diff --git a/torchquantum/operator/op_types.py b/torchquantum/operator/op_types.py
index bdf35337..98f21c6e 100644
--- a/torchquantum/operator/op_types.py
+++ b/torchquantum/operator/op_types.py
@@ -7,6 +7,7 @@
 from ..macro import C_DTYPE, F_DTYPE
 from typing import Iterable, Union, List
 from enum import IntEnum
+from torchquantum.util.quantization.clifford_quantization import CliffordQuantizer
 
 __all__ = [
     "Operator",