Fix dtype bug in BogoliubovTransform (qiskit-community#1014)

* make givens_matrix always return complex

* slater: cast to complex

* fix typo in release note

* fix docstring and comments

* undo givens_matrix change; instead, fix bug directly

* revert untouched file

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit_nature/second_q/circuit/library/bogoliubov_transform.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2022.
+# (C) Copyright IBM 2022, 2023.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -244,7 +244,7 @@ def _bogoliubov_transform_num_conserving_jw(  # pylint: disable=invalid-name
 
     # convert left rotations to right rotations
     for givens_mat, (i, j) in reversed(left_rotations):
-        givens_mat = givens_mat.T.conj()
+        givens_mat = givens_mat.T.conj().astype(complex, copy=False)
         givens_mat[:, 0] *= current_matrix[i, i]
         givens_mat[:, 1] *= current_matrix[j, j]
         new_givens_mat = givens_matrix(givens_mat[1, 1], givens_mat[1, 0])

qiskit_nature/utils/linalg.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2022.
+# (C) Copyright IBM 2022, 2023.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -129,9 +129,9 @@ def fermionic_gaussian_decomposition_jw(  # pylint: disable=invalid-name
     # compute left_unitary
     left_unitary = np.eye(n, dtype=complex)
     for j in range(n - 1):
-        # Zero out entries in column k
+        # Zero out entries in column j
         for i in range(n - 1 - j):
-            # Zero out entry in row l if needed
+            # Zero out entry in row i if needed
             if not np.isclose(current_matrix[i, j], 0.0):
                 givens_mat = givens_matrix(current_matrix[i + 1, j], current_matrix[i, j])
                 current_matrix = apply_matrix_to_slices(current_matrix, givens_mat, [i + 1, i])

releasenotes/notes/givens-fix-a12cb547cf5a1be4.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixes a bug in which :class:`~.BogoliubovTransform` would sometimes throw an error due to an inability to
+    cast complex numbers to floats.

test/second_q/circuit/library/test_bogoliubov_transform.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2022.
+# (C) Copyright IBM 2022, 2023.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -98,6 +98,19 @@ def test_bogoliubov_transform_compose_general(self, n_orbitals):
 
         self.assertTrue(Operator(circuit).equiv(Operator(bog_circuit_composed), atol=1e-8))
 
+    def test_bogoliubov_transform_jw_inverse_general_identity(self):
+        """Test Bogoliubov transform for the identity transformation."""
+        n_orbitals = 3
+        mat = np.eye(n_orbitals, 2 * n_orbitals, k=n_orbitals)
+        bog = BogoliubovTransform(mat)
+
+        register = QuantumRegister(n_orbitals)
+        circuit = QuantumCircuit(register)
+        circuit.append(bog, register)
+        circuit.append(bog.inverse(), register)
+
+        self.assertTrue(Operator(circuit).equiv(np.eye(2**n_orbitals), atol=1e-8))
+
     def test_no_side_effects(self):
         """Test that the routines don't mutate the input array."""
         n_orbitals = 5