Quantum computing

A quantum computer is a real or theoretical computer that uses special ideas from quantum physics, like superposition and entanglement. These ideas might help quantum computers solve some problems much faster than regular computers.

The smallest piece of information in a quantum computer is called a qubit. A regular computer uses bits, which can only be a 0 or a 1. But 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.

Right now, quantum computers are still mostly experiments. They only work 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 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 different subjects. Modern quantum theory started in the 1920s to explain very small particles, while digital computers were created later to help with math problems. Both became important during World War II.

As scientists began using quantum ideas for computing, the two subjects came together. In 1980, Paul Benioff made a simple model of a quantum computer. Later, Richard Feynman said that quantum computers could copy natural systems better than regular computers. In 1984, Charles Bennett and Gilles Brassard showed how quantum theory could improve information security.

Important quantum ways to solve problems appeared in the 1990s. Peter Shor found a method to break common security codes in 1994, and Grover's algorithm in 1996 helped find information more quickly. In 2019, Google AI and NASA said they made a big advance by doing a job much faster than regular supercomputers, though IBM disagreed. Today, scientists are working to make quantum computers better and more useful.

Quantum information processing

Computer engineers usually describe how a modern computer works using classical electrodynamics. In these “classical” computers, some parts like semiconductors and random number generators may use quantum behavior. But 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 need precise control of 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 get 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 need careful design to be useful.

Ethical and security implications

Quantum computing can make it hard to keep information safe. It might be able to break the codes we use today to protect secrets online, like those in banking and personal messages. Because of this, scientists are working 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.

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.

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.

Algorithms

We are making progress in finding quantum algorithms. These special ways of solving problems often use quantum circuits. Quantum algorithms can solve some problems much faster than regular computers.

For example, Shor's algorithm can break big numbers into smaller ones quickly. This could change how we keep information safe online. Other quantum algorithms help us understand complicated chemical reactions and materials. They do this by copying how tiny parts of nature work, which is very hard for regular computers.

Quantum algorithms are also being studied to help us search through information and solve tricky problems. Grover's algorithm can find something specific in a list faster than regular methods. While these ideas are exciting, we still need to improve technology and fix mistakes before we can use them in real tools.

Engineering

As of 2023, regular computers are still better than quantum computers for most everyday jobs. Even though quantum computers might solve some special math problems faster, they don’t yet help much with regular tasks. Scientists are working on many ways to build better quantum computers, but there are big challenges.

Building a big quantum computer is very hard. One big problem is keeping the tiny parts, called qubits, stable long enough to do useful work. Another problem is controlling many qubits at once, which needs very exact timing. There are also issues with keeping the quantum system cool and separate so it doesn’t lose its special properties. Researchers are trying different ways to build quantum computers, including using superconducting materials and trapping tiny particles called ions. However, each way has its own challenges, and making a big, reliable quantum computer is still a big goal for the future.

Future outlook and challenges

Quantum computers are exciting, but making big, reliable ones is still very hard. Today’s quantum systems have issues like losing information fast and not working perfectly. Scientists are working to design better systems and find ways to fix mistakes on their own. Quantum computers have proven they can do some special jobs better than regular computers, but using them for everyday tasks is still a goal for the future.

Industry and commercial development

Many big technology companies and new startups are working to build quantum computers. Companies like IBM, Google, Microsoft, and IonQ are making special parts called processors and online services. These services let scientists and students try quantum experiments from their computers. This helps make learning and using this new technology faster. Right now, quantum computers are still being tested and can’t do all jobs better than regular computers, but they are improving all the time.

Theory

Main article: Quantum complexity theory

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

Scientists believe 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 regular ones in every situation. Some problems that are easy for quantum computers might still be very hard for regular computers.