Editing Quantum computing (section)

{{Short description|Computer hardware technology that uses quantum mechanics}}
{{Use American English|date=February 2023}}
{{Use dmy dates|date=February 2021}}
[[File:Bloch sphere.svg|thumb|[[Bloch sphere]] representation of a qubit. The state <math>| \psi \rangle = \alpha |0 \rangle + \beta |1 \rangle</math> is a point on the surface of the sphere, partway between the poles, <math>|0\rangle</math> and <math>|1\rangle</math>.]]

A '''quantum computer''' is a [[computer]] that exploits [[quantum mechanical]] phenomena. On small scales, physical matter exhibits properties of [[wave-particle duality|both particles and waves]], and quantum computing takes advantage of this behavior using specialized hardware. [[Classical physics]] cannot explain the operation of these quantum devices, and a scalable quantum computer could perform some calculations [[Exponential growth|exponentially]] faster{{efn|As used in this article, "exponentially faster" has a precise [[computational complexity|complexity theoretical]] meaning. Usually, it means that as a function of input size in bits, the best known classical algorithm for a problem requires an [[exponential growth|exponentially growing]] number of steps, while a quantum algorithm uses only a polynomial number of steps.}} than any modern "classical" computer. Theoretically a large-scale quantum computer could [[post-quantum cryptography|break some widely used encryption schemes]] and aid physicists in performing [[quantum simulator|physical simulations]]; however, the current state of the art is largely experimental and impractical, with several obstacles to useful applications.

<!-- Basic principles of quantum computing -->
The basic [[unit of information]] in quantum computing, the [[qubit]] (or "quantum bit"), serves the same function as the [[bit]] in classical computing. However, unlike a classical bit, which can be in one of two states (a [[Binary number|binary]]), a qubit can exist in a [[quantum superposition|superposition]] of its two "basis" states, a state that is in an abstract sense "between" the two basis states. When [[measurement in quantum mechanics|measuring]] a qubit, the result is a [[Born rule|probabilistic output]] of a classical bit. If a quantum computer manipulates the qubit in a particular way, [[wave interference]] effects can amplify the desired measurement results. The design of [[quantum algorithms]] involves creating procedures that allow a quantum computer to perform calculations efficiently and quickly.

<!--Physical implementations-->
Quantum computers are not yet practical for real-world applications. Physically engineering high-quality qubits has proven to be challenging. If a physical qubit is not sufficiently [[isolated system|isolated]] from its environment, it suffers from [[quantum decoherence]], introducing [[noise (signal processing)|noise]] into calculations. National governments have invested heavily in experimental research aimed at developing  scalable qubits with longer coherence times and lower error rates. Example implementations include [[superconducting quantum computing|superconductors]] (which isolate an [[electrical current]] by eliminating [[electrical resistance]]) and [[trapped ion quantum computer|ion traps]] (which confine a single [[atom|atomic particle]] using [[electromagnetic fields]]).

<!-- Computability and complexity -->
In principle, a classical computer can solve the same computational problems as a quantum computer, given enough time. Quantum advantage comes in the form of [[time complexity]] rather than [[computability]], and [[quantum complexity theory]] shows that some quantum algorithms are exponentially more efficient than the best-known classical algorithms. A large-scale quantum computer could in theory solve computational problems that are not solvable within a reasonable timeframe for a classical computer. This concept of additional ability has been called "[[quantum supremacy]]". While such claims have drawn significant attention to the discipline, near-term practical use cases remain limited.