Fixes a bug with the compilation of controlled gates (#2251)

qiskit_aer/backends/aer_compiler.py

@@ -920,17 +920,17 @@ def _assemble_op(
         if ctrl_state_pos > 0:
             # Add x gates for ctrl qubits which state=0
             ctrl_state = int(name[ctrl_state_pos+2:len(name)])
-            for i in range(len(qubits)):
+            for i in range(len(qubits)-1):
                 if (ctrl_state >> i) & 1 == 0:
-                    qubits_i = [qubits[len(qubits) - 1 - i]]
+                    qubits_i = [qubits[i]]
                     aer_circ.gate("x", qubits_i, params, [], conditional_reg, aer_cond_expr,
                                   label if label else "x")
                     num_of_aer_ops += 1
             aer_circ.gate(gate_name, qubits, params, [], conditional_reg, aer_cond_expr,
                           label if label else gate_name)
-            for i in range(len(qubits)):
+            for i in range(len(qubits)-1):
                 if (ctrl_state >> i) & 1 == 0:
-                    qubits_i = [qubits[len(qubits) - 1 - i]]
+                    qubits_i = [qubits[i]]
                     aer_circ.gate("x", qubits_i, params, [], conditional_reg, aer_cond_expr,
                                   label if label else "x")
                     num_of_aer_ops += 1

releasenotes/notes/fix_controlled_gate_compile-c94fb7c41fa961db.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixes a bug in the compilation of circuits with controlled gates that resulted
+    in adding X gates on the incorrect qubits.

test/terra/backends/aer_simulator/test_circuit.py

@@ -18,10 +18,12 @@
 import numpy as np
 from qiskit import ClassicalRegister, QuantumCircuit, QuantumRegister, assemble
 from qiskit.circuit.gate import Gate
-from qiskit.circuit.library.standard_gates import HGate
+from qiskit.circuit.library.standard_gates import HGate, XGate, ZGate
+from qiskit.quantum_info import Statevector
 from test.terra.reference import ref_algorithms
 
 from test.terra.backends.simulator_test_case import SimulatorTestCase, supported_methods
+from qiskit_aer.backends import StatevectorSimulator
 
 
 @ddt
@@ -259,3 +261,27 @@ def _define(self):
             self.fail("do not reach here")
         except Exception as e:
             self.assertTrue('"params" is incorrect length' in repr(e))
+
+    def test_controlled_gates(self):
+        """Test gates with control qubits"""
+        backend = StatevectorSimulator()
+        num_qubits = 4
+        circuit = QuantumCircuit(num_qubits)
+        cccx = XGate().control(num_ctrl_qubits=3, label=None, ctrl_state="100")
+        circuit.x(2)
+        circuit.compose(cccx, range(num_qubits), inplace=True)
+        job = backend.run(circuit)
+        state = job.result().get_statevector()
+        ref_state = Statevector(circuit)
+        self.assertEqual(state, ref_state)
+
+        num_qubits = 3
+        circuit = QuantumCircuit(num_qubits)
+        cccz = ZGate().control(num_ctrl_qubits=2, label=None, ctrl_state="10")
+        circuit.x(1)
+        circuit.x(2)
+        circuit.compose(cccz, range(num_qubits), inplace=True)
+        job = backend.run(circuit)
+        state = job.result().get_statevector()
+        ref_state = Statevector(circuit)
+        self.assertEqual(state, ref_state)