Run truncation after resolving parameters (Qiskit#1428)

* run truncation after resolving parameters

Since 0.9.0, Aer reduces qubits and their instructions if they do not affect measured qubits.
On the other hand, parameterized QObj contains indices of instructions and their values.
Previously these values are set to instructions of a circuit after running truncation,
which may reduce some of instructions. Consequently, indices of parameterized QObj may address
wrong instructions in a truncated cicuit.

This fix changes the order of processing of truncation and parameterization: First, Aer
resolves paramers, and then runs truncation.

* Reword release note

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>
Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

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__':