Support QuantumCircuit.if_test in qiskit_to_tk

[vtomole]

from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister
from pytket.extensions.qiskit import qiskit_to_tk
 
qubits = QuantumRegister(1)
clbits = ClassicalRegister(1)
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 = qiskit_to_tk(circuit)

Errors with

  348     return optype
    349 except KeyError:
--> 350     raise NotImplementedError(
    351         f"Conversion of qiskit's {instruction.name} instruction is "
    352         + "currently unsupported by qiskit_to_tk. Consider "
    353         + "using QuantumCircuit.decompose() before attempting "
    354         + "conversion."
    355     )

NotImplementedError: Conversion of qiskit's if_else instruction is currently unsupported by qiskit_to_tk. Consider using QuantumCircuit.decompose() before attempting conversion.

on pytket-qiskit ==0.58.0

[CalMacCQ]

Hi, thanks for making an issue.

This would be useful for sure. We should add this in soon.

[CalMacCQ]

I haven't forgotten about this issue. I think it'd be great to handle this as some IBM devices have IfElseOp as a native operation. Its also the recommended way to do conditional logic in qiskit as far as I understand.

The best way I can think of to handle this is to handle the if and else circuits as classically controlled CircBoxes.

I made an attempt at implementing this conversion in #437 but the PR is still a bit of a mess. I don't think this will be in the 0.62 release (we are planning to release today) but it should be in 0.63 for sure.