Deprecation QubitStateVector (#6172)

[sc-72046]

doc/development/deprecations.rst

@@ -33,6 +33,12 @@ 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

@@ -98,6 +98,10 @@
 
 <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",
  [