Avoid lossing precision when scaling frequencies (#12392) (#12480)

* Avoid lossing precision when scaling frequencies Classes in pulse_instruction.py scale frequency values to GHz by multipliying `ParameterExpression` with float 1e9. This can lead to numerical errors on some systems using symengine. Instead, this scaling can be done multiplying by integer 10**9. See: #12359 (comment) * Add release note --------- Co-authored-by: Jake Lishman <[email protected]> (cherry picked from commit 96607f6) Co-authored-by: Iyán <[email protected]> Co-authored-by: Luciano Bello <[email protected]>
Qiskit · Jun 4, 2024 · 8db5490 · 8db5490
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commit 8db5490
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diff --git a/qiskit/qobj/converters/pulse_instruction.py b/qiskit/qobj/converters/pulse_instruction.py
@@ -243,7 +243,7 @@ def _convert_set_frequency(
             "name": "setf",
             "t0": time_offset + instruction.start_time,
             "ch": instruction.channel.name,
-            "frequency": instruction.frequency / 1e9,
+            "frequency": instruction.frequency / 10**9,
         }
         return self._qobj_model(**command_dict)
 
@@ -266,7 +266,7 @@ def _convert_shift_frequency(
             "name": "shiftf",
             "t0": time_offset + instruction.start_time,
             "ch": instruction.channel.name,
-            "frequency": instruction.frequency / 1e9,
+            "frequency": instruction.frequency / 10**9,
         }
         return self._qobj_model(**command_dict)
 
@@ -836,7 +836,7 @@ def _convert_setf(
         .. note::
 
             We assume frequency value is expressed in string with "GHz".
-            Operand value is thus scaled by a factor of 1e9.
+            Operand value is thus scaled by a factor of 10^9.
 
         Args:
             instruction: SetFrequency qobj instruction
@@ -845,7 +845,7 @@ def _convert_setf(
             Qiskit Pulse set frequency instructions
         """
         channel = self.get_channel(instruction.ch)
-        frequency = self.disassemble_value(instruction.frequency) * 1e9
+        frequency = self.disassemble_value(instruction.frequency) * 10**9
 
         yield instructions.SetFrequency(frequency, channel)
 
@@ -858,7 +858,7 @@ def _convert_shiftf(
         .. note::
 
             We assume frequency value is expressed in string with "GHz".
-            Operand value is thus scaled by a factor of 1e9.
+            Operand value is thus scaled by a factor of 10^9.
 
         Args:
             instruction: ShiftFrequency qobj instruction
@@ -867,7 +867,7 @@ def _convert_shiftf(
             Qiskit Pulse shift frequency schedule instructions
         """
         channel = self.get_channel(instruction.ch)
-        frequency = self.disassemble_value(instruction.frequency) * 1e9
+        frequency = self.disassemble_value(instruction.frequency) * 10**9
 
         yield instructions.ShiftFrequency(frequency, channel)
 

diff --git a/releasenotes/notes/fix-symbolic-unit-scaling-c3eb4d9be674dfd6.yaml b/releasenotes/notes/fix-symbolic-unit-scaling-c3eb4d9be674dfd6.yaml
@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed a floating-point imprecision when scaling certain pulse units
+    between seconds and nanoseconds.  If the pulse was symbolically defined,
+    an unnecessary floating-point error could be introduced by the scaling
+    for certain builds of ``symengine``, which could manifest in unexpected
+    results once the symbols were fully bound.  See `#12392 <https://github.com/Qiskit/qiskit/pull/12392>`__.