diff --git a/crates/accelerate/src/unitary_synthesis.rs b/crates/accelerate/src/unitary_synthesis.rs
index 0c578e61fce..1e77c0d451c 100644
--- a/crates/accelerate/src/unitary_synthesis.rs
+++ b/crates/accelerate/src/unitary_synthesis.rs
@@ -10,7 +10,7 @@
 // copyright notice, and modified files need to carry a notice indicating
 // that they have been altered from the originals.
 
-// #[cfg(feature = "cache_pygates")]
+#[cfg(feature = "cache_pygates")]
 use std::cell::OnceCell;
 use std::f64::consts::PI;
 use std::hash::Hash;
@@ -60,7 +60,7 @@ enum DecomposerType {
 
 #[derive(Clone, Debug)]
 enum UnitarySynthesisReturnType {
-    DAGType(DAGCircuit),
+    DAGType(Box<DAGCircuit>),
     TwoQSequenceType(TwoQubitUnitarySequence),
 }
 
@@ -185,7 +185,7 @@ fn dag_from_2q_gate_sequence(
             clbits: target_dag.cargs_interner.get_default(),
             params: new_params,
             extra_attrs: None,
-            // #[cfg(feature = "cache_pygates")]
+            #[cfg(feature = "cache_pygates")]
             py_op: OnceCell::new(),
         };
         instructions.push(pi);
@@ -423,7 +423,7 @@ fn py_run_default_main_loop(
                                     UnitarySynthesisReturnType::DAGType(synth_dag) => synth_dag,
                                     UnitarySynthesisReturnType::TwoQSequenceType(
                                         synth_sequence,
-                                    ) => dag_from_2q_gate_sequence(py, synth_sequence)?,
+                                    ) => Box::new(dag_from_2q_gate_sequence(py, synth_sequence)?),
                                 };
 
                                 let _ = out_dag.set_global_phase(add_global_phase(
@@ -1060,7 +1060,7 @@ fn synth_su4(
     };
 
     match preferred_direction {
-        None => Ok(UnitarySynthesisReturnType::DAGType(synth_dag)),
+        None => Ok(UnitarySynthesisReturnType::DAGType(Box::new(synth_dag))),
         Some(preferred_dir) => {
             let mut synth_direction: Option<Vec<u32>> = None;
             for node in synth_dag.topological_op_nodes()? {
@@ -1074,7 +1074,7 @@ fn synth_su4(
             }
             // synth direction is in the relative basis
             match synth_direction {
-                None => Ok(UnitarySynthesisReturnType::DAGType(synth_dag)),
+                None => Ok(UnitarySynthesisReturnType::DAGType(Box::new(synth_dag))),
                 Some(synth_direction) => {
                     let synth_dir = match synth_direction.as_slice() {
                         [0, 1] => true,
@@ -1090,7 +1090,7 @@ fn synth_su4(
                             decomposer_gate_params,
                         )
                     } else {
-                        Ok(UnitarySynthesisReturnType::DAGType(synth_dag))
+                        Ok(UnitarySynthesisReturnType::DAGType(Box::new(synth_dag)))
                     }
                 }
             }
@@ -1227,7 +1227,7 @@ fn reversed_synth_su4(
             let _ = target_dag.push_back(py, inst.clone());
         }
     }
-    Ok(UnitarySynthesisReturnType::DAGType(target_dag))
+    Ok(UnitarySynthesisReturnType::DAGType(Box::new(target_dag)))
 }
 
 #[pymodule]