39 lines
4.5 KiB
Plaintext
39 lines
4.5 KiB
Plaintext
---
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title: expectations
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description: API reference for qiskit.opflow.expectations
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in_page_toc_min_heading_level: 2
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python_api_type: module
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python_api_name: qiskit.opflow.expectations
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---
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<span id="module-qiskit.opflow.expectations" />
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<span id="qiskit-opflow-expectations" />
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# Expectations
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<span id="module-qiskit.opflow.expectations" />
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`qiskit.opflow.expectations`
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Expectations are converters which enable the computation of the expectation value of an Observable with respect to some state function. They traverse an Operator tree, replacing [`OperatorStateFn`](qiskit.opflow.state_fns.OperatorStateFn "qiskit.opflow.state_fns.OperatorStateFn") measurements with equivalent measurements which are more amenable to computation on quantum or classical hardware. For example, if one would like to measure the expectation value of an Operator `o` expressed as a sum of Paulis with respect to some state function, but only has access to diagonal measurements on Quantum hardware, we can create a measurement \~StateFn(o), use a [`PauliExpectation`](qiskit.opflow.expectations.PauliExpectation "qiskit.opflow.expectations.PauliExpectation") to convert it to a diagonal measurement and circuit pre-rotations to append to the state, and sample this circuit on Quantum hardware with a [`CircuitSampler`](qiskit.opflow.converters.CircuitSampler "qiskit.opflow.converters.CircuitSampler"). All in all, this would be: `my_sampler.convert(my_expect.convert(~StateFn(o)) @ my_state).eval()`.
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# Expectation Base Class
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The ExpectationBase class gives an interface for algorithms to ask for Expectations as execution settings. For example, if an algorithm contains an expectation value step within it, such as [`VQE`](qiskit.algorithms.VQE "qiskit.algorithms.VQE"), the algorithm can give the opportunity for the user to pass an ExpectationBase of their choice to be used in that expectation value step.
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| ------------------------------------------------------------------------------------------------------------ | ---------------------------------------- |
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| [`ExpectationBase`](qiskit.opflow.expectations.ExpectationBase "qiskit.opflow.expectations.ExpectationBase") | A base for Expectation value converters. |
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# Expectations
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| ------------------------------------------------------------------------------------------------------------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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| [`ExpectationFactory`](qiskit.opflow.expectations.ExpectationFactory "qiskit.opflow.expectations.ExpectationFactory") | A factory class for convenient automatic selection of an Expectation based on the Operator to be converted and backend used to sample the expectation value. |
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| [`AerPauliExpectation`](qiskit.opflow.expectations.AerPauliExpectation "qiskit.opflow.expectations.AerPauliExpectation") | An Expectation converter for using Aer’s operator snapshot to take expectations of quantum state circuits over Pauli observables. |
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| [`MatrixExpectation`](qiskit.opflow.expectations.MatrixExpectation "qiskit.opflow.expectations.MatrixExpectation") | An Expectation converter which converts Operator measurements to be matrix-based so they can be evaluated by matrix multiplication. |
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| [`PauliExpectation`](qiskit.opflow.expectations.PauliExpectation "qiskit.opflow.expectations.PauliExpectation") | An Expectation converter for Pauli-basis observables by changing Pauli measurements to a diagonal (\{Z, I}^n) basis and appending circuit post-rotations to the measured state function. |
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| [`CVaRExpectation`](qiskit.opflow.expectations.CVaRExpectation "qiskit.opflow.expectations.CVaRExpectation") | Compute the Conditional Value at Risk (CVaR) expectation value. |
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