Run truncation after resolving parameters

releasenotes/notes/resolve_parameters_before_truncation-ec7074f1f0f831e2.yaml

@@ -0,0 +1,11 @@
+---
+fixes:
+  - |
+    Fixes a bug with truncation of circuits in parameterized Qobjs. 
+    Previously parameters of parameterized QObj could be wrongly resolved
+    if unused qubits of their circuits were truncated, because indices of
+    the parameters were not updated after the instructions on unmeasured qubits
+    were removed.
+
+    See `#1427 <https://github.com/Qiskit/qiskit-aer/issues/1427>`__
+    for details. 

src/framework/qobj.hpp

@@ -129,12 +129,14 @@ Qobj::Qobj(const inputdata_t &input) {
 
   // Load circuits
   for (size_t i=0; i<num_circs; i++) {
-    // Get base circuit from qobj
-    Circuit circuit(static_cast<inputdata_t>(circs[i]), config, truncation);
     if (param_table.empty() || param_table[i].empty()) {
+      // Get base circuit from qobj
+      Circuit circuit(static_cast<inputdata_t>(circs[i]), config, truncation);
       // Non parameterized circuit
       circuits.push_back(std::move(circuit));
     } else {
+      // Get base circuit from qobj without truncation
+      Circuit circuit(static_cast<inputdata_t>(circs[i]), config, false);
       // Load different parameterizations of the initial circuit
       const auto circ_params = param_table[i];
       const size_t num_params = circ_params[0].second.size();
@@ -159,6 +161,12 @@ Qobj::Qobj(const inputdata_t &input) {
           // Update the param
           op.params[param_pos] = params.second[j];
         }
+        // Run truncation.
+        // TODO: Truncation should be performed and parameters should be resolved after it.
+        // However, parameters are associated with indices of instructions, which can be changed in truncation.
+        // Therefore, current implementation performs truncation for each parameter set.
+        if (truncation)
+          param_circuit.set_params(true);
         circuits.push_back(std::move(param_circuit));
       }
     }

test/terra/backends/test_parameterized_qobj.py

@@ -30,6 +30,7 @@
     save_expval_circuit_parameterized,
     save_expval_final_statevecs,
 )
+from qiskit.providers.aer.library import SaveStatevector
 from qiskit.providers.aer import AerSimulator, AerError
 
 
@@ -295,6 +296,37 @@ def test_run_path_multiple_circuits_mismatch_length(self):
         with self.assertRaises(AerError):
             backend.run([circuit]*3, shots=shots, parameter_binds=[parameter_binds]).result()
 
+    def test_run_path_with_truncation(self):
+        """Test parameterized circuits with truncation"""
+        backend = AerSimulator(method='statevector')
+        theta = Parameter('theta')
+        circuit = QuantumCircuit(5, 2)
+        for q in range(5):
+            circuit.ry(theta, q)
+        circuit.cx(0, 1)
+        circuit.cx(1, 2)
+        for q in range(5):
+            circuit.ry(theta, q)
+        circuit.cx(0, 1)
+        circuit.cx(1, 2)
+        circuit.append(SaveStatevector(3, label='sv', pershot=False, conditional=False), range(3))
+
+        param_map = {theta: [0.1 * i for i in range(3)]}
+        param_sets = [{theta: 0.1 * i} for i in range(3)]
+
+        resolved_circuits = [circuit.bind_parameters(param_set) for param_set in param_sets]
+
+        result = backend.run(circuit, parameter_binds=[param_map]).result()
+        self.assertSuccess(result)
+
+        result_without_parameters = backend.run(resolved_circuits).result()
+        self.assertSuccess(result_without_parameters)
+
+        for actual_result in result.results:
+            metadata = actual_result.metadata
+            self.assertEqual(metadata["active_input_qubits"], [q for q in range(3)])
+        for i in range(3):
+            self.assertEqual(result.data(i)['sv'], result_without_parameters.data(i)['sv'])
 
 
 if __name__ == '__main__':