feat: Improved support for IfElseOp in qiskit_to_tk (#463)

* refactor: use Circuit.has_implicit_wireswaps property

* add a test case for register handling

* initial rough handling of register conditions

* delete duplicate test

* delete comments

* get classical register instead of creating a new one

* rework logic of _build_if_else_circuit a bit

* expand test case

* add explanatory comment

* update error handling

* add assert to test

* add missing measurements to test case

* remove dodgy if statement for error handling

* update changelog

* fix changelog

* add additonal test case for register conditions without an else branch

* Improve single branch test

Author: CalMacCQ

docs/changelog.md

@@ -4,8 +4,13 @@
 
 # Changelog
 
+## 0.65.0 (UNRELEASED)
 
-## 0.64.0 (March 2025)
+- Improve {py:func}`qiskit_to_tk` to handle `IfElseOp` (generated by `QuantumCircuit.if_test`) 
+with register conditions. Only conditions on an entire register are supported.
+
+
+## 0.64.0
 
 - Update pytket version requirement to 2.0.1.
 

pytket/extensions/qiskit/qiskit_convert.py

@@ -58,6 +58,7 @@
     Unitary3qBox,
     UnitType,
 )
+from pytket.circuit.logic_exp import reg_eq, reg_neq
 from pytket.passes import AutoRebase
 from pytket.pauli import Pauli, QubitPauliString
 from pytket.unit_id import _TEMP_BIT_NAME
@@ -98,6 +99,7 @@
     from qiskit_ibm_runtime.models.backend_properties import Nduv
 
     from pytket.circuit import UnitID
+    from pytket.unit_id import BitRegister
     from qiskit.circuit.quantumcircuitdata import QuantumCircuitData  # type: ignore
 
 _qiskit_gates_1q = {
@@ -518,41 +520,49 @@ def _build_if_else_circuit(
     qubits: list[Qubit],
     bits: list[Bit],
 ) -> Circuit:
-    # Coniditions must be on a single bit (for now) TODO: support multiple bits.
-    if len(bits) == 1:
-        # 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()
-    else:
-        raise NotImplementedError("Conditions over multiple bits not yet supported.")
-
-    # Coniditions must be on a single bit (for now)
-    if not isinstance(if_else_op.condition[0], Clbit):
-        raise NotImplementedError(
-            "Handling of register conditions is not yet supported"
+    # 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()
+
+    if isinstance(if_else_op.condition[0], Clbit):
+        if len(bits) != 1:
+            raise NotImplementedError("Conditions on multiple bits not supported")
+        circ.add_circbox(
+            circbox=if_box,
+            args=qubits,
+            condition_bits=bits,
+            condition_value=if_else_op.condition[1],
         )
-
-    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:
-        if if_else_op.condition[1] not in {0, 1}:
-            raise ValueError(
-                "A bit must have condition value 0 or 1"
-                + f", got {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,
+                condition_value=1 ^ if_else_op.condition[1],
             )
+
+    elif isinstance(if_else_op.condition[0], ClassicalRegister):
+        pytket_bit_reg: BitRegister = circ.get_c_register(if_else_op.condition[0].name)
         circ.add_circbox(
-            circbox=else_box,
+            circbox=if_box,
             args=qubits,
-            condition_bits=bits,
-            condition_value=1 ^ if_else_op.condition[1],
+            condition=reg_eq(pytket_bit_reg, if_else_op.condition[1]),
+        )
+        if else_box is not None:
+            circ.add_circbox(
+                circbox=else_box,
+                args=qubits,
+                condition=reg_neq(pytket_bit_reg, if_else_op.condition[1]),
+            )
+    else:
+        raise TypeError(
+            "Unrecognized type used to construct IfElseOp. Expected "
+            + f"ClBit or ClassicalRegister, got {type(if_else_op.condition[0])}"
         )
+
     return circ
 
 
@@ -620,6 +630,8 @@ def add_qiskit_data(
                 # Append OpType found by stateprep helpers
                 _add_state_preparation(self.tkc, qubits, instr)
 
+            # Note: These IfElseOp/if_test type conditions are only handled
+            # for single bit conditions and conditions on entire registers.
             elif type(instr) is IfElseOp:
                 if_else_circ = _build_if_else_circuit(
                     if_else_op=instr,
@@ -954,12 +966,6 @@ 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,
@@ -987,11 +993,7 @@ def tk_to_qiskit(
     if replace_implicit_swaps:
         tkc.replace_implicit_wire_swaps()
 
-    if (
-        _has_implicit_permutation(tkcirc)
-        and perm_warning
-        and not replace_implicit_swaps
-    ):
+    if tkcirc.has_implicit_wireswaps and perm_warning and not replace_implicit_swaps:
         warnings.warn(
             "The pytket Circuit contains implicit qubit permutations"
             + " which aren't handled by default."

tests/qiskit_convert_test.py

@@ -61,6 +61,7 @@
     Unitary2qBox,
     Unitary3qBox,
     reg_eq,
+    reg_neq,
 )
 from pytket.extensions.qiskit import IBMQBackend, qiskit_to_tk, tk_to_qiskit
 from pytket.extensions.qiskit.backends import (
@@ -1274,7 +1275,61 @@ def test_ifelseop_one_branch() -> None:
     assert tket_circ_if_else == expected_circ
 
 
-def test_ifelseop_multi_bit_cond() -> None:
+# https://github.com/CQCL/pytket-qiskit/issues/452
+def test_ifelseop_reg_cond_if() -> None:
+    qreg = QuantumRegister(3, "q")
+    creg = ClassicalRegister(3, "c")
+    circuit = QuantumCircuit(creg, qreg)
+    (q0, q1, q2) = qreg
+    (c0, c1, c2) = creg
+    circuit.h(q0)
+    circuit.h(q1)
+    circuit.h(q2)
+    circuit.measure(q0, c0)
+    circuit.measure(q1, c1)
+    circuit.measure(q2, c2)
+    # Condition is on a register not a bit
+    with circuit.if_test((creg, 2)):
+        circuit.x(q0)
+        circuit.y(q1)
+        circuit.z(q2)
+    circuit.measure(q0, c0)
+    circuit.measure(q1, c1)
+    circuit.measure(q2, c2)
+
+    tkc: Circuit = qiskit_to_tk(circuit)
+    tkc.name = "test_circ"
+
+    expected_circ = Circuit()
+    expected_circ.name = "test_circ"
+    qreg_tk = expected_circ.add_q_register("q", 3)
+    creg_tk = expected_circ.add_c_register("c", 3)
+    expected_circ.H(qreg_tk[0])
+    expected_circ.H(qreg_tk[1])
+    expected_circ.H(qreg_tk[2])
+    expected_circ.Measure(qreg_tk[0], creg_tk[0])
+    expected_circ.Measure(qreg_tk[1], creg_tk[1])
+    expected_circ.Measure(qreg_tk[2], creg_tk[2])
+
+    pauli_circ = Circuit()
+    pauli_circ.name = "If"
+    pauli_qreg = pauli_circ.add_q_register("q", 3)
+    pauli_circ.X(pauli_qreg[0]).Y(pauli_qreg[1]).Z(pauli_qreg[2])
+    expected_circ.add_circbox(
+        CircBox(pauli_circ),
+        [qreg_tk[0], qreg_tk[1], qreg_tk[2]],
+        condition=reg_eq(creg_tk, 2),
+    )
+
+    expected_circ.Measure(qreg_tk[0], creg_tk[0])
+    expected_circ.Measure(qreg_tk[1], creg_tk[1])
+    expected_circ.Measure(qreg_tk[2], creg_tk[2])
+
+    assert expected_circ == tkc
+
+
+# https://github.com/CQCL/pytket-qiskit/issues/452
+def test_ifelseop_reg_cond_if_else() -> None:
     qreg = QuantumRegister(2, "q")
     creg = ClassicalRegister(2, "c")
     circuit = QuantumCircuit(creg, qreg)
@@ -1285,14 +1340,47 @@ def test_ifelseop_multi_bit_cond() -> None:
     circuit.h(q1)
     circuit.measure(q0, c0)
     circuit.measure(q1, c1)
-    with circuit.if_test((creg, 2)):
+    # Condition is on a register not a bit
+    with circuit.if_test((creg, 2)) as else_:
         circuit.x(q0)
         circuit.x(q1)
+    with else_:
+        circuit.y(q0)
+        circuit.y(q1)
     circuit.measure(q0, c0)
     circuit.measure(q1, c1)
-    # This currently gives an error as register exp not supported.
-    with pytest.raises(NotImplementedError):
-        qiskit_to_tk(circuit)
+    tkc: Circuit = qiskit_to_tk(circuit)
+    tkc.name = "test_circ"
+
+    expected_circ = Circuit()
+    expected_circ.name = "test_circ"
+    qreg_tk = expected_circ.add_q_register("q", 2)
+    creg_tk = expected_circ.add_c_register("c", 2)
+    expected_circ.H(qreg_tk[0])
+    expected_circ.H(qreg_tk[1])
+    expected_circ.Measure(qreg_tk[0], creg_tk[0])
+    expected_circ.Measure(qreg_tk[1], creg_tk[1])
+
+    x_circ2 = Circuit()
+    x_circ2.name = "If"
+    x_qreg = x_circ2.add_q_register("q", 2)
+    x_circ2.X(x_qreg[0]).X(x_qreg[1])
+    expected_circ.add_circbox(
+        CircBox(x_circ2), [qreg_tk[0], qreg_tk[1]], condition=reg_eq(creg_tk, 2)
+    )
+
+    y_circ2 = Circuit()
+    y_circ2.name = "Else"
+    y_qreg = y_circ2.add_q_register("q", 2)
+    y_circ2.Y(y_qreg[0]).Y(y_qreg[1])
+    expected_circ.add_circbox(
+        CircBox(y_circ2), [qreg_tk[0], qreg_tk[1]], condition=reg_neq(creg_tk, 2)
+    )
+
+    expected_circ.Measure(qreg_tk[0], creg_tk[0])
+    expected_circ.Measure(qreg_tk[1], creg_tk[1])
+
+    assert expected_circ == tkc
 
 
 def test_range_preds_with_conditionals() -> None: