Fix issue #1266: 'convert_observable' function raises an error for Qu…

…bitOperators containing complex coefficients (#1277)

* Checks that all coefficients in the QubitOperator are real

* Add unit test to 'test_convert_observable.py'

* Update 'qchem/CHANGELOG.md'

* Move the check of the coefficients to the 'convert_observable' function

* Apply suggestions from code review

Co-authored-by: zeyueN <48225584+zeyueN@users.noreply.github.com>

* Minor change

* Comply with CodeFactor suggestion

* Update qchem/CHANGELOG.md

Co-authored-by: zeyueN <48225584+zeyueN@users.noreply.github.com>
Co-authored-by: Maria Schuld <mariaschuld@gmail.com>

qchem/CHANGELOG.md

@@ -6,12 +6,19 @@
 
 <h3>Bug fixes</h3>
 
+* An error message is raised if a QubitOperator with complex coefficients is passed
+  to the ``convert_observable`` function. At present, the ``vqe.Hamiltonian`` class does not
+  support complex coefficients.
+  [(#1277)](https://github.com/PennyLaneAI/pennylane/pull/1277)
+
 <h3>Breaking changes</h3>
 
 <h3>Contributors</h3>
 
 This release contains contributions from (in alphabetical order):
 
+Alain Delgado Gran
+
 # Release 0.15.1
 
 <h3>Bug fixes</h3>

qchem/pennylane_qchem/qchem/structure.py

@@ -98,7 +98,7 @@ def _process_wires(wires, n_wires=None):
 
     elif isinstance(wires, dict):
 
-        if all([isinstance(w, int) for w in wires.keys()]):
+        if all(isinstance(w, int) for w in wires.keys()):
             # Assuming keys are taken from consecutive int wires. Allows for partial mapping.
             n_wires = max(wires) + 1
             labels = list(range(n_wires))  # used for completing potential partial mapping.
@@ -616,7 +616,9 @@ def _terms_to_qubit_operator(coeffs, ops, wires=None):
     all_wires = Wires.all_wires([op.wires for op in ops], sort=True)
 
     if wires is not None:
-        qubit_indexed_wires = _process_wires(wires,)
+        qubit_indexed_wires = _process_wires(
+            wires,
+        )
         if not set(all_wires).issubset(set(qubit_indexed_wires)):
             raise ValueError("Supplied `wires` does not cover all wires defined in `ops`.")
     else:
@@ -705,6 +707,12 @@ def convert_observable(qubit_observable, wires=None):
       0.16768319  0.12293305  0.17627641]
     """
 
+    if any(np.iscomplex(np.array(coef)) for coef in qubit_observable.terms.values()):
+        raise TypeError(
+            "The coefficients entering the QubitOperator must be real;"
+            " got complex coefficients in the operator {}".format(qubit_observable)
+        )
+
     return Hamiltonian(*_qubit_operator_to_terms(qubit_observable, wires=wires))
 
 

qchem/tests/test_convert_observable.py

@@ -322,6 +322,20 @@ def test_not_xyz_terms_to_qubit_operator():
         )
 
 
+def test_exception_convert_observable():
+    r"""Test that an error is raised if the QubitOperator contains complex coefficients.
+    Currently the vqe.Hamiltonian class does not support complex coefficients.
+    """
+    qubit_op = (
+        QubitOperator("Y0 Y1", 1 + 0j)
+        + QubitOperator("Z0 X1", 4.5 + 1.5j)
+        + QubitOperator("Y0 X1", 2)
+    )
+
+    with pytest.raises(TypeError, match="The coefficients entering the QubitOperator must be real"):
+        qchem.convert_observable(qubit_op)
+
+
 def test_identities_terms_to_qubit_operator():
     """Test that tensor products that contain Identity instances are handled
     correctly by the _terms_to_qubit_operator function.
@@ -333,29 +347,33 @@ def test_identities_terms_to_qubit_operator():
      [0 0 0 4]]
     """
     coeffs = [2.5, -0.5, -1.0]
-    obs_list = [qml.Identity(wires=[0]) @ qml.Identity(wires=[1]),
-            qml.Identity(wires=[0]) @ qml.PauliZ(wires=[1]),
-            qml.PauliZ(wires=[0]) @ qml.Identity(wires=[1])]
+    obs_list = [
+        qml.Identity(wires=[0]) @ qml.Identity(wires=[1]),
+        qml.Identity(wires=[0]) @ qml.PauliZ(wires=[1]),
+        qml.PauliZ(wires=[0]) @ qml.Identity(wires=[1]),
+    ]
 
     op_str = str(qchem._terms_to_qubit_operator(coeffs, obs_list))
 
     # Remove new line characters
-    op_str = op_str.replace('\n','')
+    op_str = op_str.replace("\n", "")
     assert op_str == "2.5 [] +-1.0 [Z0] +-0.5 [Z1]"
 
 
 def test_terms_to_qubit_operator_no_decomp():
     """Test the _terms_to_qubit_operator function with custom wires."""
     coeffs = np.array([0.1, 0.2])
     ops = [
-        qml.operation.Tensor(qml.PauliX(wires=['w0'])),
-        qml.operation.Tensor(qml.PauliY(wires=['w0']), qml.PauliZ(wires=['w2']))
+        qml.operation.Tensor(qml.PauliX(wires=["w0"])),
+        qml.operation.Tensor(qml.PauliY(wires=["w0"]), qml.PauliZ(wires=["w2"])),
     ]
-    op_str = str(qchem._terms_to_qubit_operator(coeffs, ops, wires=qml.wires.Wires(['w0', 'w1', 'w2'])))
+    op_str = str(
+        qchem._terms_to_qubit_operator(coeffs, ops, wires=qml.wires.Wires(["w0", "w1", "w2"]))
+    )
 
     # Remove new line characters
-    op_str = op_str.replace('\n','')
-    expected = '0.1 [X0] +0.2 [Y0 Z2]'
+    op_str = op_str.replace("\n", "")
+    expected = "0.1 [X0] +0.2 [Y0 Z2]"
     assert op_str == expected
 
 
@@ -438,7 +456,12 @@ def test_integration_mol_file_to_vqe_cost(
 
     ref_dir = os.path.join(os.path.dirname(os.path.realpath(__file__)), "test_ref_files")
     hf_file = os.path.join(ref_dir, name)
-    qubit_hamiltonian = qchem.decompose(hf_file, mapping=mapping, core=core, active=active,)
+    qubit_hamiltonian = qchem.decompose(
+        hf_file,
+        mapping=mapping,
+        core=core,
+        active=active,
+    )
 
     vqe_hamiltonian = qchem.convert_observable(qubit_hamiltonian, custom_wires)
     assert len(vqe_hamiltonian.ops) > 1  # just to check if this runs