qiskit.algorithms.eval_observables

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

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.

Parameters

• quantum_instance (Union[QuantumInstance, BaseBackend, 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]]]

Returns

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

Raises

ValueError – If a quantum_state with free parameters is provided.