments by rapidly modeling drug-like molecules.

Quantum computing could also help move the needle on global challenges such as climate change. In 2019, researchers at the German chemicals company BASF found that the technology could optimize a process for producing a crucial fertilizer ingredient whose production currently contributes up to 3 percent of global greenhouse gas emissions.

However, computer scientists note that quantum computers are only useful in solving problems rooted in quantum mechanics—including many chemical and biological processes. But in classical problems, such as optimization, it is not clear that quantum computers will yield any significant advantage, says Scott Aaronson, director of the Quantum Information Center at the University of Texas at Austin. “There’s not an obvious quantum advantage for checking your email, for playing Candy Crush, for word processing, for most of what we do,” Aaronson tells CFR.

What are the obstacles?

Experts note that today’s quantum computers fall far short of these capabilities. The most advanced quantum computers have around one thousand qubits—significantly fewer than the potentially millions necessary to achieve advanced levels of decryption. Current models are also slow, expensive, and require a large amount of physical hardware, including specialized semiconductors. Some quantum computers have to be chilled to temperatures near absolute zero to achieve superposition, leaving few companies and governments capable of storing them. 

Another major challenge is “noise,” or any disruption to a quantum computer’s environment. Because of noise, which can be caused by changes in light, heat, neighboring qubits, or any number of other variables, quantum computers have error rates far higher than their conventional peers.

Still, lured by the technology’s great potential, governments and companies are continuing to set ambitious goals for quantum advancement. IBM hopes to develop a one-hundred thousand qubit quantum computer by 2033; Google is aiming for one million.