def _row_to_dense_pauli(self, i: int) -> 'cirq.DensePauliString':
"""Return a dense Pauli string for the given row in the tableau.
Args:
i: index of the row in the tableau.
Returns:
A DensePauliString representing the row. The length of the string
is equal to the total number of qubits and each character
represents the effective single Pauli operator on that qubit. The
overall phase is captured in the coefficient.
"""
from cirq.ops.dense_pauli_string import DensePauliString
coefficient = -1 if self.rs[i] else 1
pauli_mask = ""
for k in range(self.n):
if self.xs[i, k] & (not self.zs[i, k]):
pauli_mask += "X"
elif (not self.xs[i, k]) & self.zs[i, k]:
pauli_mask += "Z"
elif self.xs[i, k] & self.zs[i, k]:
pauli_mask += "Y"
else:
pauli_mask += "I"
return DensePauliString(pauli_mask, coefficient=coefficient)
def gate(self) -> 'cirq.DensePauliString':
order: List[Optional[pauli_gates.Pauli]] = [
None, pauli_gates.X, pauli_gates.Y, pauli_gates.Z
]
from cirq.ops.dense_pauli_string import DensePauliString
return DensePauliString(
coefficient=self.coefficient,
pauli_mask=[order.index(self[q]) for q in self.qubits])
def dense(self, qubits: Sequence['cirq.Qid']) -> 'cirq.DensePauliString':
"""Returns a `cirq.DensePauliString` version of this Pauli string.
This method satisfies the invariant `P.dense(qubits).on(*qubits) == P`.
Args:
qubits: The implicit sequence of qubits used by the dense pauli
string. Specifically, if the returned dense Pauli string was
applied to these qubits (via its `on` method) then the result
would be a Pauli string equivalent to the receiving Pauli
string.
Returns:
A `cirq.DensePauliString` instance `D` such that `D.on(*qubits)`
equals the receiving `cirq.PauliString` instance `P`.
"""
from cirq.ops.dense_pauli_string import DensePauliString
if not self.keys() <= set(qubits):
raise ValueError('not self.keys() <= set(qubits)')
# pylint: disable=too-many-function-args
pauli_mask = [self.get(q, identity.I) for q in qubits]
# pylint: enable=too-many-function-args
return DensePauliString(pauli_mask, coefficient=self.coefficient)
