128 lines
6.9 KiB
Plaintext
128 lines
6.9 KiB
Plaintext
---
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title: QFI
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description: API reference for qiskit.aqua.operators.gradients.QFI
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in_page_toc_min_heading_level: 1
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python_api_type: class
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python_api_name: qiskit.aqua.operators.gradients.QFI
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---
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# qiskit.aqua.operators.gradients.QFI
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<Class id="qiskit.aqua.operators.gradients.QFI" isDedicatedPage={true} github="https://github.com/qiskit-community/qiskit-aqua/tree/stable/0.9/qiskit/aqua/operators/gradients/qfi.py" signature="QFI(qfi_method='lin_comb_full')" modifiers="class">
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Compute the Quantum Fisher Information (QFI).
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Computes the QFI given a pure, parametrized quantum state, where QFI is:
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$$
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\mathrm{QFI}_{kl}= 4 \mathrm{Re}[\langle \partial_k \psi | \partial_l \psi \rangle
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− \langle\partial_k \psi | \psi \rangle \langle\psi | \partial_l \psi \rangle].
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$$
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**Parameters**
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**qfi\_method** (`Union`\[`str`, `CircuitQFI`]) – The method used to compute the state/probability gradient. Can be either a [`CircuitQFI`](qiskit.aqua.operators.gradients.CircuitQFI "qiskit.aqua.operators.gradients.CircuitQFI") instance or one of the following pre-defined strings `'lin_comb_full'`, `` 'overlap_diag'` `` or `` 'overlap_block_diag'` ``.
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**Raises**
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**ValueError** – if `qfi_method` is neither a `CircuitQFI` object nor one of the predefined strings.
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### \_\_init\_\_
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<Function id="qiskit.aqua.operators.gradients.QFI.__init__" signature="__init__(qfi_method='lin_comb_full')">
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**Parameters**
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**qfi\_method** (`Union`\[`str`, `CircuitQFI`]) – The method used to compute the state/probability gradient. Can be either a [`CircuitQFI`](qiskit.aqua.operators.gradients.CircuitQFI "qiskit.aqua.operators.gradients.CircuitQFI") instance or one of the following pre-defined strings `'lin_comb_full'`, `` 'overlap_diag'` `` or `` 'overlap_block_diag'` ``.
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**Raises**
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**ValueError** – if `qfi_method` is neither a `CircuitQFI` object nor one of the predefined strings.
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</Function>
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## Methods
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| | |
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| --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------- |
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| [`__init__`](#qiskit.aqua.operators.gradients.QFI.__init__ "qiskit.aqua.operators.gradients.QFI.__init__")(\[qfi\_method]) | **type qfi\_method**`Union`\[`str`, `CircuitQFI`] |
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| [`convert`](#qiskit.aqua.operators.gradients.QFI.convert "qiskit.aqua.operators.gradients.QFI.convert")(operator\[, params]) | **type operator**`CircuitStateFn` |
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| [`gradient_wrapper`](#qiskit.aqua.operators.gradients.QFI.gradient_wrapper "qiskit.aqua.operators.gradients.QFI.gradient_wrapper")(operator, bind\_params\[, …]) | Get a callable function which provides the respective gradient, Hessian or QFI for given parameter values. |
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| [`parameter_expression_grad`](#qiskit.aqua.operators.gradients.QFI.parameter_expression_grad "qiskit.aqua.operators.gradients.QFI.parameter_expression_grad")(param\_expr, param) | Get the derivative of a parameter expression w\.r.t. |
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## Attributes
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| | |
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| ---------------------------------------------------------------------------------------------------------------- | --------------------- |
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| [`qfi_method`](#qiskit.aqua.operators.gradients.QFI.qfi_method "qiskit.aqua.operators.gradients.QFI.qfi_method") | Returns `CircuitQFI`. |
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### convert
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<Function id="qiskit.aqua.operators.gradients.QFI.convert" signature="convert(operator, params=None)">
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**Parameters**
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* **operator** (`CircuitStateFn`) – The operator corresponding to the quantum state |ψ(ω)〉for which we compute the QFI
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* **params** (`Union`\[`ParameterVector`, `ParameterExpression`, `List`\[`ParameterExpression`], `None`]) – The parameters we are computing the QFI wrt: ω
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**Return type**
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`ListOp`
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**Returns**
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ListOp\[ListOp] where the operator at position k,l corresponds to QFI\_kl
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</Function>
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### gradient\_wrapper
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<Function id="qiskit.aqua.operators.gradients.QFI.gradient_wrapper" signature="gradient_wrapper(operator, bind_params, grad_params=None, backend=None)">
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Get a callable function which provides the respective gradient, Hessian or QFI for given parameter values. This callable can be used as gradient function for optimizers.
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**Parameters**
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* **operator** (`OperatorBase`) – The operator for which we want to get the gradient, Hessian or QFI.
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* **bind\_params** (`Union`\[`ParameterExpression`, `ParameterVector`, `List`\[`ParameterExpression`]]) – The operator parameters to which the parameter values are assigned.
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* **grad\_params** (`Union`\[`ParameterExpression`, `ParameterVector`, `List`\[`ParameterExpression`], `Tuple`\[`ParameterExpression`, `ParameterExpression`], `List`\[`Tuple`\[`ParameterExpression`, `ParameterExpression`]], `None`]) – The parameters with respect to which we are taking the gradient, Hessian or QFI. If grad\_params = None, then grad\_params = bind\_params
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* **backend** (`Union`\[`BaseBackend`, `QuantumInstance`, `None`]) – The quantum backend or QuantumInstance to use to evaluate the gradient, Hessian or QFI.
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**Returns**
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Function to compute a gradient, Hessian or QFI. The function takes an iterable as argument which holds the parameter values.
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**Return type**
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callable(param\_values)
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</Function>
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### parameter\_expression\_grad
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<Function id="qiskit.aqua.operators.gradients.QFI.parameter_expression_grad" signature="parameter_expression_grad(param_expr, param)" modifiers="static">
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Get the derivative of a parameter expression w\.r.t. the given parameter.
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**Parameters**
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* **param\_expr** (`ParameterExpression`) – The Parameter Expression for which we compute the derivative
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* **param** (`ParameterExpression`) – Parameter w\.r.t. which we want to take the derivative
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**Return type**
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`Union`\[`ParameterExpression`, `float`]
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**Returns**
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ParameterExpression representing the gradient of param\_expr w\.r.t. param
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</Function>
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### qfi\_method
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<Attribute id="qiskit.aqua.operators.gradients.QFI.qfi_method">
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Returns `CircuitQFI`.
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**Return type**
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`CircuitQFI`
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**Returns**
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`CircuitQFI`.
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</Attribute>
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</Class>
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