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A quantum computer is a computer that represents and processes information using quantum states.

Quantum computers have the potential to complete some calculations exponentially faster than classical computers. The basic unit of information in quantum computing, the qubit (quantum bit), serves a similar function as the bit in ordinary or "classical" computing. Unlike a classical bit, which can be in one of two states (a binary), a qubit can exist in a linear combination of states known as a quantum superposition. If a quantum computer manipulates the qubit in a particular way, wave interference effects amplify the probability of the desired measurement result. Quantum algorithm design involves creating procedures that allow a quantum computer to perform this amplification. Quantum computers are not yet practical for real-world applications. Researchers have claimed that quantum devices can outperform classical computers on specific tasks, a metric referred to as quantum advantage or quantum supremacy.

Quantum mechanics and computer science formed distinct academic communities until the advent of quantum computing. As physicists applied quantum mechanical models to computational problems and swapped bits for qubits, quantum mechanics and computer science began to converge. In 1980, Paul Benioff introduced the quantum Turing machine, which used quantum theory to describe a simplified computer. As digital computers became faster, physicists faced an exponential increase in overhead when simulating quantum dynamics, prompting Yuri Manin and Richard Feynman to independently suggest that hardware based on quantum phenomena might be more efficient for computer simulation. In a 1984 paper, Charles Bennett and Gilles Brassard applied quantum theory to cryptography protocols and demonstrated that quantum key distribution could enhance information security. These algorithms did not solve practical problems, but demonstrated mathematically that more information could be obtained by querying a black box with a quantum state in superposition, sometimes referred to as quantum parallelism. Quantum computing increasingly focused on controlling decoherence through quantum error correction.

A classical computer is a quantum computer ... so we shouldn't be asking about "where do quantum speedups come from?" We should say, "Well, all computers are quantum. ...

However, any measurement can be deferred to the end of quantum computation, though this deferment may come at a computational cost, so most quantum circuits depict a network consisting only of quantum logic gates and no measurements.

A quantum computation can be described as a network of quantum logic gates and measurements.

All of these models of computation—quantum circuits, one-way quantum computation, adiabatic quantum computation, and topological quantum computation—have been shown to be equivalent to the quantum Turing machine; given a perfect implementation of one such quantum computer, it can simulate all the others with no more than polynomial overhead.

A measurement-based quantum computer decomposes computation into a sequence of Bell state measurements and single-qubit quantum gates applied to a highly entangled initial state (a cluster state), using a technique called quantum gate teleportation.

Progress in finding quantum algorithms typically focuses on the quantum circuit model, though exceptions such as the quantum adiabatic algorithm exist.

Since quantum computers can produce outputs that classical computers cannot produce efficiently, and since quantum computation is fundamentally linear algebra, so quantum algorithms that can speed up machine learning tasks may be possible. Quantum computers are naturally good for solving complex quantum many-body problems and thus may apply to applications involving quantum chemistry.

The class of problems that can be efficiently solved by a quantum computer with bounded error is called BQP, for "bounded error, quantum, polynomial time". ; that is, all problems that can be efficiently solved by a classical computer can be efficiently solved by a quantum computer, and all problems that can be efficiently solved by a quantum computer can be solved by a classical computer with polynomial space resources.

Non-local quantum computation – Method of quantum computing via entanglement Quantum bus – Device to store or transfer information in quantum computing

Quick Facts

• The basic unit of information in quantum computing, the qubit (quantum bit), serves a similar function as the bit in ordinary or "classical" computing.
• Researchers have claimed that quantum devices can outperform classical computers on specific tasks, a metric referred to as quantum advantage or quantum supremacy.
• As physicists applied quantum mechanical models to computational problems and swapped bits for qubits, quantum mechanics and computer science began to converge.
• Quantum mechanics and computer science formed distinct academic communities until the advent of quantum computing.
• Quantum computers have the potential to complete some calculations exponentially faster than classical computers.

Source material: Wikipedia - "Quantum computing". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.