Add __copy__ for Gate and QubitCircuit

src/qutip_qip/circuit/circuit.py

@@ -112,6 +112,23 @@ def __init__(
                     "{{str: gate_function}}, not {}".format(user_gates)
                 )
 
+    def _create_empty_copy(self):
+        new_qc = QubitCircuit(
+            N=self.N,
+            input_states=self.input_states,
+            output_states=self.output_states,
+            reverse_states=self.reverse_states,
+            user_gates=self.user_gates,
+            dims=self.dims,
+            num_cbits=self.num_cbits,
+        )
+        return new_qc
+
+    def __copy__(self):
+        new_qc = self._create_empty_copy()
+        new_qc.gates = [gate for gate in self.gates]
+        return new_qc
+
     def add_state(self, state, targets=None, state_type="input"):
         """
         Add an input or ouput state to the circuit. By default all the input

src/qutip_qip/operations/gateclass.py

@@ -202,6 +202,21 @@ def __init__(
             if not all_integer:
                 raise ValueError("Index of a qubit must be an integer")
 
+    def __copy__(self):
+        new_gate = type(self).__new__(self.__class__)
+        new_gate.name = self.name
+        new_gate.targets = deepcopy(self.targets)
+        new_gate.controls = deepcopy(self.controls)
+        new_gate.classical_controls = deepcopy(self.classical_controls)
+        new_gate.classical_control_value = deepcopy(
+            self.classical_control_value
+        )
+        new_gate.control_value = deepcopy(self.control_value)
+        new_gate.arg_value = deepcopy(self.arg_value)
+        new_gate.arg_label = self.arg_label
+        new_gate.latex_str = self.latex_str
+        return new_gate
+
     def get_all_qubits(self):
         """
         Return a list of all qubits that the gate operator

src/qutip_qip/transpiler/chain.py

@@ -28,8 +28,7 @@ def to_chain_structure(qc, setup="linear"):
     # FIXME This huge block has been here for a long time.
     # It could be moved to the new compiler section and carefully
     # splitted into smaller peaces.
-    qc_t = deepcopy(qc)
-    qc_t.gates = []
+    qc_t = qc._create_empty_copy()
     swap_gates = [
         "SWAP",
         "ISWAP",