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Quantum phase estimation algorithm
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<p>In <a href="/facts/Quantum_computing/nbbcgmvq">quantum computing</a>, the quantum phase estimation algorithm is a <a href="/facts/Quantum_algorithm/ceJaJ6Eq">quantum algorithm</a> to estimate the phase corresponding to an eigenvalue of a given <a href="/facts/Unitary_matrix/8fsxv3KD">unitary operator</a>. Because the eigenvalues of a unitary operator always have <a href="/facts/Absolute_value_of_a_complex_number/dwJBpEs5">unit modulus</a>, they are characterized by their phase, and therefore the algorithm can be equivalently described as retrieving either the phase or the eigenvalue itself. The algorithm was initially introduced by <a href="/facts/Alexei_Kitaev/yF6fPcey">Alexei Kitaev</a> in 1995.: 246 </p><p>Phase estimation is frequently used as a subroutine in other quantum algorithms, such as <a href="/facts/Shor%27s_algorithm/9JwEtznC">Shor's algorithm</a>,: 131 the <a href="/facts/Quantum_algorithm_for_linear_systems_of_equations/rpPIXdSe">quantum algorithm for linear systems of equations</a>, and the <a href="/facts/Quantum_counting_algorithm/aWRdAoC2">quantum counting algorithm</a>. </p>