Deprecation QubitStateVector (#6172)

[sc-72046]

doc/development/deprecations.rst

@@ -39,6 +39,12 @@ Pending deprecations
   - Deprecated in v0.37
   - Will be removed in v0.39
 
+* The ``QubitStateVector`` template is deprecated.
+  Instead, use ``StatePrep``.
+
+  - Deprecated in v0.39
+  - Will be removed in v0.40
+
 New operator arithmetic deprecations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

doc/releases/changelog-dev.md

@@ -90,6 +90,10 @@
 
 <h3>Deprecations 👋</h3>
 
+* The ``QubitStateVector`` template is deprecated.
+   Instead, use ``StatePrep``.
+   [(#6172)](https://github.com/PennyLaneAI/pennylane/pull/6172)
+
 * `Device`, `QubitDevice`, and `QutritDevice` will no longer be accessible via top-level import in v0.40.
   They will still be accessible as `qml.devices.LegacyDevice`, `qml.devices.QubitDevice`, and `qml.devices.QutritDevice`
   respectively.

pennylane/ops/qubit/state_preparation.py

@@ -15,6 +15,8 @@
 This submodule contains the discrete-variable quantum operations concerned
 with preparing a certain state on the device.
 """
+import warnings
+
 # pylint:disable=too-many-branches,abstract-method,arguments-differ,protected-access,no-member
 from typing import Optional
 
@@ -442,9 +444,19 @@ def _preprocess(state, wires, pad_with, normalize, validate_norm):
         return state
 
 
-# pylint: disable=missing-class-docstring
 class QubitStateVector(StatePrep):
-    pass  # QSV is still available
+    r"""
+    ``QubitStateVector`` is deprecated and will be removed in version 0.40. Instead, please use ``StatePrep``.
+    """
+
+    # pylint: disable=too-many-arguments
+    def __init__(self, state, wires, pad_with=None, normalize=False, validate_norm=True):
+        warnings.warn(
+            "QubitStateVector is deprecated and will be removed in version 0.40. "
+            "Instead, please use StatePrep.",
+            qml.PennyLaneDeprecationWarning,
+        )
+        super().__init__(state, wires, pad_with, normalize, validate_norm)
 
 
 class QubitDensityMatrix(Operation):

tests/drawer/test_drawable_layers.py

@@ -196,7 +196,7 @@ def test_mid_measure_custom_wires(self):
         m1 = qml.measurements.MeasurementValue([mp1], lambda v: v)
 
         def teleport(state):
-            qml.QubitStateVector(state, wires=["A"])
+            qml.StatePrep(state, wires=["A"])
             qml.Hadamard(wires="a")
             qml.CNOT(wires=["a", "B"])
             qml.CNOT(wires=["A", "a"])

tests/ops/functions/conftest.py

@@ -38,7 +38,6 @@
     qml.sum(qml.X(0), qml.X(0), qml.Z(0), qml.Z(0)),
     qml.BasisState([1], wires=[0]),
     qml.ControlledQubitUnitary(np.eye(2), control_wires=1, wires=0),
-    qml.QubitStateVector([0, 1], wires=0),
     qml.QubitChannel([np.array([[1, 0], [0, 0.8]]), np.array([[0, 0.6], [0, 0]])], wires=0),
     qml.MultiControlledX(wires=[0, 1]),
     qml.Projector([1], 0),  # the state-vector version is already tested
@@ -137,6 +136,7 @@
     PowOpObs,
     PowOperation,
     PowObs,
+    qml.QubitStateVector,
 }
 """Types that should not have actual instances created."""
 

tests/ops/qubit/test_state_prep.py

@@ -36,6 +36,12 @@ def test_adjoint_error_exception(op):
         op.adjoint()
 
 
+def test_QubitStateVector_is_deprecated():
+    """Test that QubitStateVector is deprecated."""
+    with pytest.warns(qml.PennyLaneDeprecationWarning, match="QubitStateVector is deprecated"):
+        _ = qml.QubitStateVector([1, 0, 0, 0], wires=[0, 1])
+
+
 @pytest.mark.parametrize(
     "op, mat, base",
     [