Fix issue with ScheduleBlock qpy serialization and use_symengine (#11261

)

* Fix issue with ScheduleBlock qpy serialization and use_symengine

This commit fixes an issue with the use_symengine flag on the qpy.dump()
function. Previously in certain conditions a symengine expression that
was listed in the qpy header as being encoded via symengine was
incorrectly being serialized using sympy. This would cause a failure on
deserialialization as the qpy payload was invalid and symengine could
not parse a symengine representation.

This was originally caught during the development of #10902 as that
switches the default of use_symengine to ``True`` as it makes symengine
a hard requirement. This commit splits it out so the isolated fix can be
backported to 0.45.1.

* Skip symengine test if symengine isn't installed

* Fix skip syntax

* Update releasenotes/notes/fix-schedule-qpy-use-symengine-05ae1dfab73e3ff8.yaml

Co-authored-by: Jake Lishman <jake@binhbar.com>

---------

Co-authored-by: Jake Lishman <jake@binhbar.com>
(cherry picked from commit 1dd4f54)

qiskit/qpy/binary_io/schedules.py

@@ -484,7 +484,7 @@ def _dumps_operand(operand, use_symengine):
 def _write_element(file_obj, element, metadata_serializer, use_symengine):
     if isinstance(element, ScheduleBlock):
         common.write_type_key(file_obj, type_keys.Program.SCHEDULE_BLOCK)
-        write_schedule_block(file_obj, element, metadata_serializer)
+        write_schedule_block(file_obj, element, metadata_serializer, use_symengine)
     else:
         type_key = type_keys.ScheduleInstruction.assign(element)
         common.write_type_key(file_obj, type_key)

releasenotes/notes/fix-schedule-qpy-use-symengine-05ae1dfab73e3ff8.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed an issue with :func:`.qpy.dump` which would cause the function to
+    potentially ignore the value of ``use_symengine`` when serializing a
+    :class:`.ScheduleBlock` object.

test/python/qpy/test_block_load_from_qpy.py

@@ -53,10 +53,10 @@
 class QpyScheduleTestCase(QiskitTestCase):
     """QPY schedule testing platform."""
 
-    def assert_roundtrip_equal(self, block):
+    def assert_roundtrip_equal(self, block, use_symengine=False):
         """QPY roundtrip equal test."""
         qpy_file = io.BytesIO()
-        dump(block, qpy_file)
+        dump(block, qpy_file, use_symengine=use_symengine)
         qpy_file.seek(0)
         new_block = load(qpy_file)[0]
 
@@ -278,6 +278,31 @@ def test_bell_schedule(self):
 
         self.assert_roundtrip_equal(test_sched)
 
+    @unittest.skipUnless(_optional.HAS_SYMENGINE, "Symengine required for this test")
+    def test_bell_schedule_use_symengine(self):
+        """Test complex schedule to create a Bell state."""
+        with builder.build() as test_sched:
+            with builder.align_sequential():
+                # H
+                builder.shift_phase(-1.57, DriveChannel(0))
+                builder.play(Drag(160, 0.05, 40, 1.3), DriveChannel(0))
+                builder.shift_phase(-1.57, DriveChannel(0))
+                # ECR
+                with builder.align_left():
+                    builder.play(GaussianSquare(800, 0.05, 64, 544), DriveChannel(1))
+                    builder.play(GaussianSquare(800, 0.22, 64, 544, 2), ControlChannel(0))
+                builder.play(Drag(160, 0.1, 40, 1.5), DriveChannel(0))
+                with builder.align_left():
+                    builder.play(GaussianSquare(800, -0.05, 64, 544), DriveChannel(1))
+                    builder.play(GaussianSquare(800, -0.22, 64, 544, 2), ControlChannel(0))
+                builder.play(Drag(160, 0.1, 40, 1.5), DriveChannel(0))
+                # Measure
+                with builder.align_left():
+                    builder.play(GaussianSquare(8000, 0.2, 64, 7744), MeasureChannel(0))
+                    builder.acquire(8000, AcquireChannel(0), MemorySlot(0))
+
+        self.assert_roundtrip_equal(test_sched, True)
+
     def test_with_acquire_instruction_with_kernel(self):
         """Test a schedblk with acquire instruction with kernel."""
         kernel = Kernel(