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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.

Qiskit is an open source SDK for working with quantum computers at the level of pulses, circuits and application modules. If appropriate, also use the [programming] tag. Please include the version of …
For questions regarding usage, performance, implementation, application or theory related to quantum gates.
Questions about or related to quantum states. Consider using the density-matrix tag when relevant.
For any questions whose answer is expected to require code or programming notions. For questions about a specific framework (e.g. qiskit or cirq) please also include a more specific tag. If your quest…
For questions about quantum algorithms, that is, sequences of quantum gates, operations, and measurements, whose purpose which achieve some goal. Standard examples are Shor's and Grover's algorithms.
Quantum error correction (QEC) is a collection of techniques to protect quantum information from decoherence and other quantum noise, to realise fault-tolerant quantum computation. Quantum error corre…
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…
For questions about the construction of complex circuits using elementary quantum gates.
For questions related to measurement and its effects as relevant to quantum computation and quantum information.
for questions about IBM's Quantum Service - Circuit Composer & Quantum Lab. DO NOT use this for questions about general Qiskit issues (use [qiskit] for that).
675 questions
Applies to questions of primarily educational value - styled in the format similar to that found in textbook exercises. This tag should be applied to questions that are (1) stated in the form of an ex…
a model in which a computation is a sequence of quantum gates, which are reversible transformations on an n-qubit register (the quantum-mechanical analog of an n-bit register)
For questions about vector spaces of all dimensions and linear transformations between them, including systems of linear equations, bases, dimensions, subspaces, matrices, determinants, traces, eigenv…
DO NOT use this tag. Use more specific tags such as [linear-algebra] instead.
513 questions
For questions about quantum channels or more generally quantum maps and the related formalism. For questions about unitary operations, please use quantum-gate instead.
For questions about physical implementations of a quantum algorithm or other quantum information protocol. DO NOT use for questions about simulation or emulation of quantum computers, or cloud-based q…
409 questions
Grover's search algorithm is 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 …
For questions about density matrices and related concepts and ideas, e.g. procedures for computing properties of quantum states from their density matrices.
For questions about simulating either quantum computers or simulating things on quantum computers.
For questions that involve a request for resources to learn anything related to quantum computing or quantum information.
For questions about exercises or passages from the popular quantum computing textbook *Quantum Computation and Quantum Information* by Michael Nielsen and Isaac Chuang.
342 questions
For questions regarding complexity analysis of quantum algorithms and comparisons with complexities of classical algorithms.
A stabilizer quantum error-correcting code appends ancilla qubits to qubits that we want to protect. A unitary encoding circuit rotates the global state into a subspace of a larger Hilbert space. This…
Hamiltonian simulation is a class of algorithms that, given a Hermitian matrix A, output a quantum circuit implementing an approximation to the unitary exp[iAt].
used for questions connected with information theory in classical and/or quantum sense.
276 questions
For questions about the Variational Quantum Eigensolver, a quantum/classical hybrid algorithm that can be used to find eigenvalues of a (often large) matrix H. When this algorithm is used in quantum s…
253 questions
For questions about Pauli matrices in general or Pauli gates in particular, as relevant to quantum computing and/or quantum information theory. The Pauli matrices are a set of three 2 × 2 complex matr…
243 questions
For questions about Stim, the open source tool for high-performance simulation of quantum stabilizer circuits.
For questions related to the unitarity (unitary evolution) of quantum systems, as applicable to quantum computing or quantum information.
221 questions
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…
220 questions
For questions about different types of noise a quantum computer (or qubits) may experience; how this affects the outcome of the computation; how to reduce a specific type of noise on a specific implem…
214 questions
For questions concerning how to improve quantum computers on different aspects like performance, efficiency or fault-tolerance.
213 questions
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 …
197 questions
For questions about the usage of, origin of or reasons for specific notation used in the context of quantum computing.
195 questions
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, …
For questions about matrix representations of quantum gates.
188 questions
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