Qiskit transpiler cannot handle custom multi-qubit gates

[nkanazawa1989]

What is the expected enhancement?

Because Qiskit is backend agnostic, we should be able to define custom multi qubit gate. What I want to do here is

qc = QuantumCircuit(3)
qc.append(Gate('my_custom_3q_gate', 3, []), [0,1,2])
qc.h(0)
qc.sdg(0)
qc.y(1)
...

qc.add_calibration('my_custom_3q_gate', (0,1,2), my_custom_3q_gate_schedule)

qc = transpile(qc, backend, basis_gates=['u1', 'u2', 'u3', 'cx', 'my_custom_3q_gate'])
sched = schedule(qc, backend)

But current transpiler passes don't allow to have 3q basis gates. I think this limits capability of Qiskit.

https://github.com/Qiskit/qiskit-terra/blob/6646e681501deb8aac55cdc69a8a351ec058e989/qiskit/transpiler/preset_passmanagers/level0.py#L107

[aeddins-ibm]

Just to add motivation to this old issue, I think I'm hitting the same problem when trying to simulate a noisy circuit by adding a multiqubit quantum channel to a circuit. For my use case this is simpler than building the error into a NoiseModel.

I'm using Aer's pauli_error() to define a 3+ qubit error instruction, and including that in a circuit, and later want to transpile the circuit. But AFAICT you can't transpile any circuit to which you've added a 3+ qubit quantum channel, even if that channel is added as a basis gate, which is pretty limiting for noise simulations. This is true even for optimization_level=0. I can limp by without transpiling, but it complicates other parts of the code.

So I think fixing this issue could facilitate a range of noisy simulations.

[nkanazawa1989]

Thanks for finding this. Indeed current design of pass manager is against the spirit of backend agnostic. Perhaps analyzing basis gates and conditionally inserting the unroll3Q pass is reasonable solution?

[mtreinish]

Yeah unroll3q or more should be basis and target aware. We shouldn't be unrolling gates in the target even if they're > 2q. The caveat here is we'll have to make sure that layout and routing can handle a 3q gate in the circuit. The reason that pass runs first by default is because layout and routing only works with 1q and 2q operations. This connects with https://github.com/Qiskit/qiskit-terra/issues/7812 for making sure the transpiler can work with multiqubit gates