def _find_taper_op(
self,
qubit_op: PauliSumOp,
sector_locator: Optional[Callable[[Z2Symmetries],
Optional[List[int]]]] = None,
) -> Tuple[PauliSumOp, Z2Symmetries]:
# Return operator unchanged and empty symmetries if we do not taper
tapered_qubit_op = qubit_op
z2_symmetries = self._no_symmetries
# If we were given a sector, or one might be located, we first need to find any symmetries
if self.z2symmetry_reduction is not None:
z2_symmetries = Z2Symmetries.find_Z2_symmetries(qubit_op)
if z2_symmetries.is_empty():
logger.debug("No Z2 symmetries found")
else:
# As we have symmetries, if we have a sector locator, if that provides one back
# it will override any value defined on constructor
if sector_locator is not None and self.z2symmetry_reduction == "auto":
z2symmetry_reduction = sector_locator(z2_symmetries)
if z2symmetry_reduction is not None:
self.z2symmetry_reduction = z2symmetry_reduction # Overrides any value
# We may end up that neither were we given a sector nor that the locator
# returned one. Since though we may have found valid symmetries above we should
# simply just forget about them so as not to return something we are not using.
if self.z2symmetry_reduction is None:
z2_symmetries = self._no_symmetries
# So now if we have a sector and have symmetries we found we can attempt to taper
if (self.z2symmetry_reduction is not None
and self.z2symmetry_reduction != "auto"
and not z2_symmetries.is_empty()):
# check sector definition fits to symmetries found
if len(self._z2symmetry_reduction) != len(
z2_symmetries.symmetries):
raise QiskitNatureError(
"z2symmetry_reduction tapering values list has "
"invalid length {} should be {}".format(
len(self._z2symmetry_reduction),
len(z2_symmetries.symmetries)))
# Check all operators commute with main operator's symmetry
logger.debug(
"Sanity check that operator commutes with the symmetry")
symmetry_ops = []
for symmetry in z2_symmetries.symmetries:
symmetry_ops.append(
PauliSumOp.from_list([(symmetry.to_label(), 1.0)]))
commutes = QubitConverter._check_commutes(symmetry_ops, qubit_op)
if not commutes:
raise QiskitNatureError(
"Z2 symmetry failure. The operator must commute "
"with symmetries found from it!")
z2_symmetries.tapering_values = self._z2symmetry_reduction
tapered_qubit_op = z2_symmetries.taper(
qubit_op) if commutes else None
return tapered_qubit_op, z2_symmetries
def _no_symmetries(self) -> Z2Symmetries:
return Z2Symmetries([], [], [], None)