[QAOA] Adds Cost and Mixer Layers #720

Lucaman99 · 2020-07-24T19:05:59Z

Context:

Part of the ongoing efforts to add QAOA functionality to PennyLane

Description of the Change:

Adds methods for constructing QAOA cost and mixer layers, along with tests.

Note: This functionality depends on the features introduced in a PR that is currently open (#710). I added some of this functionality to this PR (so that tests will pass). However, this PR should definitely not be merged until #710 is, at which point I will remove the extraneous functionality from this PR.

Related

#718, #712, #710

codecov · 2020-07-24T19:08:29Z

Codecov Report

Merging #720 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #720      +/-   ##
==========================================
+ Coverage   95.62%   95.63%   +0.01%     
==========================================
  Files         111      112       +1     
  Lines        6945     6968      +23     
==========================================
+ Hits         6641     6664      +23     
  Misses        304      304

Impacted Files	Coverage Δ
...ane/templates/subroutines/approx_time_evolution.py	`100.00% <ø> (ø)`
pennylane/qaoa/__init__.py	`100.00% <100.00%> (ø)`
pennylane/qaoa/layers.py	`100.00% <100.00%> (ø)`

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josh146 · 2020-07-26T03:18:17Z

@Lucaman99, if it makes more sense, we can change the 'base branch' for this PR from master to the branch used for #710. This will avoid the need to add the extraneous code.

Lucaman99 · 2020-07-27T13:26:46Z

@josh146 Awesome, I didn't realize that could be done!

josh146 · 2020-07-27T14:03:50Z

No worries! Just click on the 'edit' button on the top:

and then you can change the base branch 👍

Lucaman99 · 2020-07-30T17:11:37Z

@josh146 I tried to do this, and I don't think it's possible, since the branch in #710 is part of my personal fork.

josh146 · 2020-07-31T06:29:21Z

@josh146 I tried to do this, and I don't think it's possible, since the branch in #710 is part of my personal fork.

Oh :(

Lucaman99 · 2020-08-04T16:23:11Z

For the time being, I had to take the ApproxTimeEvolution file and add it to the PR (in order for the tests to pass). Since I didn't bother adding the corresponding tests, Codecov is failing.

ixfoduap · 2020-08-05T19:07:35Z

pennylane/qaoa/layers.py

+    return val
+
+
+def cost_layer(hamiltonian):


I find it a bit strange at first sight that the layer does not take gamma as an input. I understand that we call ApproxTimeEvolution as a lambda function, but I'm wondering if this is the best choice.

Alternatively we could use def cost_layer(hamiltonian, gamma): and then use

from pennylane.qaoa import cost_layer @qml.qnode(dev) def circuit(gamma): cost_layer(cost_h, gamma)

I'm not sure which approach is best. Curious what others think?

I am ambivalent 🤔

I see pros and cons for both approaches:

In the cost_layer(cost_h, gamma) approach, cost_layer is essentially a template, and must always be called within a QNode.

In the cost = cost_layer(cosh_h) approach, cost_layer is more like a 'template generating function'. The returned cost function can only be used within a QNode, like a template.

Probably lean more towards cost_layer(cost_h, gamma), unless you see a use-case for having the 'template generating' behaviour.

@ixfoduap @josh146 Sounds good, I'll make the change!

ixfoduap

Looks good!

My main suggestions are to include the variational parameter as input and to add tests that are independent from ApproxTimeEvolution

pennylane/qaoa/layers.py

tests/test_qaoa.py

ixfoduap · 2020-08-05T19:31:57Z

tests/test_qaoa.py

+        """Tests that the gates of the mixer layer is correct"""
+
+        alpha = 1
+        hamiltonian = qml.Hamiltonian([1, 1], [qml.PauliX(0), qml.PauliX(1)])


As far as I understand, this is testing that the layers are implementing the same sequence of gates as ApproxTimeEvolution, which I'm not sure is the best way to test since ApproxTimeEvolution may be giving the wrong answers, i.e., these tests don't add much more than the tests for ApproxTimeEvolution. Not sure if this is ok...

Maybe it's good to test an output circuit explicitly, without relying on ApproxTimeEvolution

Made these changes 🙂

Co-authored-by: ixfoduap <40441298+ixfoduap@users.noreply.github.com>

josh146

Nice work @Lucaman99 🔥

pennylane/qaoa/layers.py

josh146 · 2020-08-06T05:31:21Z

pennylane/qaoa/layers.py

+    return val
+
+
+def cost_layer(hamiltonian):


I am ambivalent 🤔

I see pros and cons for both approaches:

In the cost_layer(cost_h, gamma) approach, cost_layer is essentially a template, and must always be called within a QNode.

In the cost = cost_layer(cosh_h) approach, cost_layer is more like a 'template generating function'. The returned cost function can only be used within a QNode, like a template.

josh146 · 2020-08-06T05:31:51Z

pennylane/qaoa/layers.py

+    return val
+
+
+def cost_layer(hamiltonian):


Probably lean more towards cost_layer(cost_h, gamma), unless you see a use-case for having the 'template generating' behaviour.

pennylane/qaoa/layers.py

Co-authored-by: Josh Izaac <josh146@gmail.com>

trbromley

Thanks @Lucaman99, just left a few comments but looks good!

pennylane/qaoa/layers.py

trbromley · 2020-08-06T19:00:45Z

pennylane/qaoa/layers.py

+    return val
+
+
+def cost_layer(gamma, hamiltonian):


I know it is already an abuse of style, but the other templates use the "CapitalizedWords" class-like case style (even though they are also functions). Is there a motivation for breaking this tradition here?

This is a good point! @ixfoduap @josh146 what do you think?

@trbromley I talked to Josh, and he is Ok with either. @ixfoduap do you have a preference?

Thanks for the suggestion Tom! I view this more as a function inside qaoa than a template, so I prefer not using capitalized naming here

trbromley · 2020-08-06T19:06:34Z

tests/test_qaoa.py

+            ],
+            [
+                qaoa.xy_mixer(Graph([(0, 1), (1, 2), (2, 0)])),
+                [qml.PauliRot(1, "XX", wires=[0, 1]), qml.PauliRot(1, "YY", wires=[0, 1]),


Nice, I never saw this class before! Maybe we need to add it to https://pennylane.readthedocs.io/en/latest/introduction/operations.html

Yes, definitely! MultiRZ should probably be added as well

@trbromley @Lucaman99 Let's make sure that the documentation is fully up to date next week during the feature freeze :)

@josh146 Sounds good!

Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com>

ixfoduap · 2020-08-07T15:39:02Z

pennylane/qaoa/layers.py

+    return val
+
+
+def cost_layer(gamma, hamiltonian):


Thanks for the suggestion Tom! I view this more as a function inside qaoa than a template, so I prefer not using capitalized naming here

trbromley

Great job!

Lucaman99 added 8 commits July 22, 2020 14:44

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Lucaman99 added 5 commits July 27, 2020 12:37

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Lucaman99 added 7 commits July 30, 2020 13:18

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Lucaman99 added 3 commits July 31, 2020 11:14

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ixfoduap requested review from josh146, trbromley and ixfoduap August 5, 2020 13:28

Lucaman99 added 2 commits August 5, 2020 09:44

remove time evolution

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Merge branch 'master' into layers

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Lucaman99 marked this pull request as ready for review August 5, 2020 13:47

ixfoduap reviewed Aug 5, 2020

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Lucaman99 and others added 2 commits August 5, 2020 17:09

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Co-authored-by: ixfoduap <40441298+ixfoduap@users.noreply.github.com>

Merge branch 'master' into layers

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josh146 approved these changes Aug 6, 2020

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Co-authored-by: Josh Izaac <josh146@gmail.com>

co9olguy added the do not merge ⚠️ Do not merge the pull request until this label is removed label Aug 6, 2020

Lucaman99 added 6 commits August 6, 2020 09:25

code review

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Lucaman99 and others added 5 commits August 6, 2020 15:31

Update pennylane/qaoa/layers.py

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Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com>

Update pennylane/qaoa/layers.py

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Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com>

Merge branch 'master' into layers

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ixfoduap approved these changes Aug 7, 2020

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ixfoduap removed the do not merge ⚠️ Do not merge the pull request until this label is removed label Aug 7, 2020

ixfoduap merged commit 27b1bf6 into PennyLaneAI:master Aug 7, 2020

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[QAOA] Adds Cost and Mixer Layers #720

[QAOA] Adds Cost and Mixer Layers #720

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[QAOA] Adds Cost and Mixer Layers #720

[QAOA] Adds Cost and Mixer Layers #720

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