Quantum computing

A quantum computer is a real or theoretical computer that uses special ideas from quantum physics, like superposition and entanglement. These ideas let quantum computers possibly solve some problems much faster than regular computers. For example, a big quantum computer might be able to break some secret codes we use today and help scientists study how materials and atoms behave.

The smallest piece of information in a quantum computer is called a qubit, which is like the bit used in regular computers. But unlike a regular bit, which can only be a 0 or a 1, a qubit can be both at the same time thanks to superposition. When we check what a qubit is doing, we see either 0 or 1, but quantum computers can guide these possibilities so that the right answer ends up more likely.

Right now, quantum computers are still mostly experiments and work only on very specific problems. Making good qubits is very hard because outside interference can mess up their state, a problem called quantum decoherence. Scientists around the world are working hard to build better qubits that last longer and make fewer mistakes. Some methods they use include superconductors and ion traps. Even though some special quantum devices have shown they can do certain tasks faster than regular computers — a point called quantum supremacy — these are mainly scientific steps and not yet tools we use in everyday life.

History

For a chronological guide, see Timeline of quantum computing and communication.

For many years, quantum mechanics and computer science were separate areas of study. Modern quantum theory began in the 1920s to explain tiny particles, while digital computers developed later to help with calculations. Both became important during World War II.

As scientists started using quantum ideas for computing, the two fields joined together. In 1980, Paul Benioff created a simple quantum computer model. Later, Richard Feynman suggested that quantum computers could simulate physical systems better than regular computers. In 1984, Charles Bennett and Gilles Brassard showed how quantum theory could make information security stronger.

Important quantum algorithms appeared in the 1990s. Peter Shor created a way to break common encryption in 1994, and Grover's algorithm in 1996 helped search information faster. In 2019, Google AI and NASA claimed a big step by doing a task much faster than regular supercomputers, though IBM disagreed. Today, researchers work on making quantum computers more reliable and powerful.

Quantum information processing

Computer engineers typically describe a modern computer's operation using classical electrodynamics. In these "classical" computers, some parts like semiconductors and random number generators may use quantum behavior. However, because they aren't isolated from their environment, any quantum information quickly disappears. Programmers might use probability theory when designing algorithms, but ideas like superposition and wave interference aren't usually important.

Quantum programs, however, need precise control of coherent quantum systems. Physicists describe these systems using math, with complex numbers representing probability amplitudes, vectors representing quantum states, and matrices showing operations on these states. Programming a quantum computer means arranging these operations to compute useful results.

The basic unit of quantum information is the qubit. Unlike a classical bit, which is either 0 or 1, a qubit can be in a superposition of both states at once. This means a qubit can be in a mix of |0⟩ and |1⟩, represented as α|0⟩ + β|1⟩, where α and β are complex numbers. When measured, a qubit shows 0 or 1 with probabilities based on |α|² and |β|².

Quantum computers can potentially solve some problems much faster than classical computers by exploring many possibilities at once, though they require careful design to make this useful.

Ethical and security implications

Quantum computing can create big challenges for keeping information safe. It might be able to break the special codes we use today to protect secrets online, like those used in banking and personal messages. Because of this, scientists are working hard to create new, stronger codes that even a quantum computer can't easily break. Groups like the National Institute of Standards and Technology (NIST) are helping to choose and improve these new codes so we stay safe in the future. There are also worries about how quantum computers could be used to spy or steal data, so people are talking about how to use this technology in a way that is safe and fair.

Communication

Further information: Quantum information science

Quantum cryptography offers new ways to send information safely. For example, quantum key distribution uses special quantum states to create secure cryptographic keys. When two people share these quantum states, they can be sure that no one else has seen the message, because any unwanted person would disturb the delicate system and leave a trace.

Modern fiber-optic cables can carry quantum information over short distances. Scientists are working on better tools, like quantum repeaters, to make this work over longer distances. This could lead to new technologies, such as shared quantum computers and improved quantum sensing.

Quantum teleportation is a way for one person to send the state of a tiny part of quantum information to another person using shared quantum links and normal communication. This shows that quantum communication needs both special quantum links and regular communication.

Superdense coding is another method where one person can send two pieces of regular information by using a shared quantum link and sending a small amount of quantum information. This shows that shared quantum links can greatly increase how much information can be sent.

Algorithms

Progress in finding quantum algorithms often focuses on the quantum circuit model. Quantum algorithms can solve certain problems much faster than classical computers. For example, Shor's algorithm can factor large numbers quickly, which could affect how we keep information safe online. Other quantum algorithms help us understand complex chemical reactions and materials by simulating quantum systems, which is very hard for regular computers to do.

Quantum algorithms are also being studied for use in searching databases and solving optimization problems. Grover's algorithm, for instance, can find a specific item in a list faster than classical methods. While these speedups are promising, turning them into practical tools still requires advances in technology and error correction.

Engineering

As of 2023, classical computers are still better than quantum computers for everyday tasks. Even though quantum computers might solve some specific math problems faster, they don’t yet offer real benefits for practical use. Scientists are working on many different technologies to build better quantum computers, but there are big challenges to overcome.

Building a large quantum computer is very hard. One big challenge is making sure the tiny parts, called qubits, stay stable long enough to do useful work. Another challenge is controlling many qubits at once, which needs very precise timing. There are also problems with keeping the quantum system cool and isolated so it doesn’t lose its special properties. Researchers are exploring different ways to build quantum computers, including using superconducting materials and trapping tiny particles called ions. However, each method has its own difficulties, and making a big, reliable quantum computer remains a major goal for the future.

Future outlook and challenges

Quantum computers hold great promise, but building large, reliable ones is still very difficult. Today’s quantum systems face problems like losing information quickly and having imperfect operations. Scientists are working hard to create better designs and find ways to fix errors automatically. Although quantum computers have shown they can beat regular computers at some special tasks, using them for everyday problems in a useful way is still something we hope to achieve in the future.

Industry and commercial development

Many big technology companies and new startups have been working hard to build quantum computers. Companies like IBM, Google, Microsoft, and IonQ are creating special quantum parts called processors and online services. These services let scientists and students try out quantum experiments from their computers, which helps speed up learning and using this new technology. Right now, these quantum computers are still being tested and can’t do all jobs better than regular computers, but they are getting better all the time.

Theory

Main article: Quantum complexity theory

Quantum computers can solve the same problems as classical computers, but they might do it faster for certain tasks. For example, they could quickly break some codes that are hard for regular computers to solve.

Scientists think quantum computers are more powerful for some problems because they use special rules of physics. However, we don’t fully understand how these computers compare to classical ones in all situations. Some problems that are easy for quantum computers might still be very hard for regular computers.