Add SparseObservable support to evolved_operator_ansatz #15442

akhil-102 · 2025-12-13T20:58:50Z

PR Summary: Add SparseObservable Support to Evolved Operator Ansatz

Overview

Adds support for SparseObservable in evolved_operator_ansatz, enabling the Rust-accelerated path for better performance.

Changes

evolved_operator_ansatz.py:

Added conditional import and handling for SparseObservable
Extended Rust fast path to accept SparseObservable (in addition to SparsePauliOp)
Fixed _is_pauli_identity() to handle SparseObservable with coefficient checks
Fixed edge cases: return empty circuit when all operators are filtered out, handle string/list parameter_prefix in _remove_identities()

test_evolved_op_ansatz.py:

Added tests for SparseObservable acceptance and equivalence
New test class TestEvolvedOperatorAnsatzSparseObservable covering Rust path, identity removal, mixed operators, and equivalence with SparsePauliOp

- Add conditional import and handling for SparseObservable - Extend Rust fast path to accept SparseObservable (in addition to SparsePauliOp) - Enhance _is_pauli_identity() to handle SparseObservable with coefficient checks - Fix edge cases: return empty circuit when all operators filtered, handle string/list parameter_prefix - Add comprehensive test coverage for SparseObservable support

qiskit-bot · 2025-12-13T20:58:54Z

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One or more of the following people are relevant to this code:

@Cryoris
@Qiskit/terra-core
@ajavadia

CLAassistant · 2025-12-13T20:58:55Z

All committers have signed the CLA.

Cryoris

This PR is over-engineering the solution a lot, please check the comments below to simplify the code.

qiskit/circuit/library/n_local/evolved_operator_ansatz.py

- Use direct import of SparseObservable (no try-except) - Simplify identity check: use num_terms == 1 and len(bit_labels()) == 0 - Fix empty circuit handling: return circuit with correct num_qubits when all operators are identities - Add SparseObservable support to fast Rust path - Fix prefix handling to support both string and list types - Add comprehensive test cases for all changes

akhil-102 · 2025-12-18T06:18:02Z

Hi, I’ve updated and simplified the code according to your comments.

Cryoris

Thanks for the updates @akhil-102, the approach looks good now! Also great catch on the bug with the string-based parameter prefix. It would be great to separate the fix into a different PR, if you'd be interested in doing that -- see also the comments below.

Cryoris · 2025-12-18T08:41:13Z

test/python/circuit/library/test_evolved_op_ansatz.py

+    def setUp(self):
+        """Set up test fixtures."""
+        super().setUp()
+        try:
+            from qiskit.quantum_info import SparseObservable
+            self.SparseObservable = SparseObservable
+        except ImportError:
+            self.SparseObservable = None
+


When running the test locally, do you need this try-except for the import? If qiskit is installed we shouldn't need this and just have a global

from qiskit.quantum_info import SparseObservable

import

Cryoris · 2025-12-18T08:51:35Z

qiskit/circuit/library/n_local/evolved_operator_ansatz.py

        operators, parameter_prefix = _remove_identities(operators, parameter_prefix)

-    if any(op.num_qubits != num_qubits for op in operators):
+    # After removing identities, update num_operators to reflect the actual count


We could optimize this a bit: first, we can change to update num_operators only inside the if remove_identities statement (otherwise the number of operators won't change). Then, if num_operators is in fact 0 after the update, we can directly return an empty circuit of size num_qubits (also still inside the if-statement). With this we are sure that num_operators > 0 in the rest of the code and we don't have to check that anymore.

Cryoris · 2025-12-18T08:52:44Z

qiskit/circuit/library/n_local/evolved_operator_ansatz.py

        and evolution is None
-        and all(isinstance(op, SparsePauliOp) for op in operators)
+        and all(
+            isinstance(op, SparsePauliOp) or isinstance(op, SparseObservable)


You can merge two isinstance checks as

Suggested change

isinstance(op, SparsePauliOp) or isinstance(op, SparseObservable)

isinstance(op, (SparsePauliOp, SparseObservable))

(this also occurs somewhere else in the code)

Cryoris · 2025-12-18T09:02:18Z

qiskit/circuit/library/n_local/evolved_operator_ansatz.py


    cleaned_ops = [op for i, op in enumerate(operators) if i not in identity_ops]
-    cleaned_prefix = [prefix for i, prefix in enumerate(prefixes) if i not in identity_ops]
+    # Handle both string and list prefixes


Nice catch! This fixes a bug we had before. It would be nice to backport this fix to the 2.3 series. Would you be up for splitting this fix into a separate PR (including a test) that we merge separately from this? If not no worries, I can do a small follow up to fix it in 2.3.

Cryoris · 2025-12-18T09:03:58Z

qiskit/circuit/library/n_local/evolved_operator_ansatz.py

+        cleaned_prefix = [prefix for i, prefix in enumerate(prefixes) if i not in identity_ops]

-    return cleaned_ops, cleaned_prefix
+    return cleaned_ops, cleaned_prefix


Could we leave the final empty lines in place? 🙂

Cryoris · 2025-12-18T09:05:07Z

test/python/circuit/library/test_evolved_op_ansatz.py

+        self.assertTrue(all("theta" in name for name in param_names))
+
+
+class TestEvolvedOperatorAnsatzSparseObservable(QiskitTestCase):


We don't really need a new test class for this, could you just add the functions to the existing test class above?

Cryoris · 2025-12-18T09:06:39Z

test/python/circuit/library/test_evolved_op_ansatz.py

+        self.assertEqual(ansatz.num_qubits, 1)
+        self.assertEqual(ansatz.num_parameters, 1)
+
+    def test_sparse_observable_identity_detection(self):


I think we can remove this test, we don't usually test internal, private functions since they are subject to change and instead focus on testing the user API. Since you have other tests that check identities are being removed, I would say it's safe to say the internal function works

akhil-102 requested a review from a team as a code owner December 13, 2025 20:58

qiskit-bot added the Community PR PRs from contributors that are not 'members' of the Qiskit repo label Dec 13, 2025

akhil-102 mentioned this pull request Dec 13, 2025

Fast Rust path in evolved_operator_ansatz should support SparseObservable #15433

Open

Cryoris reviewed Dec 16, 2025

View reviewed changes

akhil-102 added 2 commits December 17, 2025 06:20

Fix import after removing try

d933668

Cryoris reviewed Dec 18, 2025

View reviewed changes

	isinstance(op, SparsePauliOp) or isinstance(op, SparseObservable)
	isinstance(op, (SparsePauliOp, SparseObservable))

		self.assertTrue(all("theta" in name for name in param_names))


		class TestEvolvedOperatorAnsatzSparseObservable(QiskitTestCase):

Add SparseObservable support to evolved_operator_ansatz #15442

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Add SparseObservable support to evolved_operator_ansatz #15442

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akhil-102 commented Dec 13, 2025

PR Summary: Add SparseObservable Support to Evolved Operator Ansatz

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Changes

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CLAassistant commented Dec 13, 2025 •

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