Source code for pennylane.transforms.optimization.merge_rotations

# Copyright 2018-2021 Xanadu Quantum Technologies Inc.

# Licensed under the Apache License, Version 2.0 (the "License");
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#     http://www.apache.org/licenses/LICENSE-2.0

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"""Transform for merging adjacent rotations of the same type in a quantum circuit."""
# pylint: disable=too-many-branches
from pennylane import apply
from pennylane.transforms import qfunc_transform
from pennylane.math import allclose, stack, cast_like, zeros, is_abstract

from pennylane.ops.qubit.attributes import composable_rotations
from pennylane.ops.op_math import Adjoint
from .optimization_utils import find_next_gate, fuse_rot_angles


[docs]@qfunc_transform def merge_rotations(tape, atol=1e-8, include_gates=None): r"""Quantum function transform to combine rotation gates of the same type that act sequentially. If the combination of two rotation produces an angle that is close to 0, neither gate will be applied. Args: qfunc (function): A quantum function. atol (float): After fusion of gates, if the fused angle :math:`\theta` is such that :math:`|\theta|\leq \text{atol}`, no rotation gate will be applied. include_gates (None or list[str]): A list of specific operations to merge. If set to ``None`` (default), all operations in the `~.pennylane.ops.qubit.attributes.composable_rotations` attribute will be merged. Otherwise, only the operations whose names match those in the list will undergo merging. Returns: function: the transformed quantum function **Example** Consider the following quantum function. .. code-block:: python def qfunc(x, y, z): qml.RX(x, wires=0) qml.RX(y, wires=0) qml.CNOT(wires=[1, 2]) qml.RY(y, wires=1) qml.Hadamard(wires=2) qml.CRZ(z, wires=[2, 0]) qml.RY(-y, wires=1) return qml.expval(qml.PauliZ(0)) The circuit before optimization: >>> dev = qml.device('default.qubit', wires=3) >>> qnode = qml.QNode(qfunc, dev) >>> print(qml.draw(qnode)(1, 2, 3)) 0: ──RX(1.00)──RX(2.00)─╭RZ(3.00)────────────┤ <Z> 1: ─╭●─────────RY(2.00)─│──────────RY(-2.00)─┤ 2: ─╰X─────────H────────╰●───────────────────┤ By inspection, we can combine the two ``RX`` rotations on the first qubit. On the second qubit, we have a cumulative angle of 0, and the gates will cancel. >>> optimized_qfunc = merge_rotations()(qfunc) >>> optimized_qnode = qml.QNode(optimized_qfunc, dev) >>> print(qml.draw(optimized_qnode)(1, 2, 3)) 0: ──RX(3.00)────╭RZ(3.00)─┤ <Z> 1: ─╭●───────────│─────────┤ 2: ─╰X─────────H─╰●────────┤ It is also possible to explicitly specify which rotations ``merge_rotations`` should be merged using the ``include_gates`` argument. For example, if in the above circuit we wanted only to merge the "RX" gates, we could do so as follows: >>> optimized_qfunc = merge_rotations(include_gates=["RX"])(qfunc) >>> optimized_qnode = qml.QNode(optimized_qfunc, dev) >>> print(qml.draw(optimized_qnode)(1, 2, 3)) 0: ──RX(3.00)───────────╭RZ(3.00)────────────┤ <Z> 1: ─╭●─────────RY(2.00)─│──────────RY(-2.00)─┤ 2: ─╰X─────────H────────╰●───────────────────┤ """ # Expand away adjoint ops expanded_tape = tape.expand(stop_at=lambda obj: not isinstance(obj, Adjoint)) list_copy = expanded_tape.operations while len(list_copy) > 0: current_gate = list_copy[0] # If a specific list of operations is specified, check and see if our # op is in it, then try to merge. If not, queue and move on. if include_gates is not None: if current_gate.name not in include_gates: apply(current_gate) list_copy.pop(0) continue # Check if the rotation is composable; if it is not, move on. if not current_gate in composable_rotations: apply(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 no such gate is found (either there simply is none, or there are other gates # "in the way", queue the operation and move on if next_gate_idx is None: apply(current_gate) list_copy.pop(0) continue # We need to use stack to get this to work and be differentiable in all interfaces cumulative_angles = stack(current_gate.parameters) # As long as there is a valid next gate, check if we can merge the angles while next_gate_idx is not None: # Get the next gate next_gate = list_copy[next_gate_idx + 1] # If next gate is of the same type, we can merge the angles if current_gate.name == next_gate.name and current_gate.wires == next_gate.wires: list_copy.pop(next_gate_idx + 1) # The Rot gate must be treated separately if current_gate.name == "Rot": if is_abstract(cumulative_angles): # jax-jit does not support cast_like cumulative_angles = cumulative_angles + stack(next_gate.parameters) else: cumulative_angles = fuse_rot_angles( cumulative_angles, cast_like(stack(next_gate.parameters), cumulative_angles), ) # Other, single-parameter rotation gates just have the angle summed else: if is_abstract(cumulative_angles): # jax-jit does not support cast_like cumulative_angles = cumulative_angles + stack(next_gate.parameters) else: cumulative_angles = cumulative_angles + cast_like( stack(next_gate.parameters), cumulative_angles ) # If it is not, we need to stop else: break # If we did merge, look now at the next gate 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 operation regardless of the angles. Otherwise, only apply if # the rotation angle is non-trivial. if is_abstract(cumulative_angles): current_gate.__class__(*cumulative_angles, wires=current_gate.wires) else: if not allclose(cumulative_angles, zeros(len(cumulative_angles)), atol=atol, rtol=0): current_gate.__class__(*cumulative_angles, wires=current_gate.wires) # Remove the first gate gate from the working list list_copy.pop(0) # Queue the measurements normally for m in tape.measurements: apply(m)