fix: improve implicit qubit permutations warning

docs/changelog.md

@@ -6,7 +6,8 @@
 
 ## 0.60.0 (Unreleased)
 
-- Revert a change made in release v0.59.0 where users are warned about implicit qubit permutations in {py:func}`tk_to_qiskit`. This avoids spamming the user with unhelpful warnings when using pytket-qiskit backends. These backends handle implicit permutations automatically.
+- Fix an unhelpful warning message about implicit swaps when using optimisation level 2 with {py:class}`AerBackend`
+- Add a boolean `perm_warn` argument to {py:func}`tk_to_qiskit` indicating whether to give a warning if the input {py:class}`Circuit` has an implicit qubit permutation.
 - Add new level 3 optimisation that uses `GreedyPauliSimp`
 
 ## 0.59.0 (November 2024)

docs/index.md

@@ -194,7 +194,7 @@ Every {py:class}`~pytket.backends.backend.Backend` in pytket has its own {py:met
 2 preserve barriers in a circuit, while optimisation level 3 will remove them.
 
 :::{list-table} **Default compilation pass for the IBMQBackend and IBMQEmulatorBackend**
-:widths: 25 25 25
+:widths: 25 25 25 25
 :header-rows: 1
 
 * - optimisation_level = 0
@@ -208,7 +208,7 @@ Every {py:class}`~pytket.backends.backend.Backend` in pytket has its own {py:met
 * - [AutoRebase [2]](inv:#*.AutoRebase)
   - [SynthesiseTket](inv:#*.SynthesiseTket)
   - [FullPeepholeOptimise](inv:#*.passes.FullPeepholeOptimise)
-  - [RemoveBarriers](inv:#*.passes.RmoveBarriers)
+  - [RemoveBarriers](inv:#*pytket._tket.passes.RemoveBarriers)
 * - LightSabre [3]
   - LightSabre [3]
   - LightSabre [3]

pytket/extensions/qiskit/backends/aer.py

@@ -286,6 +286,7 @@ def process_circuits(
         circuit_batches, batch_order = _batch_circuits(circuits, n_shots_list)
 
         replace_implicit_swaps = self.supports_state or self.supports_unitary
+        perm_warning = False
 
         for (n_shots, batch), indices in zip(circuit_batches, batch_order):
             qcs, ppcirc_strs, tkc_qubits_count = [], [], []
@@ -296,7 +297,7 @@ def process_circuits(
                 else:
                     c0, ppcirc_rep = tkc, None
 
-                qc = tk_to_qiskit(c0, replace_implicit_swaps)
+                qc = tk_to_qiskit(c0, replace_implicit_swaps, perm_warning)
 
                 if self.supports_state:
                     qc.save_state()

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
@@ -861,8 +862,16 @@ def append_tk_command_to_qiskit(
 supported_gate_rebase = AutoRebase(_protected_tket_gates)
 
 
+def _has_implicit_permutation(circ: Circuit) -> bool:
+    """Returns True if a Circuit has a non-trivial permutation
+    of qubits, false otherwise."""
+    return any(q0 != q1 for q0, q1 in circ.implicit_qubit_permutation().items())
+
+
 def tk_to_qiskit(
-    tkcirc: Circuit, replace_implicit_swaps: bool = False
+    tkcirc: Circuit,
+    replace_implicit_swaps: bool = False,
+    perm_warning: bool = True,
 ) -> QuantumCircuit:
     """
     Converts a pytket :py:class:`Circuit` to a qiskit :py:class:`qiskit.QuantumCircuit`.
@@ -871,19 +880,33 @@ def tk_to_qiskit(
     If no exact replacement can be found for a part of the circuit then an equivalent
     circuit will be returned using the tket gates which are supported in qiskit.
 
-    Please note that implicit swaps in a pytket Circuit are not handled by default.
+    Note that implicit swaps in a pytket Circuit are not handled by default.
     Consider using the replace_implicit_swaps flag to replace these implicit swaps with
     SWAP gates.
 
     :param tkcirc: A :py:class:`Circuit` to be converted
     :param replace_implicit_swaps: Implement implicit permutation by adding SWAPs
         to the end of the circuit.
+    :param perm_warnings: Warn if an input circuit has implicit qubit permutations,
+        True by default.
     :return: The converted circuit
     """
     tkc = tkcirc.copy()  # Make a local copy of tkcirc
     if replace_implicit_swaps:
         tkc.replace_implicit_wire_swaps()
 
+    if (
+        _has_implicit_permutation(tkcirc)
+        and perm_warning
+        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

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import os
+import warnings
 from collections import Counter
 from math import pi
 
@@ -62,7 +63,10 @@
     reg_eq,
 )
 from pytket.extensions.qiskit import IBMQBackend, qiskit_to_tk, tk_to_qiskit
-from pytket.extensions.qiskit.backends import qiskit_aer_backend
+from pytket.extensions.qiskit.backends import (
+    AerBackend,
+    qiskit_aer_backend,
+)
 from pytket.extensions.qiskit.qiskit_convert import _gate_str_2_optype
 from pytket.extensions.qiskit.result_convert import qiskit_result_to_backendresult
 from pytket.extensions.qiskit.tket_pass import TketAutoPass, TketPass
@@ -1166,6 +1170,19 @@ def test_symbolic_param_conv() -> None:
     )
 
 
+def test_implicit_swap_warning() -> None:
+    c = Circuit(2).H(0).SWAP(0, 1)
+    c.replace_SWAPs()
+    c.measure_all()
+    with pytest.warns(UserWarning, match="The pytket Circuit contains implicit qubit"):
+        tk_to_qiskit(c)
+
+    shots_backend = AerBackend()
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        shots_backend.run_circuit(c)
+
+
 # https://github.com/CQCL/pytket-qiskit/issues/337
 def test_nonregister_bits() -> None:
     c = Circuit(1).X(0).measure_all()