Merged with recent pennylane pr

lambeq/backend/pennylane.py

@@ -47,6 +47,7 @@
 import pennylane as qml
 import sympy
 import torch
+import sys
 
 from lambeq.backend.quantum import (circuital_to_dict,
                                     is_circuital,
@@ -165,6 +166,15 @@ def to_pennylane(diagram: Diagram, probabilities=False,
         The PennyLane circuit equivalent to the input lambeq circuit.
 
     """
+    if any(isinstance(box, Measure) for box in diagram.boxes):
+        raise ValueError('Only pure circuits, or circuits with discards'
+                         ' are currently supported.')
+
+    if diagram.is_mixed and diagram.cod:
+        # Some qubits discarded, some left open
+        print('Warning: Circuit includes both discards and open codomain'
+              ' wires. All open wires will be discarded during conversion',
+              file=sys.stderr)
 
     if not is_circuital(diagram):
         diagram = to_circuital(diagram)

lambeq/backend/quantum.py

@@ -1249,9 +1249,10 @@ def to_circuital(diagram: Diagram):
     circuit = circuit.init_and_discard()
 
     #  Cleans up any '1' kets and converts them to X|0> -> |1>
-    def remove_ket1(_, box: Box) -> Diagram | Box:
+    def remove_ketbra1(_, box: Box) -> Diagram | Box:
         ob_map: dict[Box, Diagram]
-        ob_map = {Ket(1): Ket(0) >> X}  # type: ignore[dict-item]
+        ob_map = {Ket(1): Ket(0) >> X,
+                  Bra(1): X >> Ket(0)}  # type: ignore[dict-item]
         return ob_map.get(box, box)
 
     def add_qubit(qubits: list[Layer],
@@ -1459,7 +1460,7 @@ def build_left_right(q_idx, layer, layers):
 
     circuit = Functor(target_category=quantum,  # type: ignore [assignment]
                       ob=lambda _, x: x,
-                      ar=remove_ket1)(circuit)  # type: ignore [arg-type]
+                      ar=remove_ketbra1)(circuit)  # type: ignore [arg-type]
 
     layers = circuit.layers