Fix expand_tape_state_prep.

doc/releases/changelog-dev.md

@@ -162,6 +162,9 @@
 
 <h3>Bug fixes 🐛</h3>
 
+* Fix `skip_first` option in `expand_tape_state_prep`.
+  [(#4564)](https://github.com/PennyLaneAI/pennylane/pull/4564)
+
 * `convert_to_numpy_parameters` now uses `qml.ops.functions.bind_new_parameters`. This reinitializes the operation and
   makes sure everything references the new numpy parameters.
 
@@ -174,6 +177,7 @@ This release contains contributions from (in alphabetical order):
 
 Soran Jahangiri,
 Lillian M. A. Frederiksen,
+Vincent Michaud-Rioux,
 Romain Moyard,
 Mudit Pandey,
 Matthew Silverman,

pennylane/tape/tape.py

@@ -263,7 +263,7 @@ def expand_tape_state_prep(tape, skip_first=True):
     first_op = tape.operations[0]
     new_ops = (
         [first_op]
-        if isinstance(first_op, StatePrepBase) and skip_first
+        if not isinstance(first_op, StatePrepBase) or skip_first
         else first_op.decomposition()
     )
 

tests/tape/test_tape.py

@@ -1014,10 +1014,12 @@ def test_depth_expansion(self):
             [
                 qml.BasisState([1, 0], wires=[0, 1]),
                 qml.StatePrep([0, 1, 0, 0], wires=[0, 1]),
+                qml.PauliZ(0),
             ],
             [
                 qml.BasisStatePreparation([1, 0], wires=[0, 1]),
                 qml.MottonenStatePreparation([0, 1, 0, 0], wires=[0, 1]),
+                qml.PauliZ(0),
             ],
         ),
     )
@@ -1026,12 +1028,13 @@ def test_expansion_state_prep(self, skip_first, op, decomp):
         expanding other operations in the tape.
         """
         ops = [
+            op,
             qml.PauliZ(wires=0),
             qml.Rot(0.1, 0.2, 0.3, wires=0),
             qml.BasisState([0], wires=1),
             qml.StatePrep([0, 1], wires=0),
         ]
-        tape = QuantumTape(ops=ops, measurements=[], prep=[op])
+        tape = QuantumTape(ops=ops, measurements=[])
         new_tape = expand_tape_state_prep(tape, skip_first=skip_first)
 
         true_decomposition = []