Remove decomp_depth from qml.device constructor

doc/development/deprecations.rst

@@ -21,11 +21,6 @@ Pending deprecations
  - Deprecated in v0.38
  - Will be removed in v0.39 
 
-* The ``decomp_depth`` argument in ``qml.device`` is deprecated. 
-
- - Deprecated in v0.38
- - Will be removed in v0.39
-
 * The functions ``qml.qinfo.classical_fisher`` and ``qml.qinfo.quantum_fisher`` are deprecated since they are being migrated
  to the ``qml.gradients`` module. Therefore, ``qml.gradients.classical_fisher`` and ``qml.gradients.quantum_fisher`` should be used instead.
 
@@ -82,6 +77,11 @@ Other deprecations
 Completed deprecation cycles
 ----------------------------
 
+* The ``decomp_depth`` argument in ``qml.device`` is removed. 
+
+ - Deprecated in v0.38
+ - Removed in v0.39
+
 * The ``expansion_strategy`` attribute of ``qml.QNode`` is removed.
  Users should make use of ``qml.workflow.construct_batch``, should they require fine control over the output tape(s).
 

doc/releases/changelog-dev.md

@@ -39,6 +39,8 @@
  Please use `qml.transforms.split_non_commuting` instead.
  [(#6204)](https://github.com/PennyLaneAI/pennylane/pull/6204)
 
+* The `decomp_depth` keyword argument to `qml.device` is removed.
+
 <h3>Deprecations 👋</h3>
 
 <h3>Documentation 📝</h3>

pennylane/devices/device_constructor.py

@@ -14,7 +14,6 @@
 """
 This module contains code for the main device construction delegation logic.
 """
-import warnings
 from importlib import metadata
 from sys import version_info
 
@@ -57,37 +56,36 @@ def refresh_devices():
 
 # pylint: disable=protected-access
 def device(name, *args, **kwargs):
- r"""
- Load a device and return the instance.
+ r"""Load a device and return the instance.
 
  This function is used to load a particular quantum device,
  which can then be used to construct QNodes.
 
  PennyLane comes with support for the following devices:
 
  * :mod:`'default.qubit' <pennylane.devices.default_qubit>`: a simple
- state simulator of qubit-based quantum circuit architectures.
+  state simulator of qubit-based quantum circuit architectures.
 
  * :mod:`'default.mixed' <pennylane.devices.default_mixed>`: a mixed-state
- simulator of qubit-based quantum circuit architectures.
+  simulator of qubit-based quantum circuit architectures.
 
  * ``'lightning.qubit'``: a more performant state simulator of qubit-based
- quantum circuit architectures written in C++.
+  quantum circuit architectures written in C++.
 
  * :mod:`'default.qutrit' <pennylane.devices.default_qutrit>`: a simple
- state simulator of qutrit-based quantum circuit architectures.
+  state simulator of qutrit-based quantum circuit architectures.
 
  * :mod:`'default.qutrit.mixed' <pennylane.devices.default_qutrit_mixed>`: a
- mixed-state simulator of qutrit-based quantum circuit architectures.
+  mixed-state simulator of qutrit-based quantum circuit architectures.
 
  * :mod:`'default.gaussian' <pennylane.devices.default_gaussian>`: a simple simulator
- of Gaussian states and operations on continuous-variable circuit architectures.
+  of Gaussian states and operations on continuous-variable circuit architectures.
 
  * :mod:`'default.clifford' <pennylane.devices.default_clifford>`: an efficient
- simulator of Clifford circuits.
+  simulator of Clifford circuits.
 
  * :mod:`'default.tensor' <pennylane.devices.default_tensor>`: a simulator
- of quantum circuits based on tensor networks.
+  of quantum circuits based on tensor networks.
 
  Additional devices are supported through plugins — see
  the `available plugins <https://pennylane.ai/plugins.html>`_ for more
@@ -105,12 +103,6 @@ def device(name, *args, **kwargs):
  that contains global and/or device specific configurations.
  custom_decomps (Dict[Union(str, Operator), Callable]): Custom
  decompositions to be applied by the device at runtime.
- decomp_depth (int): For when custom decompositions are specified,
- the maximum expansion depth used by the expansion function.
-
- .. warning::
-
- The ``decomp_depth`` argument is deprecated and will be removed in version 0.39.
 
  All devices must be loaded by specifying their **short-name** as listed above,
  followed by the **wires** (subsystems) you wish to initialize. The ``wires``
@@ -122,10 +114,10 @@ def device(name, *args, **kwargs):
  dev = qml.device('default.qubit', wires=5)
 
  def circuit():
- qml.Hadamard(wires=1)
- qml.Hadamard(wires=[0])
- qml.CNOT(wires=[3, 4])
- ...
+  qml.Hadamard(wires=1)
+  qml.Hadamard(wires=[0])
+  qml.CNOT(wires=[3, 4])
+  ...
 
  The ``wires`` argument can also be a sequence of unique numbers or strings, specifying custom wire labels
  that the user employs to address the wires:
@@ -135,10 +127,10 @@ def circuit():
  dev = qml.device('default.qubit', wires=['ancilla', 'q11', 'q12', -1, 1])
 
  def circuit():
- qml.Hadamard(wires='q11')
- qml.Hadamard(wires=['ancilla'])
- qml.CNOT(wires=['q12', -1])
- ...
+  qml.Hadamard(wires='q11')
+  qml.Hadamard(wires=['ancilla'])
+  qml.CNOT(wires=['q12', -1])
+  ...
 
  On some newer devices, such as ``default.qubit``, the ``wires`` argument can be omitted altogether,
  and instead the wires will be computed when executing a circuit depending on its contents.
@@ -157,8 +149,8 @@ def circuit():
 
  @qml.qnode(dev)
  def circuit(a):
- qml.RX(a, wires=0)
- return qml.sample(qml.Z(0))
+  qml.RX(a, wires=0)
+  return qml.sample(qml.Z(0))
 
  >>> circuit(0.8) # 10 samples are returned
  array([ 1, 1, 1, 1, -1, 1, 1, -1, 1, 1])
@@ -203,12 +195,12 @@ def ion_trap_cnot(wires, **_):
  'default.qubit', wires=2, custom_decomps={"CNOT" : ion_trap_cnot}
  )
 
- @qml.qnode(dev)
+ @qml.qnode(dev, expansion_strategy="device")
  def run_cnot():
  qml.CNOT(wires=[0, 1])
  return qml.expval(qml.X(1))
 
- >>> print(qml.draw(run_cnot, level="device")())
+ >>> print(qml.draw(run_cnot)())
  0: ──RY(1.57)─╭IsingXX(1.57)──RX(-1.57)──RY(-1.57)─┤
  1: ───────────╰IsingXX(1.57)──RY(-1.57)────────────┤ <X>
 
@@ -243,15 +235,6 @@ def run_cnot():
  # Pop the custom decomposition keyword argument; we will use it here
  # only and not pass it to the device.
  custom_decomps = kwargs.pop("custom_decomps", None)
- decomp_depth = kwargs.pop("decomp_depth", None)
-
- if decomp_depth is not None:
- warnings.warn(
- "The decomp_depth argument is deprecated and will be removed in version 0.39. ",
- qml.PennyLaneDeprecationWarning,
- )
- else:
- decomp_depth = 10
 
  kwargs.pop("config", None)
  options.update(kwargs)
@@ -284,12 +267,12 @@ def _safe_specifier_set(version_str):
  if custom_decomps is not None:
  if isinstance(dev, qml.devices.LegacyDevice):
  custom_decomp_expand_fn = qml.transforms.create_decomp_expand_fn(
- custom_decomps, dev, decomp_depth=decomp_depth
+ custom_decomps, dev, decomp_depth=10
  )
  dev.custom_expand(custom_decomp_expand_fn)
  else:
  custom_decomp_preprocess = qml.transforms.tape_expand._create_decomp_preprocessing(
- custom_decomps, dev, decomp_depth=decomp_depth
+ custom_decomps, dev, decomp_depth=10
  )
  dev.preprocess = custom_decomp_preprocess