fix: Warn user about implicit qubit permutations in tk_to_qiskit

docs/changelog.md

@@ -7,6 +7,7 @@
 ## Unreleased
 
 -  Fix handling of non-default registers when selecting bits in results.
+- The {py:func}`tk_to_qiskit` converter gives a warning if the input {py:class}`~pytket.circuit.Circuit` contains [implicit qubit permutations](https://docs.quantinuum.com/tket/user-guide/manual/manual_circuit.html#implicit-qubit-permutations). 
 
 ## 0.58.0 (October 2024)
 

pytket/extensions/qiskit/qiskit_convert.py

@@ -15,6 +15,7 @@
 
 """Methods to allow conversion between Qiskit and pytket circuit classes
 """
+import warnings
 from collections import defaultdict
 from collections.abc import Iterable
 from inspect import signature
@@ -855,6 +856,11 @@ def append_tk_command_to_qiskit(
 supported_gate_rebase = AutoRebase(_protected_tket_gates)
 
 
+def _has_implicit_permutation(circ: Circuit) -> bool:
+    impl_dict: dict[Qubit, Qubit] = circ.implicit_qubit_permutation()
+    return tuple(impl_dict.keys()) != tuple(impl_dict.values())
+
+
 def tk_to_qiskit(
     tkcirc: Circuit, replace_implicit_swaps: bool = False
 ) -> QuantumCircuit:
@@ -873,6 +879,15 @@ def tk_to_qiskit(
     tkc = tkcirc.copy()  # Make a local copy of tkcirc
     if replace_implicit_swaps:
         tkc.replace_implicit_wire_swaps()
+
+    if _has_implicit_permutation(tkcirc) and not replace_implicit_swaps:
+        warnings.warn(
+            "The pytket Circuit contains implicit qubit permutations"
+            + " which aren't handled by default."
+            + " Consider using the replace_implicit_swaps flag in tk_to_qiskit or"
+            + " replacing them using Circuit.replace_implicit_swaps()."
+        )
+
     qcirc = QuantumCircuit(name=tkc.name)
     qreg_sizes: dict[str, int] = {}
     for qb in tkc.qubits:

tests/qiskit_convert_test.py

@@ -1172,3 +1172,10 @@ def test_nonregister_bits() -> None:
     c.rename_units({Bit(0): Bit(1)})
     with pytest.raises(NotImplementedError):
         tk_to_qiskit(c)
+
+
+def test_implicit_swap_warning() -> None:
+    c = Circuit(2).H(0).SWAP(0, 1)
+    c.replace_SWAPs()
+    with pytest.warns(UserWarning, match="The pytket Circuit contains implicit qubit"):
+        tk_to_qiskit(c)