Fix `TrotterProduct` differentiability with `parameter-shift` #6432

andrijapau · 2024-10-22T16:46:48Z

Context:

Prior to this fix, differentiating TrotterProduct with diff_method="parameter-shift" returned zeros (which was inconsistent with results when using diff_method="backprop").

Description of the Change:

Set TrotterProduct.grad_method=None and Exp.grad_method=None. This results in,

@qml.qnode(dev, diff_method="parameter-shift")
def circ(time, coeffs):
    h = qml.dot(coeffs, [qml.PauliX(0), qml.PauliY(0), qml.PauliZ(1)])
    qml.TrotterProduct(h, time, n=10, order=4)
    return qml.expval(qml.Hadamard(0))

>>> time = qml.numpy.array(4.2)
>>> coeffs = qml.numpy.array([2.0, 2.0, 2.0])
>>> qml.jacobian(circ)(time,coeffs)
(array(3.27988133), array([3.31863668, 3.56911412, 0.        ]))

Benefits: Gradient results are now consistent with backprop.

Possible Drawbacks: None

Related GitHub Issues: Fixes #6333

[sc-74923]

github-actions · 2024-10-22T16:47:04Z

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Jaybsoni · 2024-10-22T16:54:25Z

Out of curiosity, why did we have to manually set the grad_method? Isn't the inherited gard_method variable None? In which cases should we manually specify the grad_method argument?

andrijapau · 2024-10-22T17:04:19Z

Hey @Jaybsoni,

Good question. I tagged you on the relevant thread on Slack if you want to read the full conversation regarding issues #6333 and #6331. 😅

TLDR: It seems that parameter-shift tries to update the parameters and fails to do so. Setting grad_method=None forces it to decompose the operator before differentiating it's data. This does suggest some areas of improvement for Operator.grad_method.

tests/ops/functions/conftest.py

Co-authored-by: Christina Lee <[email protected]>

albi3ro · 2024-10-25T20:14:26Z

tests/ops/functions/conftest.py

@@ -57,7 +57,9 @@
    (qml.s_prod(1.1, qml.RX(1.1, 0)), {}),
    (qml.prod(qml.PauliX(0), qml.PauliY(1), qml.PauliZ(0)), {}),
    (qml.ctrl(qml.RX(1.1, 0), 1), {}),
-    (qml.exp(qml.PauliX(0), 1.1), {}),
+    (qml.exp(qml.PauliX(0), 1.1), {"skip_differentiation": True}),
+    (qml.exp(qml.PauliX(0), 2.9j), {}),


I'd be ok with commenting this line out till we figure out what to do with the generator of Exp.

Suggested change

(qml.exp(qml.PauliX(0), 2.9j), {}),

#(qml.exp(qml.PauliX(0), 2.9j), {}),

I can xfail it and create an issue to track it?

tests/templates/test_subroutines/test_trotter.py

Jaybsoni · 2024-11-23T23:10:54Z

tests/templates/test_subroutines/test_trotter.py

    @pytest.mark.parametrize("hamiltonian", test_hamiltonians)
    def test_standard_validity(self, hamiltonian):
        """Test standard validity criteria using assert_valid."""
        time, n, order = (4.2, 10, 4)
        op = qml.TrotterProduct(hamiltonian, time, n=n, order=order)
-        qml.ops.functions.assert_valid(op)
+        qml.ops.functions.assert_valid(op, skip_differentiation=True)


Why does the assert valid test for differentiation fail? I would hope this bug fix would allow that test to pass?

@Jaybsoni , I'm slowly re-visiting this PR but the reason was due to Exp also having differentiability issues lol.

codecov · 2025-01-02T17:41:10Z

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.60%. Comparing base (37abd58) to head (ea533bd).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6432   +/-   ##
=======================================
  Coverage   99.60%   99.60%           
=======================================
  Files         476      476           
  Lines       45210    45211    +1     
=======================================
+ Hits        45033    45034    +1     
  Misses        177      177

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parameter-shift matches backprop

ec9d121

andrijapau marked this pull request as ready for review October 22, 2024 16:46

add changelog entry

96bc0a0

andrijapau self-assigned this Oct 23, 2024

andrijapau added 2 commits October 23, 2024 15:18

skip diff for test case that is not unitary

2aa4d7f

Merge branch 'master' into 6333-bug-trotter-diff

77e6e05

albi3ro reviewed Oct 25, 2024

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tests/ops/functions/conftest.py Show resolved Hide resolved

andrijapau and others added 2 commits October 25, 2024 11:43

Update tests/ops/functions/conftest.py

60518ed

Co-authored-by: Christina Lee <[email protected]>

typo in committed suggestion

8d095d7

albi3ro reviewed Oct 25, 2024

View reviewed changes

andrijapau added 2 commits October 30, 2024 17:18

skip diff and add seperate custom diff check

03853c5

comment out failing test case

c47adbb

andrijapau requested a review from lillian542 October 31, 2024 17:09

andrijapau changed the title ~~Fix TrotterProduct differentiability with parameter-shift~~ Fix TrotterProduct differentiability with parameter-shift bug Nov 7, 2024

Merge branch 'master' into 6333-bug-trotter-diff [skip-ci]

f671300

andrijapau requested a review from albi3ro November 8, 2024 16:42

Jaybsoni reviewed Nov 23, 2024

View reviewed changes

tests/templates/test_subroutines/test_trotter.py Outdated Show resolved Hide resolved

Jaybsoni reviewed Nov 23, 2024

View reviewed changes

andrijapau changed the title ~~Fix TrotterProduct differentiability with parameter-shift bug~~ [WIP] Fix TrotterProduct differentiability with parameter-shift bug Dec 12, 2024

andrijapau added the WIP 🚧 Work-in-progress label Jan 2, 2025

andrijapau changed the title ~~[WIP] Fix TrotterProduct differentiability with parameter-shift bug~~ Fix TrotterProduct differentiability with parameter-shift bug Jan 2, 2025

andrijapau changed the title ~~Fix TrotterProduct differentiability with parameter-shift bug~~ Fix TrotterProduct differentiability with parameter-shift Jan 2, 2025

andrijapau added 4 commits January 2, 2025 12:52

Merge branch 'master' into 6333-bug-trotter-diff

e84c3f5

fix: remove xfail from test_exp.py

f0753cd

fix: Update test_differentiation to not use private _coeffs

b005e6a

fix: Remove grad_method=None addition from trotter.py

ea533bd

andrijapau added 2 commits January 2, 2025 16:04

fix: Remove grad_method=None addition from exp.py

d074068

fix: Update test_trotter.py differentiation test

0314e41

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Fix `TrotterProduct` differentiability with `parameter-shift` #6432

Fix `TrotterProduct` differentiability with `parameter-shift` #6432

andrijapau commented Oct 22, 2024 •

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	(qml.exp(qml.PauliX(0), 2.9j), {}),
	#(qml.exp(qml.PauliX(0), 2.9j), {}),

Fix TrotterProduct differentiability with parameter-shift #6432

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Fix TrotterProduct differentiability with parameter-shift #6432

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Fix `TrotterProduct` differentiability with `parameter-shift` #6432

Fix `TrotterProduct` differentiability with `parameter-shift` #6432

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