---
title: PhaseEstimation (v2.2)
description: API reference for qiskit.circuit.library.PhaseEstimation in qiskit v2.2
source: https://eu-de.quantum.cloud.ibm.com/docs/en/api/qiskit/2.2/qiskit.circuit.library.PhaseEstimation
---

# PhaseEstimation

*class* `qiskit.circuit.library.PhaseEstimation(num_evaluation_qubits, unitary, iqft=None, name='QPE')`

[GitHub](https://github.com/Qiskit/qiskit/tree/stable/2.2/qiskit/circuit/library/phase_estimation.py#L22-L106)

Bases: [`QuantumCircuit`](/docs/api/qiskit/2.2/qiskit.circuit.QuantumCircuit "qiskit.circuit.quantumcircuit.QuantumCircuit")

Phase Estimation circuit.

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

$$
U|\psi\rangle = e^{2\pi i\phi} |\psi\rangle
$$

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:

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

\[2] 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.

\[3] Qiskit [textbook](https://github.com/Qiskit/textbook/blob/main/notebooks/ch-algorithms/quantum-phase-estimation.ipynb)

> **Deprecated since version 2.1**
>
> The class `qiskit.circuit.library.phase_estimation.PhaseEstimation` is deprecated as of Qiskit 2.1. It will be removed in Qiskit 3.0. Use qiskit.circuit.library.phase\_estimation instead.

**Parameters**

- **num\_evaluation\_qubits** ([*int*](https://docs.python.org/3/library/functions.html#int)) – The number of evaluation qubits.
- **unitary** ([*QuantumCircuit*](/docs/api/qiskit/2.2/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit")) – The unitary operation $U$ which will be repeated and controlled.
- **iqft** ([*QuantumCircuit*](/docs/api/qiskit/2.2/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") *| None*) – A inverse Quantum Fourier Transform, per default the inverse of [`QFT`](/docs/api/qiskit/2.2/qiskit.circuit.library.QFT "qiskit.circuit.library.QFT") is used. Note that the QFT should not include the usual swaps!
- **name** ([*str*](https://docs.python.org/3/library/stdtypes.html#str)) – The name of the circuit.

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

Reference Circuit:

![Diagram illustrating the previously described circuit.](https://eu-de.quantum.cloud.ibm.com/docs/images/api/qiskit/2.2/qiskit-circuit-library-PhaseEstimation-1.avif)

## Attributes

### name

Type: `str`

A human-readable name for the circuit.

**Example**

```python
from qiskit import QuantumCircuit

qc = QuantumCircuit(2, 2, name="my_circuit")
print(qc.name)
```

```text
my_circuit
```