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

# QFIBase

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

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

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

Deprecated: Base class for Quantum Fisher Information (QFI).

Compute the Quantum Fisher Information (QFI) given a pure, parameterized quantum state.

The QFI is:

> \[QFI]kl= Re\[〈∂kψ|∂lψ〉−〈∂kψ|ψ〉〈ψ|∂lψ〉] \* 4.

> **Deprecated since version 0.24.0**
>
> The class `qiskit.opflow.gradients.qfi_base.QFIBase` 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).

**Parameters**

**qfi\_method** ([*str*](https://docs.python.org/3/library/stdtypes.html#str)  *|*[*CircuitQFI*](/docs/api/qiskit/0.46/qiskit.opflow.gradients.CircuitQFI "qiskit.opflow.gradients.circuit_qfis.circuit_qfi.CircuitQFI")) – The method used to compute the state/probability gradient. Can be either a [`CircuitQFI`](/docs/api/qiskit/0.46/qiskit.opflow.gradients.CircuitQFI "qiskit.opflow.gradients.CircuitQFI") instance or one of the following pre-defined strings `'lin_comb_full'`, `` 'overlap_diag'` `` or `` 'overlap_block_diag'` ``.

**Raises**

[**ValueError**](https://docs.python.org/3/library/exceptions.html#ValueError) – if `qfi_method` is neither a `CircuitQFI` object nor one of the predefined strings.

## Attributes

### qfi\_method

Returns `CircuitQFI`.

**Returns**

`CircuitQFI`.