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

# QFI

*class* `qiskit.opflow.gradients.QFI(qfi_method='lin_comb_full')`

[GitHub](https://github.com/qiskit/qiskit/tree/stable/0.46/qiskit/opflow/gradients/qfi.py)

Bases: [`QFIBase`](/docs/api/qiskit/0.46/qiskit.opflow.gradients.QFIBase "qiskit.opflow.gradients.qfi_base.QFIBase")

Deprecated: Compute the Quantum Fisher Information (QFI).

Computes the QFI given a pure, parameterized quantum state, where QFI is:

$$
\mathrm{QFI}_{kl}= 4 \mathrm{Re}[\langle \partial_k \psi | \partial_l \psi \rangle
− \langle\partial_k \psi | \psi \rangle \langle\psi | \partial_l \psi \rangle].
$$

> **Deprecated since version 0.24.0**
>
> The class `qiskit.opflow.gradients.qfi.QFI` is deprecated as of qiskit-terra 0.24.0. It will be removed in the Qiskit 1.0 release. For code migration guidelines, visit [https://qisk.it/opflow\_migration](https://qisk.it/opflow_migration).

## Attributes

### qfi\_method

Returns `CircuitQFI`.

**Returns**

`CircuitQFI`.

## Methods

### convert

`convert(operator, params=None)`

**Parameters**

- **operator** ([*CircuitStateFn*](/docs/api/qiskit/0.46/qiskit.opflow.state_fns.CircuitStateFn "qiskit.opflow.state_fns.circuit_state_fn.CircuitStateFn")) – The operator corresponding to the quantum state |ψ(ω)〉for which we compute the QFI
- **params** ([*ParameterVector*](/docs/api/qiskit/0.46/qiskit.circuit.ParameterVector "qiskit.circuit.parametervector.ParameterVector")  *|*[*ParameterExpression*](/docs/api/qiskit/0.46/qiskit.circuit.ParameterExpression "qiskit.circuit.parameterexpression.ParameterExpression")  *|*[*List*](https://docs.python.org/3/library/typing.html#typing.List)*\[*[*ParameterExpression*](/docs/api/qiskit/0.46/qiskit.circuit.ParameterExpression "qiskit.circuit.parameterexpression.ParameterExpression")*] | None*) – The parameters we are computing the QFI wrt: ω If not explicitly passed, they are inferred from the operator and sorted by name.

**Returns**

ListOp\[ListOp] where the operator at position k,l corresponds to QFI\_kl

**Raises**

[**ValueError**](https://docs.python.org/3/library/exceptions.html#ValueError) – If operator is not parameterized.

**Return type**

[*ListOp*](/docs/api/qiskit/0.46/qiskit.opflow.list_ops.ListOp "qiskit.opflow.list_ops.list_op.ListOp")