# Compute an expectation value with `Estimator` primitive#

This guide shows how to get the expected value of an observable for a given quantum circuit with the `Estimator` primitive.

Note

While this guide uses Qiskit’s reference implementation, the `Estimator` primitive can be run with any provider using `BackendEstimator` .

```from qiskit.primitives import BackendEstimator
from <some_qiskit_provider> import QiskitProvider

provider = QiskitProvider()
backend = provider.get_backend('backend_name')
estimator = BackendEstimator(backend)
```

There are some providers that implement primitives natively (see this page for more details).

## Initialize observables#

The first step is to define the observables whose expected value you want to compute. Each observable can be any `BaseOperator`, like the operators from `qiskit.quantum_info`. Among them it is preferable to use `SparsePauliOp`.

```from qiskit.quantum_info import SparsePauliOp

observable = SparsePauliOp(["II", "XX", "YY", "ZZ"], coeffs=[1, 1, -1, 1])
```

## Initialize quantum circuit#

Then you need to create the `QuantumCircuit`s for which you want to obtain the expected value.

```from qiskit import QuantumCircuit

qc = QuantumCircuit(2)
qc.h(0)
qc.cx(0,1)
qc.draw("mpl")
```

Note

The `QuantumCircuit` you pass to `Estimator` must not include any measurements.

## Initialize the `Estimator`#

Then, you need to instantiate an `Estimator`.

```from qiskit.primitives import Estimator

estimator = Estimator()
```

## Run and get results#

Now that you have defined your `estimator`, you can run your estimation by calling the `run()` method, which returns an instance of `PrimitiveJob` (subclass of `JobV1`). You can get the results from the job (as a `EstimatorResult` object) with the `result()` method.

```job = estimator.run(qc, observable)
result = job.result()
print(result)
```
```EstimatorResult(values=array([4.]), metadata=[{}])
```

While this example only uses one `QuantumCircuit` and one observable, if you want to get expectation values for multiple circuits and observables you can pass a `list` of `QuantumCircuit`s and a list of `BaseOperator`s to the `run()` method. Both `list`s must have the same length.

### Get the expected value#

From these results you can extract the expected values with the attribute `values`.

`values` returns a `numpy.ndarray` whose `i`-th element is the expectation value corresponding to the `i`-th circuit and `i`-th observable.

```exp_value = result.values
print(exp_value)
```
```3.999999999999999
```

## Parameterized circuit with `Estimator`#

The `Estimator` primitive can be run with unbound parameterized circuits like the one below. You can also manually bind values to the parameters of the circuit and follow the steps of the previous example.

```from qiskit.circuit import Parameter

theta = Parameter('θ')
param_qc = QuantumCircuit(2)
param_qc.ry(theta, 0)
param_qc.cx(0,1)
print(param_qc.draw())
```
```     ┌───────┐
q_0: ┤ Ry(θ) ├──■──
└───────┘┌─┴─┐
q_1: ─────────┤ X ├
└───┘
```

The main difference with the previous case is that now you need to specify the sets of parameter values for which you want to evaluate the expectation value as a `list` of `list`s of `float`s. The `i`-th element of the outer``list`` is the set of parameter values that corresponds to the `i`-th circuit and observable.

```import numpy as np

parameter_values = [, [np.pi/6], [np.pi/2]]

job = estimator.run([param_qc]*3, [observable]*3, parameter_values=parameter_values)
values = job.result().values

for i in range(3):
print(f"Parameter: {parameter_values[i]:.5f}\t Expectation value: {values[i]}")
```
```Parameter: 0.00000   Expectation value: 2.0
Parameter: 0.52360   Expectation value: 3.0
Parameter: 1.57080   Expectation value: 4.0
```

## Change run options#

Your workflow might require tuning primitive run options, such as the amount of shots.

By default, the reference `Estimator` class performs an exact statevector calculation based on the `Statevector` class. However, this can be modified to include shot noise if the number of `shots` is set. For reproducibility purposes, a `seed` will also be set in the following examples.

There are two main ways of setting options in the `Estimator`:

### Set keyword arguments for `run()`#

If you only want to change the settings for a specific run, it can be more convenient to set the options inside the `run()` method. You can do this by passing them as keyword arguments.

```job = estimator.run(qc, observable, shots=2048, seed=123)
result = job.result()
print(result)
```
```EstimatorResult(values=array([4.]), metadata=[{'variance': 3.552713678800501e-15, 'shots': 2048}])
```
```print(result.values)
```
```3.999999998697238
```

### Modify `Estimator` options#

If you want to keep some configuration values for several runs, it can be better to change the `Estimator` options. That way you can use the same `Estimator` object as many times as you wish without having to rewrite the configuration values every time you use `run()`.

#### Modify existing `Estimator`#

If you prefer to change the options of an already-defined `Estimator`, you can use `set_options()` and introduce the new options as keyword arguments.

```estimator.set_options(shots=2048, seed=123)

job = estimator.run(qc, observable)
result = job.result()
print(result)
```
```EstimatorResult(values=array([4.]), metadata=[{'variance': 3.552713678800501e-15, 'shots': 2048}])
```
```print(result.values)
```
```3.999999998697238
```

#### Define a new `Estimator` with the options#

If you prefer to define a new `Estimator` with new options, you need to define a `dict` like this one:

```options = {"shots": 2048, "seed": 123}
```

And then you can introduce it into your new `Estimator` with the `options` argument.

```estimator = Estimator(options=options)

job = estimator.run(qc, observable)
result = job.result()
print(result)
```
```EstimatorResult(values=array([4.]), metadata=[{'variance': 3.552713678800501e-15, 'shots': 2048}])
```
```print(result.values)
```
```3.999999998697238
```