Add template to prepare quantum states of molecules using the excitation operations #1383

agran2018 · 2021-06-03T20:19:40Z

Context:
This PR adds the new template AllSinglesDoubles which uses the SingleExcitation and DoubleExcitation operations recently implemented in the PRs 1121 and 1123 to prepare correlated states of molecules.

Description of the Change:
Add the AllSinglesDoubles class in the file ~/templates/subroutines/all_singles_doubles.py and the corresponding unit tests in ~/tests/templates/test_subroutines/test_all_singles_doubles.py

Benefits:
The new template reduces significantly the number of operations and the depth of the quantum circuit with respect to the traditional UCCSD unitary.

codecov · 2021-06-03T20:31:05Z

Codecov Report

Merging #1383 (74b4a57) into master (c16d59c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1383   +/-   ##
=======================================
  Coverage   98.19%   98.20%           
=======================================
  Files         157      158    +1     
  Lines       11729    11780   +51     
=======================================
+ Hits        11517    11568   +51     
  Misses        212      212

Impacted Files	Coverage Δ
pennylane/templates/subroutines/__init__.py	`100.00% <100.00%> (ø)`
...ylane/templates/subroutines/all_singles_doubles.py	`100.00% <100.00%> (ø)`

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ixfoduap

Docstrings need changes and so do the arguments of the template, which I think should be just number of qubits and number of particles.

I'll look at tests once these changes are made, since they will affect testing

.github/CHANGELOG.md

ixfoduap · 2021-06-04T18:56:44Z

.github/CHANGELOG.md

+  The new template reduces significantly the number of operations
+  and the depth of the quantum circuit with respect to the traditional UCCSD
+  unitary. The performance of quantum chemistry simulations should benefit
+  from this new implementation. 


You can remove the last sentence, which is redundant: it's of course beneficial to significantly reduce resource requirements.

It would be good to add a short snippet of example usage, as we do for other contributions

pennylane/templates/subroutines/__init__.py

ixfoduap · 2021-06-04T19:01:51Z

pennylane/templates/subroutines/all_singles_doubles.py

+
+
+class AllSinglesDoubles(Operation):
+    r"""Apply the particle-conserving :class:`~.pennylane.SingleExcitation` and


This is not clear enough in saying that the template builds a circuit consisting of ALL such excitation gates, which conserve spin

ixfoduap · 2021-06-04T19:08:32Z

pennylane/templates/subroutines/all_singles_doubles.py

+    :class:`~.pennylane.DoubleExcitation` operations, implemented as Givens rotations,
+    to prepare quantum states of molecules beyond the Hartree-Fock approximation.
+
+    In the Jordan-Wigner representation the qubit state :math:`\vert 0 \rangle` or


I think this paragraph is trying to explain too much and it's probably better to remove.

We can direct the user to the documentation for the excitation gates, we don't need to re-explain everything here

pennylane/templates/subroutines/all_singles_doubles.py

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

…AI/pennylane into template-excitation-operations

ixfoduap

Looks good to me! 🚀

Only main suggestion is whether we want to add a shape method, but this may not be necessary since the users get a clear error when passing the wrong shape of weights

.github/CHANGELOG.md

ixfoduap · 2021-06-08T15:20:17Z

.github/CHANGELOG.md

+
+  @qml.qnode(dev)
+  def circuit(weights, hf_state, singles, doubles):
+      qml.templates.AllSinglesDoubles(weights, wires, hf_state, singles, doubles)


This is really long! I guess there's not much we can do about it, but it's a shame

Could import the template and save the qml.templates part? I think we often do that in docstrings

pennylane/templates/subroutines/all_singles_doubles.py

ixfoduap · 2021-06-08T15:27:24Z

pennylane/templates/subroutines/all_singles_doubles.py

+        # we can extract the numpy representation here
+        # since hf_state can never be differentiable


This comment doesn't fit in one line?

ixfoduap · 2021-06-08T15:27:44Z

pennylane/templates/subroutines/all_singles_doubles.py

+
+        # we can extract the numpy representation here
+        # since hf_state can never be differentiable
+        self.hf_state = qml.math.toarray(hf_state)


Nice! I need to get used to using qml.math

pennylane/templates/subroutines/all_singles_doubles.py

ixfoduap · 2021-06-08T15:36:22Z

pennylane/templates/subroutines/all_singles_doubles.py

+                    )
+                )
+
+        shape = qml.math.shape(weights)


This is optional, but we could consider adding a shape() method that the user can employ to obtain the shape of the input parameters, as is done here
https://pennylane.readthedocs.io/en/latest/_modules/pennylane/templates/layers/cv_neural_net.html#CVNeuralNetLayers.shape

Actually, I am strongly in favour of shape() being added here. It is frustrating from a user point of view for some templates to have this method, and some to not :)

If I understand correctly, do we want to add a shape() method that receives the lists singles and doubles and returns ``shape = (len(singles) + len(doubles)) ?
@mariaschuld your instructions here would be valuable :-).

Yes, the shape method would have the @staticmethod decorator and does not take self, which means that all information it needs have to be derived from the argument that shape() gets (since you cannot do self.xyz).

If the shape can be derived from anything other than singles and doubles, like the number of qubits or something the user should know about the circuit, I would prefer this - especially if you make the singles and doubles optional as discussed above!

mariaschuld · 2021-06-09T13:51:37Z

pennylane/templates/subroutines/all_singles_doubles.py

+                "'singles' and 'doubles' lists can not be both empty; got singles = {}, doubles = {}".format(
+                    singles, doubles
+                )
+            )


It's still a matter of taste, but we are trying to keep the checks to an absolute minimum, because they are used in every qnode call...

My thinking is that we should check tensor dimensions (since this is an annoying error to see), and things that are theoretically tricky. So if this check is something a user with a look at the docs can get right, maybe we should delete it?

Non-blocking comment though

Good point. My concern in removing this check is that the user may want to call this template without passing singles and doubles. In this case, the template will not raise an error, will only prepare the HF state and numbers will come out (even though the state the user is preparing is trivial). I vote to keep it :-).

I didn't know that not a evaluates to True if a=None, interesting!

mariaschuld

A very nice PR. I am only requesting changes because I'd like to

have the shape method added (plus a test)
if it makes sense, make singles and doubles an optional argument as proposed by JM
If you have concerns about any of these, I am happy to be convinced otherwise :)

pennylane/templates/subroutines/all_singles_doubles.py

Co-authored-by: Maria Schuld <mariaschuld@gmail.com>

…AI/pennylane into template-excitation-operations

pennylane/templates/subroutines/all_singles_doubles.py

mariaschuld

Sorry, one last round: I think there is a twister in the shape function, and you forgot to test it (I think) :)

Else I agree with all your arguments!

Co-authored-by: Maria Schuld <mariaschuld@gmail.com>

pennylane/templates/subroutines/all_singles_doubles.py

Co-authored-by: Maria Schuld <mariaschuld@gmail.com>

This reverts commit 7798409.

agran2018 and others added 7 commits June 2, 2021 16:42

Add new template AllSinglesDoubles

6cbfbe9

Add unit tests

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Import the template in __init__.py

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Add docstrings

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Modify docstrings

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Merge branch 'master' into template-excitation-operations

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Black unit tests file

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agran2018 mentioned this pull request Jun 3, 2021

Refactor UCCSD template using the excitation operations #1370

Closed

agran2018 self-assigned this Jun 3, 2021

agran2018 added qchem ⚛️ Related to the QChem package templates 📁 Updates and additions to the templates labels Jun 3, 2021

agran2018 added 5 commits June 3, 2021 16:50

Modify the docstrings

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Update CHANGELOG.md

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Fix the order of the template arguments in the docstring

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Add template to the documentation

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Fix name of the template in templates.rst

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ixfoduap self-requested a review June 4, 2021 16:02

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Modify docstrings according to suggestions

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Merge branch 'template-excitation-operations' of github.com:PennyLane…

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Polish docstrings

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Add example to the CHANGELOG.md file

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Merge branch 'master' into template-excitation-operations

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Modify the example in the CHANGELOG.md

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Merge branch 'master' into template-excitation-operations

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Update .github/CHANGELOG.md

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Add 'shape' method

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Modify 'shape' function

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Merge branch 'master' into template-excitation-operations

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Merge branch 'template-excitation-operations' of github.com:PennyLane…

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Add template to prepare quantum states of molecules using the excitation operations #1383

Add template to prepare quantum states of molecules using the excitation operations #1383

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		class AllSinglesDoubles(Operation):
		r"""Apply the particle-conserving :class:`~.pennylane.SingleExcitation` and

		# we can extract the numpy representation here
		# since hf_state can never be differentiable

Add template to prepare quantum states of molecules using the excitation operations #1383

Add template to prepare quantum states of molecules using the excitation operations #1383

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