From 0acdf15ecd0786aa5b8c96d9c9c3b53bb15bbe11 Mon Sep 17 00:00:00 2001
From: Daniel Puzzuoli <dan.puzzuoli@gmail.com>
Date: Mon, 22 Jan 2024 14:28:31 -0800
Subject: [PATCH] Fix perturbation label re-ordering bug (#307)

---
 .../perturbation/solve_lmde_perturbation.py   |  5 +-
 ...rbation-ordering-bug-efa3f6ce7559d2b9.yaml |  6 ++
 .../test_solve_lmde_perturbation.py           | 75 +++++++++++++++++++
 3 files changed, 84 insertions(+), 2 deletions(-)
 create mode 100644 releasenotes/notes/perturbation-ordering-bug-efa3f6ce7559d2b9.yaml

diff --git a/qiskit_dynamics/perturbation/solve_lmde_perturbation.py b/qiskit_dynamics/perturbation/solve_lmde_perturbation.py
index 745bb801a..4441865dd 100644
--- a/qiskit_dynamics/perturbation/solve_lmde_perturbation.py
+++ b/qiskit_dynamics/perturbation/solve_lmde_perturbation.py
@@ -230,8 +230,9 @@ def solve_lmde_perturbation(
         else:
             # validate perturbation_labels
             perturbations_len = len(perturbation_labels)
-            perturbation_labels = _clean_multisets(perturbation_labels)
-            if len(perturbation_labels) != perturbations_len:
+            perturbation_labels = [Multiset(x) for x in perturbation_labels]
+            cleaned_perturbation_labels = _clean_multisets(perturbation_labels)
+            if len(cleaned_perturbation_labels) != perturbations_len:
                 raise QiskitError("perturbation_labels argument contains duplicates as multisets.")
 
         expansion_labels = _merge_multiset_expansion_order_labels(
diff --git a/releasenotes/notes/perturbation-ordering-bug-efa3f6ce7559d2b9.yaml b/releasenotes/notes/perturbation-ordering-bug-efa3f6ce7559d2b9.yaml
new file mode 100644
index 000000000..84f352d4b
--- /dev/null
+++ b/releasenotes/notes/perturbation-ordering-bug-efa3f6ce7559d2b9.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes a bug in :func:`.solve_lmde_perturbation` in which the ``perturbation_labels`` argument
+    was being unexpectedly re-ordered, leading to errors when retrieving results.
+    
diff --git a/test/dynamics/perturbation/test_solve_lmde_perturbation.py b/test/dynamics/perturbation/test_solve_lmde_perturbation.py
index 1e70a556d..fca649083 100644
--- a/test/dynamics/perturbation/test_solve_lmde_perturbation.py
+++ b/test/dynamics/perturbation/test_solve_lmde_perturbation.py
@@ -576,6 +576,81 @@ def A1(t):
         self.assertAllClose(expected_D0001, results.perturbation_data.get_item([1, 1, 1, 10])[-1])
         self.assertAllClose(expected_D0011, results.perturbation_data.get_item([1, 1, 10, 10])[-1])
 
+    def test_dyson_analytic_case1_1d_reverse_order_labeled(self):
+        """This is the same numerical test as test_dyson_analytic_case1_1d, however it relabels
+        the perturbation indices as 0 -> 1, and 1 -> 0. This is an integration test that would
+        have caught multiset sorting bug that broke solve_lmde_perturbation results computation.
+        """
+
+        def generator(t):
+            return Array([[1, 0], [0, 1]], dtype=complex).data
+
+        def A0(t):
+            return Array([[0, t], [t**2, 0]], dtype=complex).data
+
+        def A1(t):
+            return Array([[t, 0], [0, t**2]], dtype=complex).data
+
+        T = np.pi * 1.2341
+
+        results = solve_lmde_perturbation(
+            perturbations=[A0, A1],
+            t_span=[0, T],
+            generator=generator,
+            y0=np.array([0.0, 1.0], dtype=complex),
+            expansion_method="dyson",
+            expansion_order=2,
+            expansion_labels=[[1, 1, 0], [1, 1, 1, 0], [1, 1, 0, 0]],
+            perturbation_labels=[[1], [0]],
+            dyson_in_frame=False,
+            integration_method=self.integration_method,
+            atol=1e-13,
+            rtol=1e-13,
+        )
+
+        T2 = T**2
+        T3 = T * T2
+        T4 = T * T3
+        T5 = T * T4
+        T6 = T * T5
+        T7 = T * T6
+        T8 = T * T7
+        T9 = T * T8
+        T10 = T * T9
+        T11 = T * T10
+
+        U = expm(np.array(generator(0)) * T)
+
+        expected_D0 = U @ np.array([[T2 / 2], [0]], dtype=complex)
+        expected_D1 = U @ np.array([[0], [T3 / 3]], dtype=complex)
+        expected_D00 = U @ np.array([[0], [T5 / 10]], dtype=complex)
+        expected_D01 = U @ (
+            np.array([[T5 / 15], [0]], dtype=complex) + np.array([[T4 / 8], [0]], dtype=complex)
+        )
+        expected_D11 = U @ np.array([[0], [T6 / 18]], dtype=complex)
+        expected_D001 = U @ (
+            np.array([[0], [T8 / 120]], dtype=complex)
+            + np.array([[0], [T7 / 56]], dtype=complex)
+            + np.array([[0], [T8 / 80]], dtype=complex)
+        )
+        expected_D0001 = U @ np.array([[(T10 / 480) + (T9 / 280)], [0]], dtype=complex)
+        expected_D0011 = U @ np.array(
+            [
+                [0],
+                [(T11 / 396) + 23 * (T10 / 8400) + (T9 / 432)],
+            ],
+            dtype=complex,
+        )
+
+        self.assertAllClose(expected_D0, results.perturbation_data.get_item([1])[-1])
+        self.assertAllClose(expected_D1, results.perturbation_data.get_item([0])[-1])
+        self.assertAllClose(expected_D00, results.perturbation_data.get_item([1, 1])[-1])
+        self.assertAllClose(expected_D01, results.perturbation_data.get_item([1, 0])[-1])
+        self.assertAllClose(expected_D11, results.perturbation_data.get_item([0, 0])[-1])
+        self.assertAllClose(expected_D001, results.perturbation_data.get_item([1, 1, 0])[-1])
+        self.assertAllClose(expected_D0001, results.perturbation_data.get_item([1, 1, 1, 0])[-1])
+        self.assertAllClose(expected_D0011, results.perturbation_data.get_item([1, 1, 0, 0])[-1])
+
     def test_dyson_analytic_case1(self):
         """Analytic test of computing dyson terms.