Source code for pennylane.transforms.optimization.single_qubit_fusion
# Copyright 2018-2021 Xanadu Quantum Technologies Inc.
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
# http://www.apache.org/licenses/LICENSE-2.0
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Transform for fusing sequences of single-qubit gates."""
# pylint: disable=too-many-branches
from typing import Sequence, Callable
from pennylane.tape import QuantumTape
from pennylane.transforms import transform
from pennylane.ops.qubit import Rot
from pennylane.math import allclose, stack, is_abstract
from pennylane.queuing import QueuingManager
from .optimization_utils import find_next_gate, fuse_rot_angles
[docs]@transform
def single_qubit_fusion(
tape: QuantumTape, atol=1e-8, exclude_gates=None
) -> (Sequence[QuantumTape], Callable):
r"""Quantum function transform to fuse together groups of single-qubit
operations into a general single-qubit unitary operation (:class:`~.Rot`).
Fusion is performed only between gates that implement the property
``single_qubit_rot_angles``. Any sequence of two or more single-qubit gates
(on the same qubit) with that property defined will be fused into one ``Rot``.
Args:
tape (QNode or QuantumTape or Callable): A quantum circuit.
atol (float): An absolute tolerance for which to apply a rotation after
fusion. After fusion of gates, if the fused angles :math:`\theta` are such that
:math:`|\theta|\leq \text{atol}`, no rotation gate will be applied.
exclude_gates (None or list[str]): A list of gates that should be excluded
from full fusion. If set to ``None``, all single-qubit gates that can
be fused will be fused.
Returns:
qnode (QNode) or quantum function (Callable) or tuple[List[QuantumTape], function]: The transformed circuit as described in :func:`qml.transform <pennylane.transform>`.
**Example**
>>> dev = qml.device('default.qubit', wires=1)
You can apply the transform directly on :class:`QNode`:
.. code-block:: python
@single_qubit_fusion
@qml.qnode(device=dev)
def qfunc(r1, r2):
qml.Hadamard(wires=0)
qml.Rot(*r1, wires=0)
qml.Rot(*r2, wires=0)
qml.RZ(r1[0], wires=0)
qml.RZ(r2[0], wires=0)
return qml.expval(qml.PauliX(0))
The single qubit gates are fused before execution.
.. details::
:title: Usage Details
Consider the following quantum function.
.. code-block:: python
def qfunc(r1, r2):
qml.Hadamard(wires=0)
qml.Rot(*r1, wires=0)
qml.Rot(*r2, wires=0)
qml.RZ(r1[0], wires=0)
qml.RZ(r2[0], wires=0)
return qml.expval(qml.PauliX(0))
The circuit before optimization:
>>> qnode = qml.QNode(qfunc, dev)
>>> print(qml.draw(qnode)([0.1, 0.2, 0.3], [0.4, 0.5, 0.6]))
0: ──H──Rot(0.1, 0.2, 0.3)──Rot(0.4, 0.5, 0.6)──RZ(0.1)──RZ(0.4)──┤ ⟨X⟩
Full single-qubit gate fusion allows us to collapse this entire sequence into a
single ``qml.Rot`` rotation gate.
>>> optimized_qfunc = single_qubit_fusion(qfunc)
>>> optimized_qnode = qml.QNode(optimized_qfunc, dev)
>>> print(qml.draw(optimized_qnode)([0.1, 0.2, 0.3], [0.4, 0.5, 0.6]))
0: ──Rot(3.57, 2.09, 2.05)──┤ ⟨X⟩
"""
# Make a working copy of the list to traverse
list_copy = tape.operations.copy()
new_operations = []
while len(list_copy) > 0:
current_gate = list_copy[0]
# If the gate should be excluded, queue it and move on regardless
# of fusion potential
if exclude_gates is not None:
if current_gate.name in exclude_gates:
new_operations.append(current_gate)
list_copy.pop(0)
continue
# Look for single_qubit_rot_angles; if not available, queue and move on.
# If available, grab the angles and try to fuse.
try:
cumulative_angles = stack(current_gate.single_qubit_rot_angles())
except (NotImplementedError, AttributeError):
new_operations.append(current_gate)
list_copy.pop(0)
continue
# Find the next gate that acts on the same wires
next_gate_idx = find_next_gate(current_gate.wires, list_copy[1:])
if next_gate_idx is None:
new_operations.append(current_gate)
list_copy.pop(0)
continue
# Before entering the loop, we check to make sure the next gate is not in the
# exclusion list. If it is, we should apply the original gate as-is, and not the
# Rot version (example in test test_single_qubit_fusion_exclude_gates).
if exclude_gates is not None:
next_gate = list_copy[next_gate_idx + 1]
if next_gate.name in exclude_gates:
new_operations.append(current_gate)
list_copy.pop(0)
continue
# Loop as long as a valid next gate exists
while next_gate_idx is not None:
next_gate = list_copy[next_gate_idx + 1]
# Check first if the next gate is in the exclusion list
if exclude_gates is not None:
if next_gate.name in exclude_gates:
break
# Try to extract the angles; since the Rot angles are implemented
# solely for single-qubit gates, and we used find_next_gate to obtain
# the gate in question, only valid single-qubit gates on the same
# wire as the current gate will be fused.
try:
next_gate_angles = stack(next_gate.single_qubit_rot_angles())
except (NotImplementedError, AttributeError):
break
cumulative_angles = fuse_rot_angles(cumulative_angles, stack(next_gate_angles))
list_copy.pop(next_gate_idx + 1)
next_gate_idx = find_next_gate(current_gate.wires, list_copy[1:])
# If we are tracing/jitting, don't perform any conditional checks and
# apply the rotation regardless of the angles.
if is_abstract(cumulative_angles):
with QueuingManager.stop_recording():
new_operations.append(Rot(*cumulative_angles, wires=current_gate.wires))
# If not tracing, check whether all angles are 0 (or equivalently, if the RY
# angle is close to 0, and so is the sum of the RZ angles
else:
if not allclose(
stack([cumulative_angles[0] + cumulative_angles[2], cumulative_angles[1]]),
[0.0, 0.0],
atol=atol,
rtol=0,
):
with QueuingManager.stop_recording():
new_operations.append(Rot(*cumulative_angles, wires=current_gate.wires))
# Remove the starting gate from the list
list_copy.pop(0)
new_tape = type(tape)(new_operations, tape.measurements, shots=tape.shots)
def null_postprocessing(results):
"""A postprocesing function returned by a transform that only converts the batch of results
into a result for a single ``QuantumTape``.
"""
return results[0]
return [new_tape], null_postprocessing
_modules/pennylane/transforms/optimization/single_qubit_fusion
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