use faster aqc objective function evaluation by default (#11479)

* use faster aqc objective function evaluation by default

* release note

* Update qiskit/transpiler/synthesis/aqc/aqc_plugin.py

Co-authored-by: Jake Lishman <jake@binhbar.com>

---------

Co-authored-by: Jake Lishman <jake@binhbar.com>

Author: ajavadia

qiskit/transpiler/synthesis/aqc/aqc_plugin.py

@@ -109,7 +109,9 @@ def run(self, unitary, **options):
         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
-        from qiskit.transpiler.synthesis.aqc.cnot_unit_objective import DefaultCNOTUnitObjective
+        from qiskit.transpiler.synthesis.aqc.fast_gradient.fast_gradient import (
+            FastCNOTUnitObjective,
+        )
 
         num_qubits = int(round(np.log2(unitary.shape[0])))
 
@@ -132,7 +134,7 @@ def run(self, unitary, **options):
         aqc = AQC(optimizer, seed)
 
         approximate_circuit = CNOTUnitCircuit(num_qubits=num_qubits, cnots=cnots)
-        approximating_objective = DefaultCNOTUnitObjective(num_qubits=num_qubits, cnots=cnots)
+        approximating_objective = FastCNOTUnitObjective(num_qubits=num_qubits, cnots=cnots)
 
         initial_point = config.get("initial_point")
         aqc.compile_unitary(

releasenotes/notes/aqc-faster-default-8d47c88fefd1b6f6.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    The AQC unitary synthesis plugin method now uses a faster objective function
+    evaluation by default, which results in substantial improvement in synthesis time.