def expectation(observable: Observable, target: QubitSetInput = None) -> ResultType:
"""Registers this function into the circuit class.
Args:
observable (Observable): the observable for the result type
target (int, Qubit, or iterable of int / Qubit, optional): Target qubits that the
result type is requested for. Default is `None`, which means the observable must
operate only on 1 qubit and it is applied to all qubits in parallel.
Returns:
ResultType: expectation as a requested result type
Examples:
>>> circ = Circuit().expectation(observable=Observable.Z(), target=0)
"""
return ResultType.Expectation(observable=observable, target=target)
def _get_expectation_value(
self,
bkcirc: braket.circuits.Circuit,
observable: Observable,
target: QubitSet,
n_shots: int,
**kwargs: KwargTypes,
) -> np.float64:
if not self.supports_expectation:
raise RuntimeError("Backend does not support expectation")
if (n_shots < self._expectation_min_shots
or n_shots > self._expectation_max_shots):
raise ValueError(
f"n_shots must be between {self._expectation_min_shots} and "
f"{self._expectation_max_shots}")
restype = ResultType.Expectation(observable, target=target)
bkcirc.add_result_type(restype)
task = self._run(bkcirc, n_shots=n_shots, **kwargs)
res = task.result()
return res.get_value_by_result_type(restype) # type: ignore