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eval_observables(quantum_instance, quantum_state, observables, expectation, threshold=1e-12)

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Pending deprecation: 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.

This function has been superseded by the qiskit.algorithms.observables_evaluator.eval_observables() function. It will be deprecated in a future release and subsequently removed after that.


  • quantum_instance (Union[QuantumInstance, Backend]) – A quantum instance used for calculations.
  • quantum_state (Union[Statevector, QuantumCircuit, OperatorBase]) – An unparametrized quantum circuit representing a quantum state that expectation values are computed against.
  • observables (Union[List[Optional[OperatorBase]], Dict[str, OperatorBase]]) – A list or a dictionary of operators whose expectation values are to be calculated.
  • expectation (ExpectationBase) – An instance of ExpectationBase which defines a method for calculating expectation values.
  • threshold (float) – A threshold value that defines which mean values should be neglected (helpful for ignoring numerical instabilities close to 0).

Return type

Union[List[Optional[Tuple[complex, complex]]], Dict[str, Tuple[complex, complex]]]


A list or a dictionary of tuples (mean, standard deviation).


ValueError – If a quantum_state with free parameters is provided.

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