# Quellcode für qiskit.circuit.library.phase_estimation

# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2020.
#
# obtain a copy of this license in the LICENSE.txt file in the root directory
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.

"""Phase estimation circuit."""

from typing import Optional

from qiskit.circuit import QuantumCircuit, QuantumRegister

from .basis_change import QFT

[Doku]class PhaseEstimation(QuantumCircuit):
r"""Phase Estimation circuit.

In the Quantum Phase Estimation (QPE) algorithm [1, 2, 3], the Phase Estimation circuit is used
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.

**References:**

: Kitaev, A. Y. (1995). Quantum measurements and the Abelian Stabilizer Problem. 1–22.
quant-ph/9511026 <http://arxiv.org/abs/quant-ph/9511026>_

: Michael A. Nielsen and Isaac L. Chuang. 2011.
Quantum Computation and Quantum Information: 10th Anniversary Edition (10th ed.).
Cambridge University Press, New York, NY, USA.

: Qiskit
textbook <https://learn.qiskit.org/course/ch-algorithms/quantum-phase-estimation>_

"""

def __init__(
self,
num_evaluation_qubits: int,
unitary: QuantumCircuit,
iqft: Optional[QuantumCircuit] = None,
name: str = "QPE",
) -> None:
"""
Args:
num_evaluation_qubits: The number of evaluation qubits.
unitary: The unitary operation :math:U which will be repeated and controlled.
iqft: A inverse Quantum Fourier Transform, per default the inverse of
:class:~qiskit.circuit.library.QFT is used. Note that the QFT should not include
the usual swaps!
name: The name of the circuit.

.. note::

The inverse QFT should not include a swap of the qubit order.

Reference Circuit:
.. plot::

from qiskit.circuit import QuantumCircuit
from qiskit.circuit.library import PhaseEstimation
from qiskit.tools.jupyter.library import _generate_circuit_library_visualization
unitary = QuantumCircuit(2)
unitary.x(0)
unitary.y(1)
circuit = PhaseEstimation(3, unitary)
_generate_circuit_library_visualization(circuit)
"""
qr_eval = QuantumRegister(num_evaluation_qubits, "eval")
qr_state = QuantumRegister(unitary.num_qubits, "q")
circuit = QuantumCircuit(qr_eval, qr_state, name=name)

if iqft is None:
iqft = QFT(num_evaluation_qubits, inverse=True, do_swaps=False).reverse_bits()

circuit.h(qr_eval)  # hadamards on evaluation qubits

for j in range(num_evaluation_qubits):  # controlled powers
circuit.compose(unitary.power(2**j).control(), qubits=[j] + qr_state[:], inplace=True)

circuit.compose(iqft, qubits=qr_eval[:], inplace=True)  # final QFT

super().__init__(*circuit.qregs, name=circuit.name)
self.compose(circuit.to_gate(), qubits=self.qubits, inplace=True)