35 lines
2.5 KiB
Plaintext
35 lines
2.5 KiB
Plaintext
---
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title: eval_observables
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description: API reference for qiskit.algorithms.eval_observables
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in_page_toc_min_heading_level: 1
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python_api_type: function
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python_api_name: qiskit.algorithms.eval_observables
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---
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# qiskit.algorithms.eval\_observables
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<Function id="qiskit.algorithms.eval_observables" isDedicatedPage={true} github="https://github.com/qiskit/qiskit/tree/stable/0.21/qiskit/algorithms/aux_ops_evaluator.py" signature="eval_observables(quantum_instance, quantum_state, observables, expectation, threshold=1e-12)">
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Accepts a list or a dictionary of operators and calculates their expectation values - means and standard deviations. They are calculated with respect to a quantum state provided. A user can optionally provide a threshold value which filters mean values falling below the threshold.
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**Parameters**
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* **quantum\_instance** (`Union`\[[`QuantumInstance`](qiskit.utils.QuantumInstance "qiskit.utils.quantum_instance.QuantumInstance"), [`Backend`](qiskit.providers.Backend "qiskit.providers.backend.Backend")]) – A quantum instance used for calculations.
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* **quantum\_state** (`Union`\[[`Statevector`](qiskit.quantum_info.Statevector "qiskit.quantum_info.states.statevector.Statevector"), [`QuantumCircuit`](qiskit.circuit.QuantumCircuit "qiskit.circuit.quantumcircuit.QuantumCircuit"), [`OperatorBase`](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")]) – An unparametrized quantum circuit representing a quantum state that expectation values are computed against.
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* **observables** (`Union`\[`List`\[`Optional`\[[`OperatorBase`](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")]], `Dict`\[`str`, [`OperatorBase`](qiskit.opflow.OperatorBase "qiskit.opflow.operator_base.OperatorBase")]]) – A list or a dictionary of operators whose expectation values are to be calculated.
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* **expectation** ([`ExpectationBase`](qiskit.opflow.expectations.ExpectationBase "qiskit.opflow.expectations.expectation_base.ExpectationBase")) – An instance of ExpectationBase which defines a method for calculating expectation values.
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* **threshold** (`float`) – A threshold value that defines which mean values should be neglected (helpful for ignoring numerical instabilities close to 0).
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**Return type**
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`Union`\[`List`\[`Optional`\[`Tuple`\[`complex`, `complex`]]], `Dict`\[`str`, `Tuple`\[`complex`, `complex`]]]
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**Returns**
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A list or a dictionary of tuples (mean, standard deviation).
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**Raises**
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**ValueError** – If a `quantum_state` with free parameters is provided.
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</Function>
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