43 lines
2.4 KiB
Plaintext
43 lines
2.4 KiB
Plaintext
---
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title: expectation_value (v0.29)
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description: API reference for qiskit.ignis.mitigation.expectation_value in qiskit v0.29
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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.ignis.mitigation.expectation_value
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---
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<span id="qiskit-ignis-mitigation-expectation-value" />
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# qiskit.ignis.mitigation.expectation\_value
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<Function id="qiskit.ignis.mitigation.expectation_value" isDedicatedPage={true} github="https://github.com/qiskit-community/qiskit-ignis/tree/stable/0.6/qiskit/ignis/mitigation/expval/utils.py" signature="expectation_value(counts, diagonal=None, qubits=None, clbits=None, meas_mitigator=None)">
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Compute the expectation value of a diagonal operator from counts.
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This computes the estimator of $\langle O \rangle = \mbox{Tr}[\rho. O]$, optionally with measurement error mitigation, of a diagonal observable $O = \sum_{x\in\{0, 1\}^n} O(x)|x\rangle\!\langle x|$.
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**Parameters**
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* **counts** – counts object
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* **diagonal** – Optional, the vector of diagonal values for summing the expectation value. If `None` the the default value is $[1, -1]^\otimes n$.
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* **qubits** – Optional, the measured physical qubits the count bitstrings correspond to. If None qubits are assumed to be $[0, ..., n-1]$.
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* **clbits** – Optional, if not None marginalize counts to the specified bits.
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* **meas\_mitigator** – Optional, a measurement mitigator to apply mitigation.
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**Returns**
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the expectation value and standard deviation.
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**Return type**
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(float, float)
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## Additional Information:
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The diagonal observable $O$ is input using the `diagonal` kwarg as a list or Numpy array $[O(0), ..., O(2^n -1)]$. If no diagonal is specified the diagonal of the Pauli operator $O = \mbox{diag}(Z^{\otimes n}) = [1, -1]^{\otimes n}$ is used.
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The `clbits` kwarg is used to marginalize the input counts dictionary over the specified bit-values, and the `qubits` kwarg is used to specify which physical qubits these bit-values correspond to as `circuit.measure(qubits, clbits)`.
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For calibrating a expval measurement error mitigator for the `meas_mitigator` kwarg see [`qiskit.ignis.mitigation.expval_meas_mitigator_circuits()`](qiskit.ignis.mitigation.expval_meas_mitigator_circuits "qiskit.ignis.mitigation.expval_meas_mitigator_circuits") and [`qiskit.ignis.mitigation.ExpvalMeasMitigatorFitter`](qiskit.ignis.mitigation.ExpvalMeasMitigatorFitter "qiskit.ignis.mitigation.ExpvalMeasMitigatorFitter").
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</Function>
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