# This code is part of Qiskit.
#
# (C) Copyright IBM 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.
"""The evolved operator ansatz."""
from __future__ import annotations
from collections.abc import Sequence
import numpy as np
from qiskit.circuit.parameter import Parameter
from qiskit.circuit.quantumregister import QuantumRegister
from qiskit.circuit.quantumcircuit import QuantumCircuit
from qiskit.exceptions import QiskitError
from qiskit.quantum_info import Operator, Pauli, SparsePauliOp
from qiskit.synthesis.evolution import LieTrotter
from .pauli_evolution import PauliEvolutionGate
from .n_local.n_local import NLocal
[docs]class EvolvedOperatorAnsatz(NLocal):
"""The evolved operator ansatz."""
def __init__(
self,
operators=None,
reps: int = 1,
evolution=None,
insert_barriers: bool = False,
name: str = "EvolvedOps",
parameter_prefix: str | Sequence[str] = "t",
initial_state: QuantumCircuit | None = None,
flatten: bool | None = None,
):
"""
Args:
operators (BaseOperator | OperatorBase | QuantumCircuit | list | None): The operators
to evolve. If a circuit is passed, we assume it implements an already evolved
operator and thus the circuit is not evolved again. Can be a single operator
(circuit) or a list of operators (and circuits).
reps: The number of times to repeat the evolved operators.
evolution (EvolutionBase | EvolutionSynthesis | None):
A specification of which evolution synthesis to use for the
:class:`.PauliEvolutionGate`, if the operator is from :mod:`qiskit.quantum_info`
or an opflow converter object if the operator is from :mod:`qiskit.opflow`.
Defaults to first order Trotterization.
insert_barriers: Whether to insert barriers in between each evolution.
name: The name of the circuit.
parameter_prefix: Set the names of the circuit parameters. If a string, the same prefix
will be used for each parameters. Can also be a list to specify a prefix per
operator.
initial_state: A :class:`.QuantumCircuit` object to prepend to the circuit.
flatten: Set this to ``True`` to output a flat circuit instead of nesting it inside multiple
layers of gate objects. By default currently the contents of
the output circuit will be wrapped in nested objects for
cleaner visualization. However, if you're using this circuit
for anything besides visualization its **strongly** recommended
to set this flag to ``True`` to avoid a large performance
overhead for parameter binding.
"""
super().__init__(
initial_state=initial_state,
parameter_prefix=parameter_prefix,
reps=reps,
insert_barriers=insert_barriers,
name=name,
flatten=flatten,
)
self._operators = None
if operators is not None:
self.operators = operators
self._evolution = evolution
# a list of which operators are parameterized, used for internal settings
self._ops_are_parameterized = None
def _check_configuration(self, raise_on_failure: bool = True) -> bool:
"""Check if the current configuration is valid."""
if not super()._check_configuration(raise_on_failure):
return False
if self.operators is None:
if raise_on_failure:
raise ValueError("The operators are not set.")
return False
return True
@property
def num_qubits(self) -> int:
if self.operators is None:
return 0
if isinstance(self.operators, list) and len(self.operators) > 0:
return self.operators[0].num_qubits
return self.operators.num_qubits
@property
def evolution(self):
"""The evolution converter used to compute the evolution.
Returns:
EvolutionBase or EvolutionSynthesis: The evolution converter used to compute the evolution.
"""
if self._evolution is None:
# pylint: disable=cyclic-import
from qiskit.opflow import PauliTrotterEvolution
return PauliTrotterEvolution()
return self._evolution
@evolution.setter
def evolution(self, evol) -> None:
"""Sets the evolution converter used to compute the evolution.
Args:
evol (EvolutionBase | EvolutionSynthesis): An evolution synthesis object or
opflow converter object to construct the evolution.
"""
self._invalidate()
self._evolution = evol
@property
def operators(self):
"""The operators that are evolved in this circuit.
Returns:
list: The operators to be evolved (and circuits) contained in this ansatz.
"""
return self._operators
@operators.setter
def operators(self, operators=None) -> None:
"""Set the operators to be evolved.
operators (Optional[Union[OperatorBase, QuantumCircuit, list]): The operators to evolve.
If a circuit is passed, we assume it implements an already evolved operator and thus
the circuit is not evolved again. Can be a single operator (circuit) or a list of
operators (and circuits).
"""
operators = _validate_operators(operators)
self._invalidate()
self._operators = operators
self.qregs = [QuantumRegister(self.num_qubits, name="q")]
# TODO: the `preferred_init_points`-implementation can (and should!) be improved!
@property
def preferred_init_points(self):
"""Getter of preferred initial points based on the given initial state."""
if self._initial_state is None:
return None
else:
# If an initial state was set by the user, then we want to make sure that the VQE does
# not start from a random point. Thus, we return an all-zero initial point for the
# optimizer which is used (unless it gets overwritten by a higher-priority setting at
# runtime of the VQE).
# However, in order to determine the correct length, we must build the QuantumCircuit
# first, because otherwise the operators may not be set yet.
self._build()
return np.zeros(self.reps * len(self.operators), dtype=float)
def _evolve_operator(self, operator, time):
from qiskit.opflow import OperatorBase, EvolutionBase
# pylint: disable=cyclic-import
from qiskit.circuit.library.hamiltonian_gate import HamiltonianGate
if isinstance(operator, OperatorBase):
if not isinstance(self.evolution, EvolutionBase):
raise QiskitError(
"If qiskit.opflow operators are evolved the evolution must be a "
f"qiskit.opflow.EvolutionBase, not a {type(self.evolution)}."
)
evolved = self.evolution.convert((time * operator).exp_i())
return evolved.reduce().to_circuit()
# if the operator is specified as matrix use exact matrix exponentiation
if isinstance(operator, Operator):
gate = HamiltonianGate(operator, time)
# otherwise, use the PauliEvolutionGate
else:
evolution = LieTrotter() if self._evolution is None else self._evolution
gate = PauliEvolutionGate(operator, time, synthesis=evolution)
evolved = QuantumCircuit(operator.num_qubits)
if not self.flatten:
evolved.append(gate, evolved.qubits)
else:
evolved.compose(gate.definition, evolved.qubits, inplace=True)
return evolved
def _build(self):
if self._is_built:
return
# need to check configuration here to ensure the operators are not None
self._check_configuration()
coeff = Parameter("c")
circuits = []
for op in self.operators:
# if the operator is already the evolved circuit just append it
if isinstance(op, QuantumCircuit):
circuits.append(op)
else:
# check if the operator is just the identity, if yes, skip it
if _is_pauli_identity(op):
continue
evolved = self._evolve_operator(op, coeff)
circuits.append(evolved)
self.rotation_blocks = []
self.entanglement_blocks = circuits
super()._build()
def _validate_operators(operators):
if not isinstance(operators, list):
operators = [operators]
if len(operators) > 1:
num_qubits = operators[0].num_qubits
if any(operators[i].num_qubits != num_qubits for i in range(1, len(operators))):
raise ValueError("All operators must act on the same number of qubits.")
return operators
def _validate_prefix(parameter_prefix, operators):
if isinstance(parameter_prefix, str):
return len(operators) * [parameter_prefix]
if len(parameter_prefix) != len(operators):
raise ValueError("The number of parameter prefixes must match the operators.")
return parameter_prefix
def _is_pauli_identity(operator):
from qiskit.opflow import PauliOp, PauliSumOp
if isinstance(operator, PauliSumOp):
operator = operator.to_pauli_op()
if isinstance(operator, SparsePauliOp):
if len(operator.paulis) == 1:
operator = operator.paulis[0] # check if the single Pauli is identity below
else:
return False
if isinstance(operator, PauliOp):
operator = operator.primitive
if isinstance(operator, Pauli):
return not np.any(np.logical_or(operator.x, operator.z))
return False