def test_is_pauli_word(self):
"""Test for determining whether input ``Observable`` instance is a Pauli word."""
observable_1 = PauliX(0)
observable_2 = PauliZ(1) @ PauliX(2) @ PauliZ(4)
observable_3 = PauliX(1) @ Hadamard(4)
observable_4 = Hadamard(0)
assert is_pauli_word(observable_1)
assert is_pauli_word(observable_2)
assert not is_pauli_word(observable_3)
assert not is_pauli_word(observable_4)
def diagonalize_pauli_word(pauli_word):
"""Transforms the Pauli word to diagonal form in the computational basis.
**Usage example:**
>>> diagonalize_pauli_word(PauliX('a') @ PauliY('b') @ PauliZ('c'))
Tensor(PauliZ(wires=['a']), PauliZ(wires=['b']), PauliZ(wires=['c']))
Args:
pauli_word (Observable): the Pauli word to diagonalize in computational basis
Returns:
Observable: the Pauli word diagonalized in the computational basis
Raises:
TypeError: if the input is not a Pauli word, i.e., a Pauli operator, identity, or `Tensor`
instances thereof
"""
if not is_pauli_word(pauli_word):
raise TypeError(
"Input must be a Pauli word, instead got: {}.".format(pauli_word))
paulis_with_identity = (qml.PauliX, qml.PauliY, qml.PauliZ, qml.Identity)
diag_term = None
if isinstance(pauli_word, Tensor):
for sigma in pauli_word.obs:
if sigma.name != "Identity":
if diag_term is None:
diag_term = qml.PauliZ(wires=sigma.wires)
else:
diag_term @= qml.PauliZ(wires=sigma.wires)
elif isinstance(pauli_word, paulis_with_identity):
sigma = pauli_word
if sigma.name != "Identity":
diag_term = qml.PauliZ(wires=sigma.wires)
if diag_term is None:
diag_term = qml.Identity(pauli_word.wires.tolist()[0])
return diag_term