def evolution_for_pauli(self, pauli_op: PauliOp) -> PrimitiveOp:
r"""
Compute evolution Operator for a single Pauli using a ``PauliBasisChange``.
Args:
pauli_op: The ``PauliOp`` to evolve.
Returns:
A ``PrimitiveOp``, either the evolution ``CircuitOp`` or a ``PauliOp`` equal to the
identity if pauli_op is the identity.
"""
def replacement_fn(cob_instr_op, dest_pauli_op):
z_evolution = dest_pauli_op.exp_i()
# Remember, circuit composition order is mirrored operator composition order.
return cob_instr_op.adjoint().compose(z_evolution).compose(
cob_instr_op)
# Note: PauliBasisChange will pad destination with identities
# to produce correct CoB circuit
sig_bits = np.logical_or(pauli_op.primitive.z, pauli_op.primitive.x)
a_sig_bit = int(
max(np.extract(sig_bits,
np.arange(pauli_op.num_qubits)[::-1])))
destination = (I.tensorpower(a_sig_bit)) ^ (Z * pauli_op.coeff)
cob = PauliBasisChange(destination_basis=destination,
replacement_fn=replacement_fn)
return cast(PrimitiveOp, cob.convert(pauli_op))
def convert(self, operator: OperatorBase) -> OperatorBase:
"""Accepts an Operator and returns a new Operator with the Pauli measurements replaced by
diagonal Pauli post-rotation based measurements so they can be evaluated by sampling and
averaging.
Args:
operator: The operator to convert.
Returns:
The converted operator.
"""
if isinstance(operator, ListOp):
return operator.traverse(self.convert).reduce()
if isinstance(operator, OperatorStateFn) and operator.is_measurement:
# Change to Pauli representation if necessary
if (isinstance(operator.primitive, (ListOp, PrimitiveOp))
and not isinstance(operator.primitive, PauliSumOp) and
{"Pauli", "SparsePauliOp"} < operator.primitive_strings()):
logger.warning(
"Measured Observable is not composed of only Paulis, converting to "
"Pauli representation, which can be expensive.")
# Setting massive=False because this conversion is implicit. User can perform this
# action on the Observable with massive=True explicitly if they so choose.
pauli_obsv = operator.primitive.to_pauli_op(massive=False)
operator = StateFn(pauli_obsv,
is_measurement=True,
coeff=operator.coeff)
if self._grouper and isinstance(operator.primitive,
(ListOp, PauliSumOp)):
grouped = self._grouper.convert(operator.primitive)
operator = StateFn(grouped,
is_measurement=True,
coeff=operator.coeff)
# Convert the measurement into diagonal basis (PauliBasisChange chooses
# this basis by default).
cob = PauliBasisChange(
replacement_fn=PauliBasisChange.measurement_replacement_fn)
return cob.convert(operator).reduce()
return operator