diff --git a/src/qc_compiler/autotuning.py b/src/qc_compiler/autotuning.py
index 7b17c8c..b909ade 100644
--- a/src/qc_compiler/autotuning.py
+++ b/src/qc_compiler/autotuning.py
@@ -86,13 +86,13 @@ class AutotuneResult:
         top_k_configs: Top-k configurations by estimated fidelity.
     """
 
-    best_config: TranspileConfig = None
+    best_config: TranspileConfig | None = None
     best_estimated_fidelity: float = 0.0
     all_results: dict[str, float] = field(default_factory=dict)
     measured_fidelities: dict[str, float] = field(default_factory=dict)
     circuits_evaluated: int = 0
     search_space_size: int = 0
-    best_circuit: QuantumCircuit = None
+    best_circuit: QuantumCircuit | None = None
     top_k_configs: list[TranspileConfig] = field(default_factory=list)
 
     @property
diff --git a/src/qc_compiler/fusion.py b/src/qc_compiler/fusion.py
index 8927912..d2e00bc 100644
--- a/src/qc_compiler/fusion.py
+++ b/src/qc_compiler/fusion.py
@@ -58,10 +58,10 @@ class FusionResult:
         improvement: Fidelity improvement (after - before).
     """
 
-    original_circuit: QuantumCircuit = None
-    optimized_circuit: QuantumCircuit = None
-    original_metrics: CircuitMetrics = None
-    optimized_metrics: CircuitMetrics = None
+    original_circuit: QuantumCircuit | None = None
+    optimized_circuit: QuantumCircuit | None = None
+    original_metrics: CircuitMetrics | None = None
+    optimized_metrics: CircuitMetrics | None = None
     chains_fused: int = 0
     total_gates_before: int = 0
     total_gates_after: int = 0
diff --git a/src/qc_compiler/scheduling.py b/src/qc_compiler/scheduling.py
index 5779342..a3b2d37 100644
--- a/src/qc_compiler/scheduling.py
+++ b/src/qc_compiler/scheduling.py
@@ -53,7 +53,7 @@ class ScheduleResult:
         depth_optimized: Circuit depth with the chosen method.
     """
 
-    circuit: QuantumCircuit = None
+    circuit: QuantumCircuit | None = None
     idle_times: dict[int, float] = field(default_factory=dict)
     estimated_fidelity_asap: float = 0.0
     estimated_fidelity_alap: float = 0.0
diff --git a/src/qc_compiler/transpiler.py b/src/qc_compiler/transpiler.py
index 7686f7f..8d30485 100644
--- a/src/qc_compiler/transpiler.py
+++ b/src/qc_compiler/transpiler.py
@@ -79,8 +79,8 @@ class QCompilerResult:
         config: The OptimizerConfig used.
     """
 
-    original_circuit: QuantumCircuit = None
-    optimized_circuit: QuantumCircuit = None
+    original_circuit: QuantumCircuit | None = None
+    optimized_circuit: QuantumCircuit | None = None
     fidelity_before: float = 0.0
     fidelity_after: float = 0.0
     fusion_result: FusionResult | None = None
@@ -91,7 +91,7 @@ class QCompilerResult:
     batch_plan: BatchPlan | None = None
     subcircuits: list[QuantumCircuit] | None = None
     passes_applied: list[str] = field(default_factory=list)
-    config: OptimizerConfig = None
+    config: OptimizerConfig | None = None
 
     @property
     def fidelity_improvement(self) -> float: