# This code is part of a Qiskit project.
#
# (C) Copyright IBM 2021, 2024.
#
# 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.
"""An algorithm to implement a Trotterization real time-evolution."""
from __future__ import annotations
from qiskit import QuantumCircuit
from qiskit.circuit.library import PauliEvolutionGate
from qiskit.circuit.parametertable import ParameterView
from qiskit.primitives import BaseEstimator
from qiskit.quantum_info import Pauli, SparsePauliOp
from qiskit.synthesis import ProductFormula, LieTrotter
from qiskit_algorithms.time_evolvers.time_evolution_problem import TimeEvolutionProblem
from qiskit_algorithms.time_evolvers.time_evolution_result import TimeEvolutionResult
from qiskit_algorithms.time_evolvers.real_time_evolver import RealTimeEvolver
from qiskit_algorithms.observables_evaluator import estimate_observables
[docs]class TrotterQRTE(RealTimeEvolver):
"""Quantum Real Time Evolution using Trotterization.
The type of Trotterization is defined by the :class:`~qiskit.synthesis.ProductFormula`
provided to the algorithm.
Examples:
.. code-block:: python
from qiskit.quantum_info import Pauli, SparsePauliOp
from qiskit import QuantumCircuit
from qiskit_algorithms import TrotterQRTE, TimeEvolutionProblem
from qiskit.primitives import Estimator
operator = SparsePauliOp([Pauli("X"), Pauli("Z")])
initial_state = QuantumCircuit(1)
time = 1
evolution_problem = TimeEvolutionProblem(operator, time, initial_state)
# LieTrotter with 1 rep
estimator = Estimator()
trotter_qrte = TrotterQRTE(estimator=estimator)
evolved_state = trotter_qrte.evolve(evolution_problem).evolved_state
"""
def __init__(
self,
product_formula: ProductFormula | None = None,
estimator: BaseEstimator | None = None,
num_timesteps: int = 1,
*,
insert_barriers: bool = False,
) -> None:
"""
Args:
product_formula: A Lie-Trotter-Suzuki product formula. If ``None`` provided (default),
the :class:`~qiskit.synthesis.LieTrotter` first order product formula with a single
repetition is used. ``reps`` should be 1 to obtain a number of time-steps equal to
``num_timesteps`` and an evaluation of :attr:`.TimeEvolutionProblem.aux_operators`
at every time-step. If ``reps`` is larger than 1, the true number of time-steps will
be ``num_timesteps * reps``.
estimator: An estimator primitive used for calculating expectation values of
``TimeEvolutionProblem.aux_operators``.
num_timesteps: The number of time-steps the full evolution time is divided into
(repetitions of ``product_formula``).
insert_barriers: If True, insert a barrier after the initial state and after each Trotter
step.
"""
self.product_formula = product_formula
self.num_timesteps = num_timesteps
self.estimator = estimator
self._insert_barriers = insert_barriers
@property
def product_formula(self) -> ProductFormula:
"""Returns a product formula."""
return self._product_formula
@product_formula.setter
def product_formula(self, product_formula: ProductFormula | None):
"""Sets a product formula. If ``None`` provided, sets the Lie-Trotter first order product
formula with a single repetition."""
if product_formula is None:
product_formula = LieTrotter(reps=1)
self._product_formula = product_formula
@property
def estimator(self) -> BaseEstimator | None:
"""
Returns an estimator.
"""
return self._estimator
@estimator.setter
def estimator(self, estimator: BaseEstimator) -> None:
"""
Sets an estimator.
"""
self._estimator = estimator
@property
def num_timesteps(self) -> int:
"""Returns the number of timesteps."""
return self._num_timesteps
@num_timesteps.setter
def num_timesteps(self, num_timesteps: int) -> None:
"""
Sets the number of time-steps.
Raises:
ValueError: If num_timesteps is not positive.
"""
if num_timesteps <= 0:
raise ValueError(
f"Number of time steps must be positive integer, {num_timesteps} provided"
)
self._num_timesteps = num_timesteps
[docs] @classmethod
def supports_aux_operators(cls) -> bool:
"""
Whether computing the expectation value of auxiliary operators is supported.
Returns:
``True`` if ``aux_operators`` expectations in the ``TimeEvolutionProblem`` can be
evaluated, ``False`` otherwise.
"""
return True
[docs] def evolve(self, evolution_problem: TimeEvolutionProblem) -> TimeEvolutionResult:
"""
Evolves a quantum state for a given time using the Trotterization method
based on a product formula provided. The result is provided in the form of a quantum
circuit. If auxiliary operators are included in the ``evolution_problem``, they are
evaluated on the ``init_state`` and on the evolved state at every step (``num_timesteps``
times) using an estimator primitive provided.
Args:
evolution_problem: Instance defining evolution problem. For the included Hamiltonian,
``Pauli`` or ``SparsePauliOp`` are supported by TrotterQRTE.
Returns:
Evolution result that includes an evolved state as a quantum circuit and, optionally,
auxiliary operators evaluated for a resulting state on an estimator primitive.
Raises:
ValueError: If ``t_param`` is not set to ``None`` in the ``TimeEvolutionProblem``
(feature not currently supported).
ValueError: If ``aux_operators`` provided in the time evolution problem but no estimator
provided to the algorithm.
ValueError: If the ``initial_state`` is not provided in the ``TimeEvolutionProblem``.
ValueError: If an unsupported Hamiltonian type is provided.
"""
if evolution_problem.aux_operators is not None and self.estimator is None:
raise ValueError(
"The time evolution problem contained ``aux_operators`` but no estimator was "
"provided. The algorithm continues without calculating these quantities. "
)
# ensure the hamiltonian is a sparse pauli op
hamiltonian = evolution_problem.hamiltonian
if not isinstance(hamiltonian, (Pauli, SparsePauliOp)):
raise ValueError(
f"TrotterQRTE only accepts Pauli | SparsePauliOp, {type(hamiltonian)} " "provided."
)
if isinstance(hamiltonian, Pauli):
hamiltonian = SparsePauliOp(hamiltonian)
t_param = evolution_problem.t_param
free_parameters = hamiltonian.parameters
if t_param is not None and free_parameters != ParameterView([t_param]):
raise ValueError(
f"Hamiltonian time parameters ({free_parameters}) do not match "
f"evolution_problem.t_param ({t_param})."
)
# make sure PauliEvolutionGate does not implement more than one Trotter step
dt = evolution_problem.time / self.num_timesteps # pylint: disable=invalid-name
if evolution_problem.initial_state is not None:
initial_state = evolution_problem.initial_state
else:
raise ValueError("``initial_state`` must be provided in the ``TimeEvolutionProblem``.")
evolved_state = QuantumCircuit(initial_state.num_qubits)
evolved_state.append(initial_state, evolved_state.qubits)
if self._insert_barriers:
evolved_state.barrier()
if evolution_problem.aux_operators is not None:
observables = []
observables.append(
estimate_observables(
self.estimator,
evolved_state,
evolution_problem.aux_operators,
None,
evolution_problem.truncation_threshold,
)
)
else:
observables = None
if t_param is None:
# the evolution gate
single_step_evolution_gate = PauliEvolutionGate(
hamiltonian, dt, synthesis=self.product_formula
)
for n in range(self.num_timesteps):
# if hamiltonian is time-dependent, bind new time-value at every step to construct
# evolution for next step
if t_param is not None:
time_value = (n + 1) * dt
bound_hamiltonian = hamiltonian.assign_parameters([time_value])
single_step_evolution_gate = PauliEvolutionGate(
bound_hamiltonian,
dt,
synthesis=self.product_formula,
)
evolved_state.append(single_step_evolution_gate, evolved_state.qubits)
if self._insert_barriers:
evolved_state.barrier()
if evolution_problem.aux_operators is not None:
observables.append(
estimate_observables(
self.estimator,
evolved_state,
evolution_problem.aux_operators,
None,
evolution_problem.truncation_threshold,
)
)
evaluated_aux_ops = None
if evolution_problem.aux_operators is not None:
evaluated_aux_ops = observables[-1]
return TimeEvolutionResult(
evolved_state, evaluated_aux_ops, observables # type: ignore[arg-type]
)