added insert_barrier to TrotterQRTE (qiskit-community#128)

Co-authored-by: Elena Peña Tapia <[email protected]>
Co-authored-by: Steve Wood <[email protected]>

qiskit_algorithms/time_evolvers/trotterization/trotter_qrte.py

@@ -1,6 +1,6 @@
 # This code is part of a Qiskit project.
 #
-# (C) Copyright IBM 2021, 2023.
+# (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
@@ -58,6 +58,8 @@ def __init__(
         product_formula: ProductFormula | None = None,
         estimator: BaseEstimator | None = None,
         num_timesteps: int = 1,
+        *,
+        insert_barriers: bool = False,
     ) -> None:
         """
         Args:
@@ -67,15 +69,18 @@ def __init__(
                 ``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``.
-            num_timesteps: The number of time-steps the full evolution time is divided into
-                (repetitions of ``product_formula``)
             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:
@@ -192,6 +197,8 @@ def evolve(self, evolution_problem: TimeEvolutionProblem) -> TimeEvolutionResult
 
         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 = []
@@ -225,6 +232,8 @@ def evolve(self, evolution_problem: TimeEvolutionProblem) -> TimeEvolutionResult
                     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(

releasenotes/notes/add-barriers-trotter-248cccabbf9deed1.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - |
+    Added input argument `insert_barriers` to `TrotterQRTE` to add barriers between Trotter layers.

test/time_evolvers/test_trotter_qrte.py

@@ -1,6 +1,6 @@
 # This code is part of a Qiskit project.
 #
-# (C) Copyright IBM 2021, 2023.
+# (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
@@ -211,6 +211,34 @@ def test_trotter_qrte_trotter_hamiltonian_errors(self, operator, initial_state):
         """Test TrotterQRTE with raising errors for evolution problem content."""
         self._run_error_test(initial_state, operator, None, None, None, None)
 
+    @data(True, False)
+    def test_barriers(self, insert_barrier):
+        """Test TrotterQRTE to insert barriers correctly."""
+        initial_state = QuantumCircuit(1)
+        initial_state.x(0)
+
+        expected_circuit = QuantumCircuit(1)
+        expected_circuit.append(initial_state, expected_circuit.qubits)
+        if insert_barrier:
+            expected_circuit.barrier()
+        expected_circuit.rx(1, 0)
+        expected_circuit.ry(1, 0)
+        if insert_barrier:
+            expected_circuit.barrier()
+        expected_circuit.rx(1, 0)
+        expected_circuit.ry(1, 0)
+        if insert_barrier:
+            expected_circuit.barrier()
+
+        operator = SparsePauliOp(["X", "Y"])
+        evolution_problem = TimeEvolutionProblem(operator, 1, initial_state)
+        trotter_qrte = TrotterQRTE(num_timesteps=2, insert_barriers=insert_barrier)
+        evolution_result = trotter_qrte.evolve(evolution_problem)
+
+        self.assertEqual(
+            expected_circuit.decompose(reps=3), evolution_result.evolved_state.decompose(reps=5)
+        )
+
     @staticmethod
     def _run_error_test(initial_state, operator, aux_ops, estimator, t_param, param_value_dict):
         time = 1