A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.

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For questions on quantum algorithms. That is, algorithms which in theory can be executed by quantum computers, usually the computers providing 'universal' quantum computation.
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For questions regarding usage, performance, implementation, application or theory related to quantum gates.
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Quantum systems can mathematically be described by their 'quantum state'. When the system is closed/isolated, the state is 'pure' and can be written as a sum (i.e. 'superposition') of basis vectors. W…
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For questions relating to the process of assembling quantum programs that can be executed on a quantum computer. For questions using or about a specific programming language, consider using a more spe…
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Quantum Software Development Kit for writing quantum computing experiments, programs, and applications. If appropriate, also use the [programming] tag.
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For questions about quantum information theory. In physics and computer science, quantum information is information that is held in the state of a quantum system. Quantum information is the basic ent…
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For questions about the principle and application of quantum entanglement. It is a physical phenomenon which occurs when pairs or groups of particles are generated, interact, or share spatial proximit…
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For questions about the construction of complex circuits using elementary quantum gates.
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for questions about mathematics relevant to quantum computing and/or quantum information theory. DO NOT use this tag for general mathematics questions.
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for questions about IBM's quantum processors which are accessible online. DO NOT use this for questions about general Qiskit issues. As of July 2018, there are five processors on the IBM …
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For questions related to measurement and its effects as relevant to quantum computation and quantum information.
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a collection of techniques to protect quantum information from decoherence and other quantum noise, to realise fault-tolerant quantum computation. Quantum error corre…
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For questions that involve a request for resources to learn anything related to quantum computing or quantum information.
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an algorithm that can perform a search in the order of square root of the input size. This is a provable speed up over the best classical algorithm, which requires a time …
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For questions regarding complexity analysis of quantum algorithms and comparisons with complexities of classical algorithms.
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For questions about practical computers or processors that run on a quantum architecture. This is for questions about the machines themselves, not just any computing that could take place on one. You …
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For questions about finding (short) gate sequences to implement a specific unitary operation, for example decomposing a complicated multi-qubit gate into a sequence of basic gates. It might apply to o…
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For questions about simulating either quantum computers or simulating things on quantum computers.
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For questions related to building of quantum computers and discussion of the hardware aspects.
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a matrix that can be used to describe a quantum system in a mixed state, a statistical ensemble of several quantum states. This should be contrasted with a single state vector that…
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For questions related to physical realizations of quantum bits i.e. quantum systems living in a 2-dimensional Hilbert space.
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For questions about exercises or passages from the popular quantum computing textbook *Quantum Computation and Quantum Information* by Michael Nielsen and Isaac Chuang.
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In quantum mechanics, a quantum operation is a mathematical formalism used to describe a broad class of transformations that a quantum mechanical system can undergo. The quantum operation formalism de…
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For questions about the relation between quantum computing and classical computing, such as their relative performance.
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a domain-specific programming language used to express quantum algorithms. This tag should be used for questions related to creating/programming quantum algorithms using the 'Q#' language. If th…
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In quantum information theory, a quantum channel is a communication channel which can transmit quantum information, as well as classical information. An example of quantum information is the state of …
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For questions about cryptography as it relates directly to quantum computing. Not for general cryptographic methods.
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For questions about matrix representations of quantum gates.
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For questions about either: comparing the performance of a quantum algorithm with a classical algorithm (or set of classical algorithms) independent of devices; or the ratio of time to solution of a …
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a class of algorithms that, given a Hermitian matrix A, output a quantum circuit implementing an approximation to the unitary exp[iAt].
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a process by which quantum information (e.g. the exact state of an atom or photon) can be transmitted (exactly, in principle) from one location to another, with the help of cl…
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For questions related to the Bloch sphere. In quantum mechanics, the Bloch sphere is a geometrical representation of the pure state space of a two-level quantum mechanical system (qubit), named after …
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Shor's algorithm, named after American mathematician Peter Shor, is a quantum algorithm for integer factorization, formulated in 1994. Informally, it solves the following problem: given an integer N, …
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an abstract object generalising a scalar or vector and can be represented by a number, a 1D array, 2D matrix or higher order generalisations thereof. A tensor product is a product defined …
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a fundamental principle of quantum mechanics. It states that, much like waves in classical physics, any two (or more) quantum states can be added together ("superposed") and t…
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For questions related to experiments with actual quantum computing devices and how to interpret their results. Also appropriate if the question can only be reasonable answered with an experiment (or e…