qiskit/opflow/operator_globals.py

# This code is part of Qiskit.
#
# (C) Copyright IBM 2020, 2023.
#
# 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.

"""
Operator Globals
"""

from qiskit.quantum_info import Pauli
from qiskit.circuit.library import CXGate, SGate, TGate, HGate, SwapGate, CZGate

from qiskit.opflow.primitive_ops.pauli_op import PauliOp
from qiskit.opflow.primitive_ops.circuit_op import CircuitOp
from qiskit.opflow.state_fns.dict_state_fn import DictStateFn
from qiskit.utils.deprecation import deprecate_func

# Digits of precision when returning values from eval functions. Without rounding, 1e-17 or 1e-32
# values often show up in place of 0, etc.
# Note: care needs to be taken in rounding otherwise some behavior may not be as expected. E.g
# evolution is used in QAOA variational form and difference when optimizing may be small - round
# the outcome too much and a small difference may become none and the optimizer gets stuck where
# otherwise it would not.
EVAL_SIG_DIGITS = 18

# Immutable convenience objects


@deprecate_func(
    since="0.24.0",
    additional_msg="For code migration guidelines, visit https://qisk.it/opflow_migration.",
)
def make_immutable(obj):
    r"""Deprecate\: Delete the __setattr__ property to make the object mostly immutable."""

    # TODO figure out how to get correct error message
    # def throw_immutability_exception(self, *args):
    #     raise OpflowError('Operator convenience globals are immutable.')

    obj.__setattr__ = None
    return obj


# 1-Qubit Paulis
X = make_immutable(PauliOp(Pauli("X")))
Y = make_immutable(PauliOp(Pauli("Y")))
Z = make_immutable(PauliOp(Pauli("Z")))
I = make_immutable(PauliOp(Pauli("I")))

# Clifford+T, and some other common non-parameterized gates
CX = make_immutable(CircuitOp(CXGate()))
S = make_immutable(CircuitOp(SGate()))
H = make_immutable(CircuitOp(HGate()))
T = make_immutable(CircuitOp(TGate()))
Swap = make_immutable(CircuitOp(SwapGate()))
CZ = make_immutable(CircuitOp(CZGate()))

# 1-Qubit states
Zero = make_immutable(DictStateFn("0"))
One = make_immutable(DictStateFn("1"))
Plus = make_immutable(H.compose(Zero))
Minus = make_immutable(H.compose(X).compose(Zero))