Fix lack of alternative for Optimizer in AQC (#11099) (#11148)

* Add alternative and deprecation warning

* Add deprecation test

* Apply Julien's suggestion

Co-authored-by: Julien Gacon <gaconju@gmail.com>

* Apply suggestions from Julien's code review

* Move optimizer setting to init

* Fix ddt import

* Fix lint

* Add reno

---------

Co-authored-by: Julien Gacon <gaconju@gmail.com>
(cherry picked from commit f0d3937)

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

qiskit/transpiler/synthesis/aqc/aqc.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2022.
+# (C) Copyright IBM 2022, 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
@@ -10,30 +10,84 @@
 # copyright notice, and modified files need to carry a notice indicating
 # that they have been altered from the originals.
 """A generic implementation of Approximate Quantum Compiler."""
-from typing import Optional
+from __future__ import annotations
+
+from functools import partial
+
+from collections.abc import Callable
+from typing import Protocol
 
 import numpy as np
+from scipy.optimize import OptimizeResult, minimize
 
-from qiskit.algorithms.optimizers import L_BFGS_B, Optimizer
+from qiskit.algorithms.optimizers import Optimizer
 from qiskit.quantum_info import Operator
+from qiskit.utils.deprecation import deprecate_arg
+
 from .approximate import ApproximateCircuit, ApproximatingObjective
 
 
+class Minimizer(Protocol):
+    """Callable Protocol for minimizer.
+
+    This interface is based on `SciPy's optimize module
+    <https://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.minimize.html>`__.
+
+     This protocol defines a callable taking the following parameters:
+
+         fun
+             The objective function to minimize.
+         x0
+             The initial point for the optimization.
+         jac
+             The gradient of the objective function.
+         bounds
+             Parameters bounds for the optimization. Note that these might not be supported
+             by all optimizers.
+
+     and which returns a SciPy minimization result object.
+    """
+
+    def __call__(
+        self,
+        fun: Callable[[np.ndarray], float],
+        x0: np.ndarray,  # pylint: disable=invalid-name
+        jac: Callable[[np.ndarray], np.ndarray] | None = None,
+        bounds: list[tuple[float, float]] | None = None,
+    ) -> OptimizeResult:
+        """Minimize the objective function.
+
+        This interface is based on `SciPy's optimize module <https://docs.scipy.org/doc
+        /scipy/reference/generated/scipy.optimize.minimize.html>`__.
+
+        Args:
+            fun: The objective function to minimize.
+            x0: The initial point for the optimization.
+            jac: The gradient of the objective function.
+            bounds: Parameters bounds for the optimization. Note that these might not be supported
+                by all optimizers.
+
+        Returns:
+             The SciPy minimization result object.
+        """
+        ...  # pylint: disable=unnecessary-ellipsis
+
+
 class AQC:
     """
-    A generic implementation of Approximate Quantum Compiler. This implementation is agnostic of
+    A generic implementation of the Approximate Quantum Compiler. This implementation is agnostic of
     the underlying implementation of the approximate circuit, objective, and optimizer. Users may
     pass corresponding implementations of the abstract classes:
 
-    * Optimizer is an instance of :class:`~qiskit.algorithms.optimizers.Optimizer` and used to run
-      the optimization process. A choice of optimizer may affect overall convergence, required time
+    * The *optimizer* is an implementation of the :class:`~.Minimizer` protocol, a callable used to run
+      the optimization process. The choice of optimizer may affect overall convergence, required time
       for the optimization process and achieved objective value.
 
-    * Approximate circuit represents a template which parameters we want to optimize.  Currently,
+    * The *approximate circuit* represents a template which parameters we want to optimize.  Currently,
       there's only one implementation based on 4-rotations CNOT unit blocks:
       :class:`.CNOTUnitCircuit`. See the paper for more details.
 
-    * Approximate objective is tightly coupled with the approximate circuit implementation and
+    * The *approximate objective* is tightly coupled with the approximate circuit implementation and
       provides two methods for computing objective function and gradient with respect to approximate
       circuit parameters. This objective is passed to the optimizer. Currently, there are two
       implementations based on 4-rotations CNOT unit blocks: :class:`.DefaultCNOTUnitObjective` and
@@ -48,28 +102,44 @@ class AQC:
       also allocates a number of temporary memory buffers comparable in size to the target matrix.
     """
 
+    @deprecate_arg(
+        "optimizer",
+        deprecation_description=(
+            "Setting the `optimizer` argument to an instance "
+            "of `qiskit.algorithms.optimizers.Optimizer` "
+        ),
+        additional_msg=("Please, submit a callable that follows the `Minimizer` protocol instead."),
+        predicate=lambda optimizer: isinstance(optimizer, Optimizer),
+        since="0.45.0",
+    )
     def __init__(
         self,
-        optimizer: Optional[Optimizer] = None,
-        seed: Optional[int] = None,
+        optimizer: Minimizer | Optimizer | None = None,
+        seed: int | None = None,
     ):
         """
         Args:
             optimizer: an optimizer to be used in the optimization procedure of the search for
-                the best approximate circuit. By default, :obj:`.L_BFGS_B` is used with max
-                iterations set to 1000.
-            seed: a seed value to be user by a random number generator.
+                the best approximate circuit. By default, the scipy minimizer with the
+                ``L-BFGS-B`` method is used with max iterations set to 1000.
+            seed: a seed value to be used by a random number generator.
         """
         super().__init__()
-        self._optimizer = optimizer
+        self._optimizer = optimizer or partial(
+            minimize, args=(), method="L-BFGS-B", options={"maxiter": 1000}
+        )
+        # temporary fix -> remove after deprecation period of Optimizer
+        if isinstance(self._optimizer, Optimizer):
+            self._optimizer = self._optimizer.minimize
+
         self._seed = seed
 
     def compile_unitary(
         self,
         target_matrix: np.ndarray,
         approximate_circuit: ApproximateCircuit,
         approximating_objective: ApproximatingObjective,
-        initial_point: Optional[np.ndarray] = None,
+        initial_point: np.ndarray | None = None,
     ) -> None:
         """
         Approximately compiles a circuit represented as a unitary matrix by solving an optimization
@@ -96,13 +166,11 @@ def compile_unitary(
         # set the matrix to approximate in the algorithm
         approximating_objective.target_matrix = su_matrix
 
-        optimizer = self._optimizer or L_BFGS_B(maxiter=1000)
-
         if initial_point is None:
             np.random.seed(self._seed)
             initial_point = np.random.uniform(0, 2 * np.pi, approximating_objective.num_thetas)
 
-        opt_result = optimizer.minimize(
+        opt_result = self._optimizer(
             fun=approximating_objective.objective,
             x0=initial_point,
             jac=approximating_objective.gradient,

qiskit/transpiler/synthesis/aqc/aqc_plugin.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2021.
+# (C) Copyright IBM 2021, 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
@@ -12,6 +12,7 @@
 """
 An AQC synthesis plugin to Qiskit's transpiler.
 """
+from functools import partial
 import numpy as np
 
 from qiskit.converters import circuit_to_dag
@@ -104,7 +105,7 @@ def run(self, unitary, **options):
 
         # Runtime imports to avoid the overhead of these imports for
         # plugin discovery and only use them if the plugin is run/used
-        from qiskit.algorithms.optimizers import L_BFGS_B
+        from scipy.optimize import minimize
         from qiskit.transpiler.synthesis.aqc.aqc import AQC
         from qiskit.transpiler.synthesis.aqc.cnot_structures import make_cnot_network
         from qiskit.transpiler.synthesis.aqc.cnot_unit_circuit import CNOTUnitCircuit
@@ -125,7 +126,8 @@ def run(self, unitary, **options):
             depth=depth,
         )
 
-        optimizer = config.get("optimizer", L_BFGS_B(maxiter=1000))
+        default_optimizer = partial(minimize, args=(), method="L-BFGS-B", options={"maxiter": 1000})
+        optimizer = config.get("optimizer", default_optimizer)
         seed = config.get("seed")
         aqc = AQC(optimizer, seed)
 

releasenotes/notes/fix-aqc-optimizer-typehint-34b54c6278d23f79.yaml

@@ -0,0 +1,21 @@
+---
+fixes:
+  - |
+    The use of the (deprecated) ``Optimizer`` class on :class:`~.AQC` did not have a
+    non-deprecated alternative path, which should have been introduced in
+    the original ``qiskit-algorithms`` deprecation PR
+    [#10406](https://github.com/Qiskit/qiskit/pull/10406).
+    It now accepts a callable that implements the :class:`~.Minimizer` protocol,
+    as explicitly stated in the deprecation warning. The callable can look like the
+    following example:
+
+      .. code-block:: python
+
+          from scipy.optimize import minimize
+          from qiskit.transpiler.synthesis.aqc.aqc import AQC
+
+          optimizer = partial(minimize, args=(), method="L-BFGS-B", options={"maxiter": 200})
+          aqc = AQC(optimizer=optimizer)
+
+
+

test/python/transpiler/aqc/test_aqc.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2022.
+# (C) Copyright IBM 2022, 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
@@ -12,9 +12,15 @@
 """
 Tests AQC framework using hardcoded and randomly generated circuits.
 """
+from functools import partial
+
 import unittest
 from test.python.transpiler.aqc.sample_data import ORIGINAL_CIRCUIT, INITIAL_THETAS
+
+from ddt import ddt, data
 import numpy as np
+from scipy.optimize import minimize
+
 from qiskit.algorithms.optimizers import L_BFGS_B
 from qiskit.quantum_info import Operator
 from qiskit.test import QiskitTestCase
@@ -25,10 +31,12 @@
 from qiskit.transpiler.synthesis.aqc.fast_gradient.fast_gradient import FastCNOTUnitObjective
 
 
+@ddt
 class TestAqc(QiskitTestCase):
     """Main tests of approximate quantum compiler."""
 
-    def test_aqc(self):
+    @data(True, False)
+    def test_aqc(self, uses_default):
         """Tests AQC on a hardcoded circuit/matrix."""
 
         seed = 12345
@@ -38,9 +46,11 @@ def test_aqc(self):
             num_qubits=num_qubits, network_layout="spin", connectivity_type="full", depth=0
         )
 
-        optimizer = L_BFGS_B(maxiter=200)
-
-        aqc = AQC(optimizer=optimizer, seed=seed)
+        if uses_default:
+            aqc = AQC(seed=seed)
+        else:
+            optimizer = partial(minimize, args=(), method="L-BFGS-B", options={"maxiter": 200})
+            aqc = AQC(optimizer=optimizer, seed=seed)
 
         target_matrix = ORIGINAL_CIRCUIT
         approximate_circuit = CNOTUnitCircuit(num_qubits, cnots)
@@ -55,7 +65,16 @@ def test_aqc(self):
 
         approx_matrix = Operator(approximate_circuit).data
         error = 0.5 * (np.linalg.norm(approx_matrix - ORIGINAL_CIRCUIT, "fro") ** 2)
-        self.assertTrue(error < 1e-3)
+        self.assertLess(error, 1e-3)
+
+    def test_aqc_deprecation(self):
+        """Tests that AQC raises deprecation warning."""
+
+        seed = 12345
+        optimizer = L_BFGS_B(maxiter=200)
+
+        with self.assertRaises(DeprecationWarning):
+            _ = AQC(optimizer=optimizer, seed=seed)
 
     def test_aqc_fastgrad(self):
         """
@@ -70,7 +89,7 @@ def test_aqc_fastgrad(self):
             num_qubits=num_qubits, network_layout="spin", connectivity_type="full", depth=0
         )
 
-        optimizer = L_BFGS_B(maxiter=200)
+        optimizer = partial(minimize, args=(), method="L-BFGS-B", options={"maxiter": 200})
         aqc = AQC(optimizer=optimizer, seed=seed)
 
         # Make multi-control CNOT gate matrix.
@@ -103,7 +122,7 @@ def test_aqc_determinant_minus_one(self):
             num_qubits=num_qubits, network_layout="spin", connectivity_type="full", depth=0
         )
 
-        optimizer = L_BFGS_B(maxiter=200)
+        optimizer = partial(minimize, args=(), method="L-BFGS-B", options={"maxiter": 200})
         aqc = AQC(optimizer=optimizer, seed=seed)
 
         target_matrix = np.eye(2**num_qubits, dtype=int)

test/python/transpiler/aqc/test_aqc_plugin.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2021.
+# (C) Copyright IBM 2021, 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
@@ -12,11 +12,12 @@
 """
 Tests AQC plugin.
 """
+from functools import partial
 
 import numpy as np
+from scipy.optimize import minimize
 
 from qiskit import QuantumCircuit
-from qiskit.algorithms.optimizers import SLSQP
 from qiskit.converters import dag_to_circuit, circuit_to_dag
 from qiskit.quantum_info import Operator
 from qiskit.test import QiskitTestCase
@@ -68,12 +69,13 @@ def test_plugin_setup(self):
 
     def test_plugin_configuration(self):
         """Tests plugin with a custom configuration."""
+        optimizer = partial(minimize, args=(), method="SLSQP")
         config = {
             "network_layout": "sequ",
             "connectivity_type": "full",
             "depth": 0,
             "seed": 12345,
-            "optimizer": SLSQP(),
+            "optimizer": optimizer,
         }
         transpiler_pass = UnitarySynthesis(
             basis_gates=["rx", "ry", "rz", "cx"], method="aqc", plugin_config=config