feat: handle IfElseOp in qiskit_to_tk

docs/changelog.md

@@ -4,6 +4,10 @@
 
 # Changelog
 
+## 0.63.0 (January 2025) - UNRELEASED
+
+- Support conversion of qiskit circuits containing `IfElseOp` in the {py:func}`qiskit_to_tk` converter.
+
 ## 0.62.0 (December 2024)
 
 - AerBackend now rejects circuits with too many qubits

pytket/extensions/qiskit/qiskit_convert.py

@@ -76,6 +76,7 @@
     Clbit,
     ControlledGate,
     Gate,
+    IfElseOp,
     Instruction,
     InstructionSet,
     Measure,
@@ -479,6 +480,70 @@ def _build_circbox(instr: Instruction, circuit: QuantumCircuit) -> CircBox:
     return CircBox(subc)
 
 
+# Used for handling of IfElseOp
+# docs -> https://docs.quantum.ibm.com/api/qiskit/qiskit.circuit.IfElseOp
+# Examples -> https://docs.quantum.ibm.com/guides/classical-feedforward-and-control-flow
+# pytket-qiskit issue -> https://github.com/CQCL/pytket-qiskit/issues/415
+def _pytket_boxes_from_ifelseop(
+    if_else_op: IfElseOp, qregs: list[QuantumRegister], cregs: list[ClassicalRegister]
+) -> tuple[CircBox, Optional[CircBox]]:
+    # Extract the QuantumCircuit implementing true_body
+    if_qc: QuantumCircuit = if_else_op.blocks[0]
+
+    if_builder = CircuitBuilder(qregs, cregs)
+    if_builder.add_qiskit_data(if_qc)
+    if_circuit = if_builder.circuit()
+    if_circuit.name = "If"
+    # Remove blank wires to ensure CircBox is the correct size.
+    if_circuit.remove_blank_wires()
+
+    # The false_body arg is optional
+    if len(if_else_op.blocks) == 2:
+        else_qc: QuantumCircuit = if_else_op.blocks[1]
+        else_builder = CircuitBuilder(qregs, cregs)
+        else_builder.add_qiskit_data(else_qc)
+        else_circuit = else_builder.circuit()
+        else_circuit.name = "Else"
+        else_circuit.remove_blank_wires()
+        return CircBox(if_circuit), CircBox(else_circuit)
+
+    # If no false_body is specified IfElseOp.blocks is of length 1.
+    # In this case we return a CircBox implementing true_body and None.
+    return CircBox(if_circuit), None
+
+
+def _build_if_else_circuit(
+    if_else_op: IfElseOp,
+    qregs: list[QuantumRegister],
+    cregs: list[ClassicalRegister],
+    qubits: list[Qubit],
+    bits: list[Bit],
+) -> Circuit:
+    # Get two CircBox objects which implement the true_body and false_body.
+    if_box, else_box = _pytket_boxes_from_ifelseop(if_else_op, qregs, cregs)
+    # else_box can be None if no false_body is specified.
+
+    circ_builder = CircuitBuilder(qregs, cregs)
+    circ = circ_builder.circuit()
+
+    circ.add_circbox(
+        circbox=if_box,
+        args=qubits,
+        condition_bits=bits,
+        condition_value=if_else_op.condition[1],
+    )
+    # If we have an else_box defined, add it to the circuit
+    if else_box is not None:
+        circ.add_circbox(
+            circbox=else_box,
+            args=qubits,
+            condition_bits=bits,
+            # TODO: handle conditions over multiple bits/registers?
+            condition_value=not bool(if_else_op.condition[1]),
+        )
+    return circ
+
+
 class CircuitBuilder:
     def __init__(
         self,
@@ -522,16 +587,16 @@ def add_qiskit_data(
             bits: list[Bit] = [self.cbmap[bit] for bit in cargs]
 
             condition_kwargs = {}
-            if instr.condition is not None:
+            if instr.condition is not None and type(instr) is not IfElseOp:
                 condition_kwargs = _get_pytket_condition_kwargs(
                     instruction=instr,
                     cregmap=self.cregmap,
                     circuit=circuit,
                 )
 
             optype = None
-            if type(instr) not in (PauliEvolutionGate, UnitaryGate):
-                # Handling of PauliEvolutionGate and UnitaryGate below
+            if type(instr) not in (PauliEvolutionGate, UnitaryGate, IfElseOp):
+                # Handling of PauliEvolutionGate, UnitaryGate and IfElseOp below
                 optype = _optype_from_qiskit_instruction(instruction=instr)
 
             if optype == OpType.QControlBox:
@@ -543,6 +608,16 @@ def add_qiskit_data(
                 # Append OpType found by stateprep helpers
                 _add_state_preparation(self.tkc, qubits, instr)
 
+            elif type(instr) is IfElseOp:
+                if_else_circ = _build_if_else_circuit(
+                    if_else_op=instr,
+                    qregs=self.qregs,
+                    cregs=self.cregs,
+                    qubits=qubits,
+                    bits=bits,
+                )
+                self.tkc.append(if_else_circ)
+
             elif type(instr) is PauliEvolutionGate:
                 qpo = _qpo_from_peg(instr, qubits)
                 empty_circ = Circuit(len(qargs))

tests/qiskit_convert_test.py

@@ -1193,6 +1193,65 @@ def test_nonregister_bits() -> None:
         tk_to_qiskit(c)
 
 
+# https://github.com/CQCL/pytket-qiskit/issues/415
+def test_ifelseop_two_branches() -> None:
+    qubits = QuantumRegister(2)
+    clbits = ClassicalRegister(2)
+    circuit = QuantumCircuit(qubits, clbits)
+    (q0, q1) = qubits
+    (c0, c1) = clbits
+
+    circuit.h(q0)
+    circuit.measure(q0, c0)
+
+    with circuit.if_test((c0, 1)) as else_:
+        circuit.h(q1)
+    with else_:
+        circuit.x(q1)
+    circuit.measure(q1, c1)
+
+    tkc = qiskit_to_tk(circuit)
+    assert tkc.n_gates_of_type(OpType.Conditional) == 2
+
+
+# https://github.com/CQCL/pytket-qiskit/issues/415
+def test_ifelseop_one_branch() -> None:
+    qubits = QuantumRegister(1, "q1")
+    clbits = ClassicalRegister(1, "c1")
+    circuit = QuantumCircuit(qubits, clbits)
+    (q0,) = qubits
+    (c0,) = clbits
+
+    circuit.h(q0)
+    circuit.measure(q0, c0)
+    with circuit.if_test((c0, 1)):
+        circuit.x(q0)
+    circuit.measure(q0, c0)
+
+    tket_circ_if_else = qiskit_to_tk(circuit)
+    tket_circ_if_else.name = "test_circ"
+
+    # Manually build the expected pytket Circuit.
+    # Validate against tket_circ_if_else.
+    expected_circ = Circuit()
+    expected_circ.name = "test_circ"
+    q1_tk = expected_circ.add_q_register("q1", 1)
+    c1_tk = expected_circ.add_c_register("c1", 1)
+    expected_circ.H(q1_tk[0])
+    expected_circ.Measure(q1_tk[0], c1_tk[0])
+    x_circ = Circuit()
+    x_circ.name = "If"
+    xq1 = x_circ.add_q_register("q1", 1)
+    x_circ.X(xq1[0])
+    expected_circ.add_circbox(
+        CircBox(x_circ), [q1_tk[0]], condition_bits=[c1_tk[0]], condition_value=1
+    )
+
+    expected_circ.Measure(q1_tk[0], c1_tk[0])
+
+    assert tket_circ_if_else == expected_circ
+
+
 def test_range_preds_with_conditionals() -> None:
     # https://github.com/CQCL/pytket-qiskit/issues/375
     c = Circuit(1, 1)