globalphase decomposes to nothing

doc/releases/changelog-dev.md

@@ -43,6 +43,9 @@
 * Simplified the logic for re-arranging states before returning.
   [(#4817)](https://github.com/PennyLaneAI/pennylane/pull/4817)
 
+* `GlobalPhase` now decomposes to nothing, in case devices do not support global phases.
+  [(#4855)](https://github.com/PennyLaneAI/pennylane/pull/4855)
+
 <h3>Breaking changes 💔</h3>
 
 * The `prep` keyword argument has been removed from `QuantumScript` and `QuantumTape`.

pennylane/ops/identity.py

@@ -328,6 +328,29 @@ def compute_diagonalizing_gates(
         """
         return []
 
+    @staticmethod
+    def compute_decomposition(phi, wires=None):  # pylint:disable=arguments-differ,unused-argument
+        r"""Representation of the operator as a product of other operators (static method).
+
+        .. math:: O = O_1 O_2 \dots O_n.
+
+        .. seealso:: :meth:`~.Identity.decomposition`.
+
+        Args:
+            phi (TensorLike): the global phase
+            wires (Any, Wires): A single wire that the operator acts on.
+
+        Returns:
+            list[Operator]: decomposition into lower level operations
+
+        **Example:**
+
+        >>> qml.GlobalPhase.compute_decomposition(wires=0)
+        []
+
+        """
+        return []
+
     def matrix(self, wire_order=None):
         n_wires = len(wire_order) if wire_order else len(self.wires)
         return self.compute_matrix(self.data[0], n_wires=n_wires)

tests/ops/qubit/test_parametric_ops.py

@@ -4522,6 +4522,13 @@ def test_global_phase_compute_sparse_matrix(phi, n_wires):
     assert np.allclose(sparse_matrix.todense(), expected.todense())
 
 
+def test_decomposition():
+    """Test the decomposition of the GlobalPhase operation."""
+
+    assert qml.GlobalPhase.compute_decomposition(1.23) == []
+    assert qml.GlobalPhase(1.23).decomposition() == []
+
+
 control_data = [
     (qml.Rot(1, 2, 3, wires=0), Wires([])),
     (qml.RX(1.23, wires=0), Wires([])),