qiskit_to_tk: KeyError for certain gates

[nquetschlich]

The qiskit_to_tk transformation fails when certain gates are present in the original Qiskit.QuantumCircuit and throws a respective KeyError.

One example can be seen here:

from qiskit import QuantumCircuit
from qiskit.circuit.random import random_circuit
from pytket.extensions.qiskit import qiskit_to_tk

qc = random_circuit(8, depth=10, measure=False, seed=10)
qc_tket = qiskit_to_tk(qc)

Resulting in: KeyError: <class 'qiskit.circuit.library.standard_gates.xx_plus_yy.XXPlusYYGate'>

I could imagine that the problem occurs for gates which are not part of the openQASM2.0 gates. But there are even gates from that gateset which lead to the same error, for example here:

from qiskit import QuantumCircuit
from pytket.extensions.qiskit import qiskit_to_tk

qc = QuantumCircuit(3)
qc.rccx(0,1,2)
qc_tket = qiskit_to_tk(qc)

Resulting in: KeyError: <class 'qiskit.circuit.library.standard_gates.x.RCCXGate'>

(based on pytket v1.16.0 and qiskit v0.43.1)

[CalMacCQ]

Thanks for reporting this.

The KeyError is happening here as a dict is used to map qiskit Instruction s to pytket OpTypes. Admittedly the error message could be more informative (should be better imo).

In pytket we don't currently have OpTypes for XXPlusYYGate or RCCX.

A few unsupported gates were discussed here
#88

In the next release we will support CSX, CCZ and most qiskit ControlledGates for which the base gate is supported by the converter (CS, CSdg, C3sx etc). I don't believe RCCX will work though.

I'm not sure how to support these gates (nicely) in the converter without adding new OpTypes to pytket. Maybe we could consider adding the RCCX.

Edit: I guess we could use CnRx(pi) for this? Although the decomposition may be worse than RCCX. We do have a definition for this in C++ but its not currently exposed to pytket.