Fix QubitCircuit dtype conversion with barrier operations

src/deepquantum/utils.py

@@ -44,6 +44,8 @@ def wrapped_function(*args, **kwargs):
 
 def apply_complex_fix(fn: Any, tensors_dict: dict[str, torch.Tensor]) -> dict[str, torch.Tensor]:
     """Apply the function to the tensors in the dictionary and convert the result to complex dtype."""
+    if len(tensors_dict) == 0:
+        return tensors_dict
     first_tensor = next(iter(tensors_dict.values()))
     probe = fn(torch.empty(0, dtype=first_tensor.real.dtype, device=first_tensor.device))
     target_dtype = dq.dtype_map.get(probe.dtype, probe.dtype)

tests/test_circuit.py

@@ -3,6 +3,17 @@
 import deepquantum as dq
 
 
+def test_qubit_circuit_to_with_barrier():
+    cir = dq.QubitCircuit(2)
+    cir.h(0)
+    cir.barrier()
+    cir.rx(1, 0.1)
+
+    cir.to(torch.double)
+    state = cir()
+    assert state.dtype == torch.cdouble
+
+
 def test_qubit_mps():
     nqubit = 3
     cir = dq.QubitCircuit(nqubit, mps=True, chi=64)