Correct a type of variance from complex to real (Qiskit#1767)

Previously Aer Estimator wrongly returned a complex value as variance of estimation values. This commit fixes this behavior to return a real value.
hhorii · Mar 29, 2023 · 1b866c6 · 1b866c6
1 parent bdd49de
commit 1b866c6
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Showing 3 changed files with 8 additions and 2 deletions.
diff --git a/qiskit_aer/primitives/estimator.py b/qiskit_aer/primitives/estimator.py
@@ -402,7 +402,7 @@ def _compute_with_approximation(
                 expectation_values.append(rng.normal(combined_expval, standard_error))
                 metadata.append(
                     {
-                        "variance": combined_var,
+                        "variance": np.real_if_close(combined_var).item(),
                         "shots": shots,
                         "simulator_metadata": result.results[i].metadata,
                     }
@@ -495,7 +495,7 @@ def run(self, results: list[ExperimentResult]) -> tuple[float, dict]:
             combined_var += np.dot(variances, coeffs**2)
         metadata = {
             "shots": shots,
-            "variance": combined_var,
+            "variance": np.real_if_close(combined_var).item(),
             "simulator_metadata": simulator_metadata,
         }
         return combined_expval, metadata

diff --git a/releasenotes/notes/estimator-variance-type-2b04ff7bcd305920.yaml b/releasenotes/notes/estimator-variance-type-2b04ff7bcd305920.yaml
@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fixed a bug where the variance in metadata in EstimatorResult was complex and now returns float.
diff --git a/test/terra/primitives/test_estimator.py b/test/terra/primitives/test_estimator.py
@@ -259,6 +259,7 @@ def test_with_shots_option_with_approximation(self, abelian_grouping):
         ).result()
         self.assertIsInstance(result, EstimatorResult)
         np.testing.assert_allclose(result.values, [-1.3088991960117797])
+        self.assertIsInstance(result.metadata[0]["variance"], float)
 
     def test_with_shots_option_without_approximation(self):
         """test with shots option."""
@@ -268,6 +269,7 @@ def test_with_shots_option_without_approximation(self):
         ).result()
         self.assertIsInstance(result, EstimatorResult)
         np.testing.assert_allclose(result.values, [-1.2895828299114598])
+        self.assertIsInstance(result.metadata[0]["variance"], float)
 
 
 if __name__ == "__main__":