undo_swaps

[KetpuntoG]

Context: The Permute operation uses many SWAP gates to be implemented. In (#1324) a study was proposed to make this operator native to default_qubit in order to make the circuit more efficient. Instead, I have created a transformation that removes all the SWAP gates from the circuit by playing with the wires of the operations. I think it is a more efficient and flexible solution.

Description of the Change:
With a single reading of the circuit from right to left, all SWAPS in the circuit can be eliminated.

Example:

Without undo_swaps:

import pennylane as qml
dev = qml.device('default.qubit', wires=3)

@qml.qnode(dev)
def qfunc():
    qml.Hadamard(wires=0)
    qml.PauliX(wires=1)
    qml.SWAP(wires=[0,1])
    qml.SWAP(wires=[0,2])
    qml.PauliY(wires=0)
    return qml.expval(qml.PauliZ(0))

print(qml.draw(qfunc)())

0: ──H──╭SWAP──╭SWAP──Y──┤ ⟨Z⟩
1: ──X──╰SWAP──│────────┤
2: ──────────╰SWAP─────┤

With undo_swaps:

@qml.qnode(dev)
@qml.transforms.undo_swaps
def qfunc():
    qml.Hadamard(wires=0)
    qml.PauliX(wires=1)
    qml.SWAP(wires=[0,1])
    qml.SWAP(wires=[0,2])
    qml.PauliY(wires=0)
    return qml.expval(qml.PauliZ(0))

print(qml.draw(qfunc)())

0: ──Y──┤ ⟨Z⟩
1: ──H──┤
2: ──X──┤

Benefits:
The computational cost of the circuit calculation is reduced.

Related GitHub Issues:
(#1324)

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #1960 (556f350) into master (d7fe61b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1960   +/-   ##
=======================================
  Coverage   98.81%   98.81%           
=======================================
  Files         225      226    +1     
  Lines       17312    17346   +34     
=======================================
+ Hits        17107    17141   +34     
  Misses        205      205           

Impacted Files	Coverage Δ
pennylane/transforms/__init__.py	100.00% <ø> (ø)
pennylane/transforms/optimization/__init__.py	100.00% <100.00%> (ø)
pennylane/transforms/optimization/undo_swaps.py	100.00% <100.00%> (ø)

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[josh146]

very nice solution!

[KetpuntoG]

very nice solution!

Thanks @josh146 😊

[KetpuntoG]

[sc-11831]

[dime10]

Hey @KetpuntoG, thank you for contributing this transform, great idea for an optimization :)

A few minor comments below.