Fix NumPy(Minimum)Eigensolver for sparse matrices (#9575)

* add reno * fix sparse calculation
Qiskit · Feb 13, 2023 · 1fb00e6 · 1fb00e6
1 parent 41e0c3a
commit 1fb00e6
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Showing 2 changed files with 10 additions and 4 deletions.
diff --git a/qiskit/algorithms/eigensolvers/numpy_eigensolver.py b/qiskit/algorithms/eigensolvers/numpy_eigensolver.py
@@ -120,10 +120,10 @@ def _solve(self, operator: BaseOperator | PauliSumOp) -> tuple[np.ndarray, np.nd
                     "Trying dense computation",
                     type(operator),
                 )
-            try:
-                op_matrix = operator.to_matrix()
-            except AttributeError as ex:
-                raise AlgorithmError(f"Unsupported operator type `{type(operator)}`.") from ex
+                try:
+                    op_matrix = operator.to_matrix()
+                except AttributeError as ex:
+                    raise AlgorithmError(f"Unsupported operator type `{type(operator)}`.") from ex
 
         if isinstance(op_matrix, scisparse.csr_matrix):
             # If matrix is diagonal, the elements on the diagonal are the eigenvalues. Solve by sorting.

diff --git a/releasenotes/notes/fix-numpy-eigensolver-sparse-0e255d7b13b5e43b.yaml b/releasenotes/notes/fix-numpy-eigensolver-sparse-0e255d7b13b5e43b.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed a bug in :class:`~.minimum_eigensolvers.NumPyMinimumEigensolver` and 
+    :class:`~.eigensolvers.NumPyEigensolver` where operators that support conversion
+    to sparse matrices, such as :class:`.SparsePauliOp`, were converted to dense matrices anyways.