Allow circuits using only SI units to pass through TimeUnitConversion (#14027)

* Allow circuits using only SI units to pass through TimeUnitConversion

In f0576fa, the `TimeUnitConversion` pass was modified so that it
raises an exception if a circuit has only SI unit delays with no `dt`
value set, but the intent seemed to be catch cases where SI units were
used alongside instructions with `dt` units. Here the conditions for
raising an exception are modified to allow circuits with all SI unit
delays (which usually only happens when working with a backend like
`AerSimulator`) to pass through.

Closes #14016

* Add TimeUnitConverion tests

* Update test description

* Remove comment

* lint test file

* Refactor TimeUnitConversion for readability

* Move time unit converison tests to test_transpiler

* lint

* lint

Author: wshanks

qiskit/transpiler/passes/scheduling/time_unit_conversion.py

@@ -126,15 +126,19 @@ def run(self, dag: DAGCircuit):
             # Check what units are used in other instructions: dt or SI or mixed
             units_other = inst_durations.units_used()
             unified_unit = self._unified(units_other)
-            if unified_unit == "SI" and not has_dt:
-                time_unit = "s"
-            elif unified_unit == "dt" and not has_si:
-                time_unit = "dt"
-            else:
+            has_si = has_si or unified_unit in {"SI", "mixed"}
+            has_dt = has_dt or unified_unit in {"dt", "mixed"}
+            if has_si and has_dt:
                 raise TranspilerError(
                     "Fail to unify time units. SI units "
                     "and dt unit must not be mixed when dt is not supplied."
                 )
+            if has_si:
+                time_unit = "s"
+            else:
+                # Either dt units were used or no units were used and we
+                # default to dt
+                time_unit = "dt"
         else:
             time_unit = "dt"
 
@@ -154,7 +158,7 @@ def run(self, dag: DAGCircuit):
     @staticmethod
     def _unified(unit_set: Set[str]) -> str:
         if not unit_set:
-            return "dt"
+            return "none"
 
         if len(unit_set) == 1 and "dt" in unit_set:
             return "dt"

test/python/compiler/test_transpiler.py

@@ -1372,6 +1372,28 @@ def test_delay_converts_to_dt(self):
         out = transpile(qc, dt=1e-9, seed_transpiler=42)
         self.assertEqual(out.data[0].operation.unit, "dt")
 
+    def test_delay_converts_to_seconds(self):
+        """Test that a delay instruction is converted to units of seconds when there is no dt."""
+        qc = QuantumCircuit(2)
+        qc.delay(1000, [0], unit="us")
+        qc.x(0)
+
+        # No backend
+        out = transpile([qc, qc], seed_transpiler=42)
+        self.assertEqual(out[0].data[0].operation.unit, "s")
+        self.assertEqual(out[1].data[0].operation.unit, "s")
+        self.assertEqual(out[0].data[0].operation.params[0], 1e-3)
+        self.assertEqual(out[1].data[0].operation.params[0], 1e-3)
+
+        # Backend without dt
+        backend = GenericBackendV2(num_qubits=4)
+        backend.target.dt = None
+        out = transpile([qc, qc], backend, seed_transpiler=42)
+        self.assertEqual(out[0].data[0].operation.unit, "s")
+        self.assertEqual(out[1].data[0].operation.unit, "s")
+        self.assertEqual(out[0].data[0].operation.params[0], 1e-3)
+        self.assertEqual(out[1].data[0].operation.params[0], 1e-3)
+
     def test_delay_converts_expr_to_dt(self):
         """Test that a delay instruction with a duration expression of type Duration
         is converted to units of dt given a backend."""
@@ -1513,6 +1535,21 @@ def test_rejects_negative_delay_expr(self):
                 seed_transpiler=42,
             )
 
+    def test_rejects_mixed_units_delay_without_target_dt(self):
+        """Test that delay instructions with SI and dt units are rejected without dt."""
+        qc = QuantumCircuit(2)
+        qc.delay(10, 1, unit="dt")
+        qc.delay(10, 1, unit="ns")
+
+        backend = GenericBackendV2(num_qubits=2)
+        backend.target.dt = None
+        with self.assertRaisesRegex(TranspilerError, ".*SI units and dt unit must not be mixed"):
+            transpile(
+                qc,
+                backend=backend,
+                seed_transpiler=42,
+            )
+
     def test_rejects_mixed_units_delay_expr_without_target_dt(self):
         """Test that a delay instruction with wall time and cycles without target DT
         is rejected."""