Adds global phase to the PhaseShift decomposition

pennylane/ops/qubit/parametric_ops_single_qubit.py

@@ -499,7 +499,7 @@ def compute_decomposition(phi, wires):
         [RZ(1.234, wires=[0])]
 
         """
-        return [RZ(phi, wires=wires)]
+        return [RZ(phi, wires=wires), qml.GlobalPhase(phi / 2)]
 
     def adjoint(self):
         return PhaseShift(-self.data[0], wires=self.wires)

pennylane/transforms/optimization/optimization_utils.py

@@ -190,7 +190,7 @@ def fuse_rot_angles(angles_1, angles_2):
 
 
 def _fuse_global_phases(operations):
-    """Fuse all the global phase operations into single one
+    """Fuse all the global phase operations into single one.
 
     Args:
         operations (list[Operation]): list of operations to be iterated over

tests/ops/qubit/test_parametric_ops.py

@@ -264,7 +264,7 @@ def test_phase_decomposition(self, phi, tol):
         op = qml.PhaseShift(phi, wires=0)
         res = op.decomposition()
 
-        assert len(res) == 1
+        assert len(res) == 2
 
         assert res[0].name == "RZ"
 
@@ -273,6 +273,10 @@ def test_phase_decomposition(self, phi, tol):
 
         decomposed_matrix = res[0].matrix()
         global_phase = np.exp(-1j * phi / 2)[..., np.newaxis, np.newaxis]
+
+        assert res[1].name == "GlobalPhase"
+        assert np.allclose(qml.matrix(res[1]), np.exp(-1j * phi / 2))
+
         assert np.allclose(decomposed_matrix, global_phase * op.matrix(), atol=tol, rtol=0)
 
     def test_phase_decomposition_broadcasted(self, tol):
@@ -281,7 +285,7 @@ def test_phase_decomposition_broadcasted(self, tol):
         op = qml.PhaseShift(phi, wires=0)
         res = op.decomposition()
 
-        assert len(res) == 1
+        assert len(res) == 2
 
         assert res[0].name == "RZ"
 
@@ -291,6 +295,9 @@ def test_phase_decomposition_broadcasted(self, tol):
         decomposed_matrix = res[0].matrix()
         global_phase = np.exp(-1j * phi / 2)[..., np.newaxis, np.newaxis]
 
+        assert res[1].name == "GlobalPhase"
+        assert np.allclose(qml.matrix(res[1]), np.exp(-1j * phi / 2))
+
         assert np.allclose(decomposed_matrix, global_phase * op.matrix(), atol=tol, rtol=0)
 
     def test_Rot_decomposition(self):

tests/transforms/test_batch_transform.py

@@ -214,9 +214,11 @@ def my_transform(self, tape):
         spy_expand.assert_called()
 
         input_tape = spy_transform.call_args[0][1]
-        assert len(input_tape.operations) == 1
+        assert len(input_tape.operations) == 2
         assert input_tape.operations[0].name == "RZ"
+        assert input_tape.operations[1].name == "GlobalPhase"
         assert input_tape.operations[0].parameters == [0.5]
+        assert input_tape.operations[1].parameters == [0.25]
 
     @pytest.mark.parametrize("perform_expansion", [True, False])
     def test_expand_fn_with_kwarg(self, mocker, perform_expansion):
@@ -249,9 +251,17 @@ def my_transform(self, tape, **kwargs):
         spy_expand.assert_called()  # The expand_fn of transform_fn always is called
 
         input_tape = spy_transform.call_args[0][1]
-        assert len(input_tape.operations) == 1
-        assert input_tape.operations[0].name == ("RZ" if perform_expansion else "PhaseShift")
-        assert input_tape.operations[0].parameters == [0.5]
+
+        if perform_expansion:
+            assert len(input_tape.operations) == 2
+            assert input_tape.operations[0].name == "RZ"
+            assert input_tape.operations[0].parameters == [0.5]
+            assert input_tape.operations[1].name == "GlobalPhase"
+            assert input_tape.operations[1].parameters == [0.25]
+        else:
+            assert len(input_tape.operations) == 1
+            assert input_tape.operations[0].name == "PhaseShift"
+            assert input_tape.operations[0].parameters == [0.5]
 
     @pytest.mark.parametrize("perform_expansion", [True, False])
     def test_expand_qnode_with_kwarg(self, mocker, perform_expansion):
@@ -287,9 +297,17 @@ def qnode(x):
         spy_transform.assert_called()
         spy_expand.assert_called()  # The expand_fn of transform_fn always is called
         input_tape = spy_transform.call_args[0][1]
-        assert len(input_tape.operations) == 1
-        assert input_tape.operations[0].name == ("RZ" if perform_expansion else "PhaseShift")
-        assert input_tape.operations[0].parameters == [0.5]
+
+        if perform_expansion:
+            assert len(input_tape.operations) == 2
+            assert input_tape.operations[0].name == "RZ"
+            assert input_tape.operations[0].parameters == [0.5]
+            assert input_tape.operations[1].name == "GlobalPhase"
+            assert input_tape.operations[1].parameters == [0.25]
+        else:
+            assert len(input_tape.operations) == 1
+            assert input_tape.operations[0].name == "PhaseShift"
+            assert input_tape.operations[0].parameters == [0.5]
 
     def test_parametrized_transform_tape(self):
         """Test that a parametrized transform can be applied

tests/transforms/test_compile.py

@@ -30,6 +30,7 @@
     merge_rotations,
     single_qubit_fusion,
 )
+from pennylane.transforms.optimization.optimization_utils import _fuse_global_phases
 
 
 def build_qfunc(wires):
@@ -250,7 +251,7 @@ def test_compile_decompose_into_basis_gates(self, wires):
 
         pipeline = [partial(commute_controlled, direction="left"), cancel_inverses, merge_rotations]
 
-        basis_set = ["CNOT", "RX", "RY", "RZ"]
+        basis_set = ["CNOT", "RX", "RY", "RZ", "GlobalPhase"]
 
         transformed_qfunc = compile(qfunc, pipeline=pipeline, basis_set=basis_set)
         transformed_qnode = qml.QNode(transformed_qfunc, dev)
@@ -273,6 +274,7 @@ def test_compile_decompose_into_basis_gates(self, wires):
             "CNOT",
             "RY",
             "CNOT",
+            "GlobalPhase",
         ]
 
         wires_expected = [
@@ -289,9 +291,11 @@ def test_compile_decompose_into_basis_gates(self, wires):
             Wires([wires[1], wires[2]]),
             Wires(wires[2]),
             Wires([wires[1], wires[2]]),
+            Wires([]),
         ]
 
-        compare_operation_lists(transformed_qnode.qtape.operations, names_expected, wires_expected)
+        tansformed_ops = _fuse_global_phases(transformed_qnode.qtape.operations)
+        compare_operation_lists(tansformed_ops, names_expected, wires_expected)
 
     def test_compile_template(self):
         """Test that functions with templates are correctly expanded and compiled."""

tests/transforms/test_tape_expand.py

@@ -228,9 +228,10 @@ def generator(self):
         tape = qml.tape.QuantumScript.from_queue(q)
         new_tape = qml.transforms.expand_nonunitary_gen(tape)
         assert tape.operations[:2] == new_tape.operations[:2]
-        exp_op = new_tape.operations[2]
+        exp_op, gph_op = new_tape.operations[2:4]
         assert exp_op.name == "RZ" and exp_op.data == (2.1,) and exp_op.wires == qml.wires.Wires(1)
-        assert tape.operations[3:] == new_tape.operations[3:]
+        assert gph_op.name == "GlobalPhase" and gph_op.data == (2.1 * 0.5,)
+        assert tape.operations[3:] == new_tape.operations[4:]
 
     def test_expand_nonunitary_generator(self):
         """Test that a tape with single-parameter operations with
@@ -246,9 +247,10 @@ def test_expand_nonunitary_generator(self):
         new_tape = qml.transforms.expand_nonunitary_gen(tape)
 
         assert tape.operations[:2] == new_tape.operations[:2]
-        exp_op = new_tape.operations[2]
+        exp_op, gph_op = new_tape.operations[2:4]
         assert exp_op.name == "RZ" and exp_op.data == (2.1,) and exp_op.wires == qml.wires.Wires(1)
-        assert tape.operations[3:] == new_tape.operations[3:]
+        assert gph_op.name == "GlobalPhase" and gph_op.data == (2.1 * 0.5,)
+        assert tape.operations[3:] == new_tape.operations[4:]
 
     def test_decompose_all_nonunitary_generator(self):
         """Test that decompositions actually only contain unitarily
@@ -332,8 +334,9 @@ class NonDiffPhaseShift(qml.PhaseShift):
 
         assert new_tape.operations[0].name == "RZ"
         assert new_tape.operations[0].grad_method == "A"
-        assert new_tape.operations[1].name == "RY"
-        assert new_tape.operations[2].name == "CNOT"
+        assert new_tape.operations[1].name == "GlobalPhase"
+        assert new_tape.operations[2].name == "RY"
+        assert new_tape.operations[3].name == "CNOT"
 
     def test_nontrainable_nondiff(self, mocker):
         """Test that a circuit with non-differentiable