Source code for qiskit.aqua.operators.primitive_ops.pauli_op
# This code is part of Qiskit.
#
# (C) Copyright IBM 2020, 2021.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.
""" PauliOp Class """
from typing import Union, Set, Dict, cast, List, Optional
import logging
import numpy as np
from scipy.sparse import spmatrix
from qiskit import QuantumCircuit
from qiskit.circuit import ParameterExpression, Instruction
from qiskit.quantum_info import Pauli
from qiskit.circuit.library import RZGate, RYGate, RXGate, XGate, YGate, ZGate, IGate
from ..operator_base import OperatorBase
from .primitive_op import PrimitiveOp
from ..list_ops.summed_op import SummedOp
from ..list_ops.tensored_op import TensoredOp
from ..legacy.weighted_pauli_operator import WeightedPauliOperator
from ... import AquaError
logger = logging.getLogger(__name__)
PAULI_GATE_MAPPING = {'X': XGate(), 'Y': YGate(), 'Z': ZGate(), 'I': IGate()}
[docs]class PauliOp(PrimitiveOp):
""" Class for Operators backed by Terra's ``Pauli`` module.
"""
def __init__(self,
primitive: Union[Pauli],
coeff: Union[int, float, complex, ParameterExpression] = 1.0) -> None:
"""
Args:
primitive: The Pauli which defines the behavior of the underlying function.
coeff: A coefficient multiplying the primitive.
Raises:
TypeError: invalid parameters.
"""
if not isinstance(primitive, Pauli):
raise TypeError(
'PauliOp can only be instantiated with Paulis, not {}'.format(type(primitive)))
super().__init__(primitive, coeff=coeff)
@property
def num_qubits(self) -> int:
return len(self.primitive)
[docs] def add(self, other: OperatorBase) -> OperatorBase:
if not self.num_qubits == other.num_qubits:
raise ValueError(
'Sum over operators with different numbers of qubits, {} and {}, is not well '
'defined'.format(self.num_qubits, other.num_qubits))
if isinstance(other, PauliOp) and self.primitive == other.primitive:
return PauliOp(self.primitive, coeff=self.coeff + other.coeff)
return SummedOp([self, other])
[docs] def equals(self, other: OperatorBase) -> bool:
if not isinstance(other, PauliOp) or not self.coeff == other.coeff:
return False
return self.primitive == other.primitive
def _expand_dim(self, num_qubits: int) -> 'PauliOp':
return PauliOp(Pauli('I'*num_qubits).expand(self.primitive), coeff=self.coeff)
[docs] def tensor(self, other: OperatorBase) -> OperatorBase:
# Both Paulis
if isinstance(other, PauliOp):
# Copying here because Terra's Pauli kron is in-place.
op_copy = Pauli((other.primitive.z, other.primitive.x)) # type: ignore
# NOTE!!! REVERSING QISKIT ENDIANNESS HERE
return PauliOp(op_copy.expand(self.primitive), coeff=self.coeff * other.coeff)
# pylint: disable=cyclic-import,import-outside-toplevel
from .circuit_op import CircuitOp
if isinstance(other, CircuitOp):
return self.to_circuit_op().tensor(other)
return TensoredOp([self, other])
[docs] def permute(self, permutation: List[int]) -> 'PauliOp':
"""Permutes the sequence of Pauli matrices.
Args:
permutation: A list defining where each Pauli should be permuted. The Pauli at index
j of the primitive should be permuted to position permutation[j].
Returns:
A new PauliOp representing the permuted operator. For operator (X ^ Y ^ Z) and
indices=[1,2,4], it returns (X ^ I ^ Y ^ Z ^ I).
Raises:
AquaError: if indices do not define a new index for each qubit.
"""
pauli_string = self.primitive.__str__()
length = max(permutation) + 1 # size of list must be +1 larger then its max index
new_pauli_list = ['I'] * length
if len(permutation) != self.num_qubits:
raise AquaError("List of indices to permute must have the same size as Pauli Operator")
for i, index in enumerate(permutation):
new_pauli_list[-index - 1] = pauli_string[-i - 1]
return PauliOp(Pauli(label=''.join(new_pauli_list)), self.coeff)
[docs] def compose(self, other: OperatorBase,
permutation: Optional[List[int]] = None, front: bool = False) -> OperatorBase:
new_self, other = self._expand_shorter_operator_and_permute(other, permutation)
new_self = cast(PauliOp, new_self)
if front:
return other.compose(new_self)
# If self is identity, just return other.
if not any(new_self.primitive.x + new_self.primitive.z): # type: ignore
return other * new_self.coeff # type: ignore
# Both Paulis
if isinstance(other, PauliOp):
p_a = new_self.primitive.dot(other.primitive) # type: ignore
product, phase = p_a[:], (-1j) ** p_a.phase # type: ignore
return PrimitiveOp(product, coeff=new_self.coeff * other.coeff * phase)
# pylint: disable=cyclic-import,import-outside-toplevel
from .circuit_op import CircuitOp
from ..state_fns.circuit_state_fn import CircuitStateFn
if isinstance(other, (CircuitOp, CircuitStateFn)):
return new_self.to_circuit_op().compose(other)
return super(PauliOp, new_self).compose(other)
[docs] def to_matrix(self, massive: bool = False) -> np.ndarray:
OperatorBase._check_massive('to_matrix', True, self.num_qubits, massive)
return self.primitive.to_matrix() * self.coeff # type: ignore
[docs] def to_spmatrix(self) -> spmatrix:
""" Returns SciPy sparse matrix representation of the Operator.
Returns:
CSR sparse matrix representation of the Operator.
Raises:
ValueError: invalid parameters.
"""
return self.primitive.to_matrix(sparse=True) * self.coeff # type: ignore
def __str__(self) -> str:
prim_str = str(self.primitive)
if self.coeff == 1.0:
return prim_str
else:
return "{} * {}".format(self.coeff, prim_str)
[docs] def eval(self,
front: Optional[Union[str, Dict[str, complex], np.ndarray, OperatorBase]] = None
) -> Union[OperatorBase, float, complex]:
if front is None:
return self.to_matrix_op()
# pylint: disable=import-outside-toplevel,cyclic-import
from ..state_fns.state_fn import StateFn
from ..state_fns.dict_state_fn import DictStateFn
from ..state_fns.circuit_state_fn import CircuitStateFn
from ..list_ops.list_op import ListOp
from .circuit_op import CircuitOp
new_front = None
# For now, always do this. If it's not performant, we can be more granular.
if not isinstance(front, OperatorBase):
front = StateFn(front, is_measurement=False)
if isinstance(front, ListOp) and front.distributive:
new_front = front.combo_fn([self.eval(front.coeff * front_elem) # type: ignore
for front_elem in front.oplist])
else:
if self.num_qubits != front.num_qubits:
raise ValueError(
'eval does not support operands with differing numbers of qubits, '
'{} and {}, respectively.'.format(
self.num_qubits, front.num_qubits))
if isinstance(front, DictStateFn):
new_dict = {} # type: Dict
corrected_x_bits = self.primitive.x[::-1] # type: ignore
corrected_z_bits = self.primitive.z[::-1] # type: ignore
for bstr, v in front.primitive.items():
bitstr = np.asarray(list(bstr)).astype(int).astype(bool)
new_b_str = np.logical_xor(bitstr, corrected_x_bits)
new_str = ''.join(map(str, 1 * new_b_str))
z_factor = np.product(1 - 2 * np.logical_and(bitstr, corrected_z_bits))
y_factor = np.product(np.sqrt(1 - 2 * np.logical_and(corrected_x_bits,
corrected_z_bits) + 0j))
new_dict[new_str] = (v * z_factor * y_factor) + new_dict.get(new_str, 0)
new_front = StateFn(new_dict, coeff=self.coeff * front.coeff)
elif isinstance(front, StateFn) and front.is_measurement:
raise ValueError('Operator composed with a measurement is undefined.')
# Composable types with PauliOp
elif isinstance(front, (PauliOp, CircuitOp, CircuitStateFn)):
new_front = self.compose(front)
# Covers VectorStateFn and OperatorStateFn
elif isinstance(front, OperatorBase):
new_front = self.to_matrix_op().eval(front.to_matrix_op()) # type: ignore
return new_front
[docs] def exp_i(self) -> OperatorBase:
""" Return a ``CircuitOp`` equivalent to e^-iH for this operator H. """
# if only one qubit is significant, we can perform the evolution
corrected_x = self.primitive.x[::-1] # type: ignore
corrected_z = self.primitive.z[::-1] # type: ignore
# pylint: disable=import-outside-toplevel,no-member
sig_qubits = np.logical_or(corrected_x, corrected_z)
if np.sum(sig_qubits) == 0:
# e^I is just a global phase, but we can keep track of it! Should we?
# For now, just return identity
return PauliOp(self.primitive)
if np.sum(sig_qubits) == 1:
sig_qubit_index = sig_qubits.tolist().index(True)
coeff = np.real(self.coeff) \
if not isinstance(self.coeff, ParameterExpression) \
else self.coeff
# Y rotation
if corrected_x[sig_qubit_index] and corrected_z[sig_qubit_index]:
rot_op = PrimitiveOp(RYGate(2 * coeff))
# Z rotation
elif corrected_z[sig_qubit_index]:
rot_op = PrimitiveOp(RZGate(2 * coeff))
# X rotation
elif corrected_x[sig_qubit_index]:
rot_op = PrimitiveOp(RXGate(2 * coeff))
# pylint: disable=cyclic-import
from ..operator_globals import I
left_pad = I.tensorpower(sig_qubit_index)
right_pad = I.tensorpower(self.num_qubits - sig_qubit_index - 1)
# Need to use overloaded operators here in case left_pad == I^0
return left_pad ^ rot_op ^ right_pad
else:
from ..evolutions.evolved_op import EvolvedOp
return EvolvedOp(self)
[docs] def commutes(self, other_op: OperatorBase) -> bool:
""" Returns whether self commutes with other_op.
Args:
other_op: An ``OperatorBase`` with which to evaluate whether self commutes.
Returns:
A bool equaling whether self commutes with other_op
"""
if not isinstance(other_op, PauliOp):
return False
# Don't use compose because parameters will break this
self_bits = self.primitive.z + 2 * self.primitive.x # type: ignore
other_bits = other_op.primitive.z + 2 * other_op.primitive.x # type: ignore
return all((self_bits * other_bits) * (self_bits - other_bits) == 0)
[docs] def to_circuit(self) -> QuantumCircuit:
# If Pauli equals identity, don't skip the IGates
is_identity = sum(self.primitive.x + self.primitive.z) == 0 # type: ignore
# Note: Reversing endianness!!
qc = QuantumCircuit(len(self.primitive))
for q, pauli_str in enumerate(reversed(self.primitive.to_label())): # type: ignore
gate = PAULI_GATE_MAPPING[pauli_str]
if not pauli_str == 'I' or is_identity:
qc.append(gate, qargs=[q])
return qc
[docs] def to_instruction(self) -> Instruction:
# TODO should we just do the following because performance of adding and deleting IGates
# doesn't matter?
# (Reduce removes extra IGates).
# return PrimitiveOp(self.primitive.to_instruction(), coeff=self.coeff).reduce()
return self.to_circuit().to_instruction()
[docs] def to_legacy_op(self, massive: bool = False) -> WeightedPauliOperator:
if isinstance(self.coeff, ParameterExpression):
try:
coeff = float(self.coeff)
except TypeError as ex:
raise TypeError('Cannot convert Operator with unbound parameter {} to Legacy '
'Operator'.format(self.coeff)) from ex
else:
coeff = cast(float, self.coeff)
return WeightedPauliOperator(paulis=[(coeff, self.primitive)]) # type: ignore