def __init__(
self,
observable: Union['cirq.BaseDensePauliString', Iterable['cirq.Pauli']],
key: Union[str, 'cirq.MeasurementKey'] = '',
) -> None:
"""Inits PauliMeasurementGate.
Args:
observable: Pauli observable to measure. Any `Iterable[cirq.Pauli]`
is a valid Pauli observable (with a +1 coefficient by default).
If you wish to measure pauli observables with coefficient -1,
then pass a `cirq.DensePauliString` as observable.
key: The string key of the measurement.
Raises:
ValueError: If the observable is empty.
"""
if not observable:
raise ValueError(f'Pauli observable {observable} is empty.')
if not all(
isinstance(p, pauli_gates.Pauli)
for p in cast(Iterable['cirq.Gate'], observable)):
raise ValueError(
f'Pauli observable {observable} must be Iterable[`cirq.Pauli`].'
)
coefficient = (observable.coefficient if isinstance(
observable, dps.BaseDensePauliString) else 1)
if coefficient not in [+1, -1]:
raise ValueError(
f'`cirq.DensePauliString` observable {observable} must have coefficient +1/-1.'
)
self._observable = dps.DensePauliString(observable,
coefficient=coefficient)
self.key = key # type: ignore
def with_observable(
self, observable: Union['cirq.BaseDensePauliString',
Iterable['cirq.Pauli']]
) -> 'PauliMeasurementGate':
"""Creates a pauli measurement gate with the new observable and same key."""
if (observable if isinstance(observable, dps.BaseDensePauliString) else
dps.DensePauliString(observable)) == self._observable:
return self
return PauliMeasurementGate(observable, key=self.key)
def observable(self) -> 'cirq.DensePauliString':
"""Pauli observable which should be measured by the gate."""
return dense_pauli_string.DensePauliString(self._observable)
def dense_pauli_string(self, pauli: Pauli) -> 'cirq.DensePauliString':
from cirq.ops import dense_pauli_string
pauli_tuple = self.pauli_tuple(pauli)
coefficient = -1 if pauli_tuple[1] else 1
return dense_pauli_string.DensePauliString(str(pauli_tuple[0]), coefficient=coefficient)