71 lines
2.2 KiB
Plaintext
71 lines
2.2 KiB
Plaintext
---
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title: QFI
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description: API reference for qiskit.opflow.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.opflow.gradients.QFI
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---
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# QFI
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<Class id="qiskit.opflow.gradients.QFI" isDedicatedPage={true} github="https://github.com/qiskit/qiskit/tree/stable/0.19/qiskit/opflow/gradients/qfi.py" signature="QFI(qfi_method='lin_comb_full')" modifiers="class">
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Bases: `qiskit.opflow.gradients.qfi_base.QFIBase`
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Compute the Quantum Fisher Information (QFI).
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Computes the QFI given a pure, parameterized 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.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'` ``.
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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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## Methods Defined Here
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### convert
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<Function id="qiskit.opflow.gradients.QFI.convert" signature="QFI.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 If not explicitly passed, they are inferred from the operator and sorted by name.
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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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**Raises**
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**ValueError** – If operator is not parameterized.
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</Function>
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## Attributes
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### qfi\_method
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<Attribute id="qiskit.opflow.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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