65 lines
2.6 KiB
Plaintext
65 lines
2.6 KiB
Plaintext
---
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title: phase_estimation (latest version)
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description: API reference for qiskit.circuit.library.phase_estimation in the latest version of qiskit
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in_page_toc_min_heading_level: 1
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python_api_type: class
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python_api_name: qiskit.circuit.library.phase_estimation
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---
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<span id="phase-estimation" />
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# phase\_estimation
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<Class id="qiskit.circuit.library.phase_estimation" isDedicatedPage={true} github="https://github.com/Qiskit/qiskit/tree/stable/1.3/qiskit/circuit/library/phase_estimation.py#L108-L177" signature="qiskit.circuit.library.phase_estimation(num_evaluation_qubits, unitary, name='QPE')" modifiers="class">
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Bases:
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Phase Estimation circuit.
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In the Quantum Phase Estimation (QPE) algorithm \[1, 2, 3], the Phase Estimation circuit is used to estimate the phase $\phi$ of an eigenvalue $e^{2\pi i\phi}$ of a unitary operator $U$, provided with the corresponding eigenstate $|\psi\rangle$. That is
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$$
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U|\psi\rangle = e^{2\pi i\phi} |\psi\rangle
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$$
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This estimation (and thereby this circuit) is a central routine to several well-known algorithms, such as Shor’s algorithm or Quantum Amplitude Estimation.
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**Parameters**
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* **num\_evaluation\_qubits** ([*int*](https://docs.python.org/3/library/functions.html#int "(in Python v3.13)")) – The number of evaluation qubits.
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* **unitary** ([*QuantumCircuit*](qiskit.circuit.QuantumCircuit "qiskit.circuit.quantumcircuit.QuantumCircuit")) – The unitary operation $U$ which will be repeated and controlled.
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* **name** ([*str*](https://docs.python.org/3/library/stdtypes.html#str "(in Python v3.13)")) – The name of the circuit.
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**Return type**
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[*QuantumCircuit*](qiskit.circuit.QuantumCircuit "qiskit.circuit.quantumcircuit.QuantumCircuit")
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**Reference Circuit:**
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```python
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from qiskit.circuit import QuantumCircuit
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from qiskit.circuit.library import phase_estimation
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unitary = QuantumCircuit(2)
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unitary.x(0)
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unitary.y(1)
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circuit = phase_estimation(3, unitary)
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circuit.draw('mpl')
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```
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**References:**
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**\[1]: Kitaev, A. Y. (1995). Quantum measurements and the Abelian Stabilizer Problem. 1–22.**
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[quant-ph/9511026](http://arxiv.org/abs/quant-ph/9511026)
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**\[2]: Michael A. Nielsen and Isaac L. Chuang. 2011.**
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Quantum Computation and Quantum Information: 10th Anniversary Edition (10th ed.). Cambridge University Press, New York, NY, USA.
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**\[3]: Qiskit**
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[textbook](https://github.com/Qiskit/textbook/blob/main/notebooks/ch-algorithms/quantum-phase-estimation.ipynb)
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</Class>
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