From ffbf31a6cd71d1720e860645f07097240a6b1fc2 Mon Sep 17 00:00:00 2001
From: Callum Macpherson <93673602+CalMacCQ@users.noreply.github.com>
Date: Fri, 8 Nov 2024 10:58:13 +0000
Subject: [PATCH] fix: Warn user about implicit qubit permutations in
 `tk_to_qiskit` (#412)

* fix: warn user if Circuit contains IQP

* add test

* add changelog entry

* remove unused import

* organise imports for ruff

* fix a typo

* improve `_has_implicit_permutation` function

Co-authored-by: Alec Edgington <54802828+cqc-alec@users.noreply.github.com>

* fix typo

---------

Co-authored-by: Alec Edgington <54802828+cqc-alec@users.noreply.github.com>
---
 docs/changelog.md                          |  1 +
 pytket/extensions/qiskit/qiskit_convert.py | 14 ++++++++++++++
 tests/qiskit_convert_test.py               |  7 +++++++
 3 files changed, 22 insertions(+)

diff --git a/docs/changelog.md b/docs/changelog.md
index 0e06e304..3ea66662 100644
--- a/docs/changelog.md
+++ b/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)
 
diff --git a/pytket/extensions/qiskit/qiskit_convert.py b/pytket/extensions/qiskit/qiskit_convert.py
index 49dcb584..51924e17 100644
--- a/pytket/extensions/qiskit/qiskit_convert.py
+++ b/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 +856,10 @@ def append_tk_command_to_qiskit(
 supported_gate_rebase = AutoRebase(_protected_tket_gates)
 
 
+def _has_implicit_permutation(circ: Circuit) -> bool:
+    return any(q0 != q1 for q0, q1 in circ.implicit_qubit_permutation().items())
+
+
 def tk_to_qiskit(
     tkcirc: Circuit, replace_implicit_swaps: bool = False
 ) -> QuantumCircuit:
@@ -873,6 +878,15 @@ 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:
diff --git a/tests/qiskit_convert_test.py b/tests/qiskit_convert_test.py
index cdde50d3..631f8ee0 100644
--- a/tests/qiskit_convert_test.py
+++ b/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)