---
title: EstimationProblem (v0.46)
description: API reference for qiskit.algorithms.EstimationProblem in qiskit v0.46
source: https://eu-de.quantum.cloud.ibm.com/docs/en/api/qiskit/0.46/qiskit.algorithms.EstimationProblem
---

# EstimationProblem

*class* `qiskit.algorithms.EstimationProblem(state_preparation, objective_qubits, grover_operator=None, post_processing=None, is_good_state=None)`

[GitHub](https://github.com/qiskit/qiskit/tree/stable/0.46/qiskit/algorithms/amplitude_estimators/estimation_problem.py)

Bases: [`object`](https://docs.python.org/3/library/functions.html#object)

The estimation problem is the input to amplitude estimation algorithm.

This class contains all problem-specific information required to run an amplitude estimation algorithm. That means, it minimally contains the state preparation and the specification of the good state. It can further hold some post processing on the estimation of the amplitude or a custom Grover operator.

**Parameters**

- **state\_preparation** ([*QuantumCircuit*](/docs/api/qiskit/0.46/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit")) – A circuit preparing the input state, referred to as $\mathcal{A}$.
- **objective\_qubits** ([*int*](https://docs.python.org/3/library/functions.html#int)  *|*[*list*](https://docs.python.org/3/library/stdtypes.html#list)*\[*[*int*](https://docs.python.org/3/library/functions.html#int)*]*) – A single qubit index or a list of qubit indices to specify which qubits to measure. The `is_good_state` function is applied on the bitstring of these objective qubits.
- **grover\_operator** ([*QuantumCircuit*](/docs/api/qiskit/0.46/qiskit.circuit.QuantumCircuit "qiskit.circuit.QuantumCircuit") *| None*) – The Grover operator $\mathcal{Q}$ used as unitary in the phase estimation circuit.
- **post\_processing** (*Callable\[\[*[*float*](https://docs.python.org/3/library/functions.html#float)*],* [*float*](https://docs.python.org/3/library/functions.html#float)*] | None*) – A mapping applied to the result of the algorithm $0 \leq a \leq 1$, usually used to map the estimate to a target interval. Defaults to the identity.
- **is\_good\_state** (*Callable\[\[*[*str*](https://docs.python.org/3/library/stdtypes.html#str)*],* [*bool*](https://docs.python.org/3/library/functions.html#bool)*] | None*) – A function to check whether a string represents a good state. Defaults to all objective qubits being in state $|1\rangle$.

## Attributes

### grover\_operator

Get the $\mathcal{Q}$ operator, or Grover operator.

If the Grover operator is not set, we try to build it from the $\mathcal{A}$ operator and objective\_qubits. This only works if objective\_qubits is a list of integers.

**Returns**

The Grover operator, or None if neither the Grover operator nor the $\mathcal{A}$ operator is set.

### has\_good\_state

Check whether an [`is_good_state`](#qiskit.algorithms.EstimationProblem.is_good_state "qiskit.algorithms.EstimationProblem.is_good_state") function is set.

Some amplitude estimators, such as [`AmplitudeEstimation`](/docs/api/qiskit/0.46/qiskit.algorithms.AmplitudeEstimation "qiskit.algorithms.AmplitudeEstimation") do not support a custom implementation of the [`is_good_state`](#qiskit.algorithms.EstimationProblem.is_good_state "qiskit.algorithms.EstimationProblem.is_good_state") function, and can only handle the default.

**Returns**

`True`, if a custom [`is_good_state`](#qiskit.algorithms.EstimationProblem.is_good_state "qiskit.algorithms.EstimationProblem.is_good_state") is set, otherwise returns `False`.

### is\_good\_state

Checks whether a bitstring represents a good state.

**Returns**

Handle to the `is_good_state` callable.

### objective\_qubits

Get the criterion for a measurement outcome to be in a ‘good’ state.

**Returns**

The criterion as list of qubit indices.

### post\_processing

Apply post processing to the input value.

**Returns**

A handle to the post processing function. Acts as identity by default.

### state\_preparation

Get the $\mathcal{A}$ operator encoding the amplitude $a$.

**Returns**

The $\mathcal{A}$ operator as QuantumCircuit.

## Methods

### rescale

`rescale(scaling_factor)`

Rescale the good state amplitude in the estimation problem.

**Parameters**

**scaling\_factor** ([*float*](https://docs.python.org/3/library/functions.html#float)) – The scaling factor in \[0, 1].

**Returns**

A rescaled estimation problem.

**Return type**

[*EstimationProblem*](#qiskit.algorithms.EstimationProblem "qiskit.algorithms.amplitude_estimators.estimation_problem.EstimationProblem")