47 lines
2.1 KiB
Plaintext
47 lines
2.1 KiB
Plaintext
---
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title: QFIBase
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description: API reference for qiskit.opflow.gradients.QFIBase
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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.QFIBase
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---
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# QFIBase
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<Class id="qiskit.opflow.gradients.QFIBase" isDedicatedPage={true} github="https://github.com/qiskit/qiskit/tree/stable/0.45/qiskit/opflow/gradients/qfi_base.py" signature="qiskit.opflow.gradients.QFIBase(qfi_method='lin_comb_full')" modifiers="class">
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Bases: [`DerivativeBase`](qiskit.opflow.gradients.DerivativeBase "qiskit.opflow.gradients.derivative_base.DerivativeBase")
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Deprecated: Base class for Quantum Fisher Information (QFI).
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Compute the Quantum Fisher Information (QFI) given a pure, parameterized quantum state.
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The QFI is:
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> \[QFI]kl= Re\[〈∂kψ|∂lψ〉−〈∂kψ|ψ〉〈ψ|∂lψ〉] \* 4.
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<Admonition title="Deprecated since version 0.24.0" type="danger">
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The class `qiskit.opflow.gradients.qfi_base.QFIBase` is deprecated as of qiskit-terra 0.24.0. It will be removed no earlier than 3 months after the release date. For code migration guidelines, visit [https://qisk.it/opflow\_migration](https://qisk.it/opflow_migration).
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</Admonition>
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**Parameters**
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**qfi\_method** ([*str*](https://docs.python.org/3/library/stdtypes.html#str "(in Python v3.12)") *|*[*CircuitQFI*](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`](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**](https://docs.python.org/3/library/exceptions.html#ValueError "(in Python v3.12)") – if `qfi_method` is neither a `CircuitQFI` object nor one of the predefined strings.
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## Attributes
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### qfi\_method
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<Attribute id="qiskit.opflow.gradients.QFIBase.qfi_method">
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Returns `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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