Fixed a bug in pauli truncation and add relevant test

src/framework/circuit.hpp

@@ -683,14 +683,19 @@ void Circuit::set_params(bool truncation) {
 void Circuit::remap_qubits(Op &op) const {
   // truncate save_expval
   if (op.type == OpType::save_expval || op.type == OpType::save_expval_var) {
+    // map each qubit to its location in the pauli strings
+    std::unordered_map<uint_t, uint_t> ops_pauli_qubit_map;
+    for (size_t i = 0; i < op.qubits.size(); ++i) {
+      ops_pauli_qubit_map[op.qubits[i]] = op.qubits.size() - 1 - i;
+    }
     int_t nparams = op.expval_params.size();
     for (int_t i = 0; i < nparams; i++) {
       std::string &pauli = std::get<0>(op.expval_params[i]);
       std::string new_pauli;
       new_pauli.resize(qubitmap_.size());
       for (auto q = qubitmap_.cbegin(); q != qubitmap_.cend(); q++) {
         new_pauli[qubitmap_.size() - 1 - q->second] =
-            pauli[pauli.size() - 1 - q->first];
+            pauli[ops_pauli_qubit_map[q->first]];
       }
       pauli = new_pauli;
     }

test/terra/backends/aer_simulator/test_save_expval.py

@@ -271,3 +271,21 @@ def test_save_expval_var_stabilizer_pauli_cache_blocking(self, method, device, p
             blocking_qubits=2,
             max_parallel_threads=1,
         )
+
+    @supported_methods(
+        [
+            "automatic",
+            "stabilizer",
+            "statevector",
+            "density_matrix",
+            "matrix_product_state",
+            "tensor_network",
+        ],
+    )
+    def test_save_expval_truncation(self, method, device):
+        """Test save expval when qubits are not in use and are truncated"""
+        circ = QuantumCircuit(3)
+        circ.x(1)
+        oper = qi.Pauli("ZZ")
+        qubits = [1, 2]  # qubit 0 will be truncated
+        self._test_save_expval(circ, oper, qubits, False, method=method, device=device)