Fix the exact simulator to error on classical conditions (#200)

Partially addresses #180

This also adds a test to make sure the simulator rejects a circuit
with classical control flow, for now.

circuit_knitting_toolbox/utils/simulation.py

@@ -49,6 +49,10 @@ def simulate_statevector_outcomes(qc: QuantumCircuit, /) -> dict[int, float]:
     current = defaultdict(list)
     current[0].append((1.0, Statevector.from_int(0, 2**qc.num_qubits)))
     for inst in qc.data:
+        if inst.operation.condition_bits:
+            raise ValueError(
+                "Operations conditioned on classical bits are currently not supported."
+            )
         opname = inst.operation.name
         if opname in ("measure", "reset"):
             # The current instruction is not unitary: it's either a measurement
@@ -111,7 +115,7 @@ def simulate_statevector_outcomes(qc: QuantumCircuit, /) -> dict[int, float]:
             if len(inst.clbits) != 0:
                 raise ValueError(
                     "Circuit cannot contain a non-measurement operation on classical bit(s)."
-                )  # pragma: no cover
+                )
             # Evolve each statevector according to the current instruction
             for svs in current.values():
                 for _, sv in svs:

test/utils/test_simulation.py

@@ -74,6 +74,32 @@ def test_simulate_statevector_outcomes(self):
             assert r[0] == pytest.approx(0.5)
             assert r[1] == pytest.approx(0.5)
 
+        with self.subTest("Circuit with control flow"):
+            qc = QuantumCircuit(2, 1)
+            with qc.for_loop(range(5)):
+                qc.h(0)
+                qc.cx(0, 1)
+                qc.measure(0, 0)
+                qc.break_loop().c_if(0, True)
+            with pytest.raises(ValueError) as e_info:
+                simulate_statevector_outcomes(qc)
+            assert (
+                e_info.value.args[0]
+                == "Circuit cannot contain a non-measurement operation on classical bit(s)."
+            )
+
+        with self.subTest("Circuit with condition bits"):
+            qc = QuantumCircuit(2, 1)
+            qc.h(0)
+            qc.measure(0, 0)
+            qc.x(1).c_if(0, True)
+            with pytest.raises(ValueError) as e_info:
+                simulate_statevector_outcomes(qc)
+            assert (
+                e_info.value.args[0]
+                == "Operations conditioned on classical bits are currently not supported."
+            )
+
     def test_exact_sampler(self):
         with self.subTest("Mid-circuit measurement, etc."):
             qc = QuantumCircuit(2, 2)