qiskit/releasenotes/notes/0.15/phase-estimation-circuit-c2...

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A new class :class:`~qiskit.circuit.library.PhaseEstimation` has been
added to :mod:`qiskit.circuit.library`. This circuit library class is
the circuit used in the original formulation of the phase estimation
algorithm in
`arXiv:quant-ph/9511026 <https://arxiv.org/pdf/quant-ph/9511026.pdf>`__.
Phase estimation is the task to to estimate the phase :math:`\phi` of an
eigenvalue :math:`e^{2\pi i\phi}` of a unitary operator :math:`U`, provided
with the corresponding eigenstate :math:`|psi\rangle`. That is
.. math::
U|\psi\rangle = e^{2\pi i\phi} |\psi\rangle
This estimation (and thereby this circuit) is a central routine to several
well-known algorithms, such as Shor's algorithm or Quantum Amplitude
Estimation.