Fix the order bug in Estimator (#1936)

* Fix the order bug

* add reno

---------

Co-authored-by: Jun Doi <doichan@jp.ibm.com>

qiskit_aer/primitives/estimator.py

@@ -382,20 +382,16 @@ def _compute_with_approximation(
             self._transpile_circuits(circuits)
             experiment_manager = _ExperimentManager()
             for i, j, value in zip(circuits, observables, parameter_values):
+                self._validate_parameter_length(value, i)
                 if (i, j) in experiment_manager.keys:
-                    self._validate_parameter_length(value, i)
-                    experiment_manager.append(
-                        key=(i, j),
-                        parameter_bind=dict(zip(self._parameters[i], value)),
-                    )
+                    key_index = experiment_manager.keys.index((i, j))
+                    circuit = experiment_manager.experiment_circuits[key_index]
                 else:
-                    self._validate_parameter_length(value, i)
                     circuit = (
                         self._circuits[i].copy()
                         if self._skip_transpilation
                         else self._transpiled_circuits[i].copy()
                     )
-
                     observable = self._observables[j]
                     if shots is None:
                         circuit.save_expectation_value(observable, self._layouts[i])
@@ -404,11 +400,11 @@ def _compute_with_approximation(
                             circuit.save_expectation_value(
                                 pauli, self._layouts[i], label=str(term_ind)
                             )
-                    experiment_manager.append(
-                        key=(i, j),
-                        parameter_bind=dict(zip(self._parameters[i], value)),
-                        experiment_circuit=circuit,
-                    )
+                experiment_manager.append(
+                    key=(i, j),
+                    parameter_bind=dict(zip(self._parameters[i], value)),
+                    experiment_circuit=circuit,
+                )
 
             self._cache[key] = experiment_manager
         result = self._backend.run(
@@ -616,24 +612,22 @@ def __len__(self):
     @property
     def experiment_indices(self):
         """indices of experiments"""
-        return sum(self._input_indices, [])
+        return np.argsort(sum(self._input_indices, [])).tolist()
 
     def append(
         self,
         key: tuple[int, int],
         parameter_bind: dict[ParameterExpression, float],
-        experiment_circuit: QuantumCircuit | None = None,
+        experiment_circuit: QuantumCircuit,
     ):
         """append experiments"""
-        if experiment_circuit is not None:
-            self.experiment_circuits.append(experiment_circuit)
-
-        if key in self.keys:
+        if key in self.keys and parameter_bind:
             key_index = self.keys.index(key)
             for k, vs in self.parameter_binds[key_index].items():
                 vs.append(parameter_bind[k])
             self._input_indices[key_index].append(self._num_experiment)
         else:
+            self.experiment_circuits.append(experiment_circuit)
             self.keys.append(key)
             self.parameter_binds.append({k: [v] for k, v in parameter_bind.items()})
             self._input_indices.append([self._num_experiment])

releasenotes/notes/estimator-order-bug-a341d82075f47046.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Fixed a bug that caused results to be incorrectly ordered or errors in
+    :class:`~.Estimator` with ``approximation=True``.

test/terra/primitives/test_estimator.py

@@ -20,7 +20,7 @@
 
 import numpy as np
 from ddt import data, ddt
-from qiskit.circuit import QuantumCircuit
+from qiskit.circuit import Parameter, QuantumCircuit
 from qiskit.circuit.library import RealAmplitudes
 from qiskit.exceptions import QiskitError
 from qiskit.opflow import PauliSumOp
@@ -308,6 +308,21 @@ def test_warn_shots_none_without_approximation(self):
         np.testing.assert_allclose(result.values, [-1.313831587508902])
         self.assertIsInstance(result.metadata[0]["variance"], float)
 
+    def test_result_order(self):
+        """Test to validate the order."""
+        qc1 = QuantumCircuit(1)
+        qc1.measure_all()
+
+        param = Parameter("a")
+        qc2 = QuantumCircuit(1)
+        qc2.ry(np.pi / 2 * param, 0)
+        qc2.measure_all()
+
+        estimator = Estimator(approximation=True)
+        job = estimator.run([qc1, qc2, qc1, qc1, qc2], ["Z"] * 5, [[], [1], [], [], [1]])
+        result = job.result()
+        np.testing.assert_allclose(result.values, [1, 0, 1, 1, 0], atol=1e-10)
+
 
 if __name__ == "__main__":
     unittest.main()