add comment on controlled unitary failure

qualtran/_infra/controlled.py

@@ -458,12 +458,19 @@ def _unitary_(self):
             # subbloq is a cirq gate, use the cirq-style API to derive a unitary.
             import cirq
 
+            # TODO It would be ideal to use `tensor_contract` always,
+            #      but at the moment it's about 5-10x slower than `cirq.unitary`.
+            #      So we default to `cirq.unitary`, and only use `tensor_contract` if it fails.
+            #      https://github.com/quantumlib/Qualtran/issues/1336
+            # TODO `cirq.ControlledGate` fails to correctly verify `subbloq` using
+            #      a compute-uncompute `And` pair is unitary.
+            #      https://github.com/quantumlib/Qualtran/issues/1488
             try:
                 return cirq.unitary(
                     cirq.ControlledGate(self.subbloq, control_values=self.ctrl_spec.to_cirq_cv())
                 )
             except ValueError:
-                pass
+                pass  # use the tensor contraction instead
         if all(reg.side == Side.THRU for reg in self.subbloq.signature):
             # subbloq has only THRU registers, so the tensor contraction corresponds
             # to a unitary matrix.