IBMQEmulatorBackend and IBMQBackend don't properly enforce MaxNQubitsPredicate

[CalMacCQ]

If I ask IBMQBackend and IBMQEmulatorBackend which predicates they require with Backend.required_predicates I get the following...

[NoFastFeedforwardPredicate,
 NoSymbolsPredicate,
 GateSetPredicate:{ Rz noop Reset X SX Barrier CX Measure }]

I would've expected MaxNQubitsPredicate to appear on this list.

As its not enforced I can run a 7 qubit circuit on a 5 qubit emulator which shouldn't be possible.

from pytket import Circuit
from pytket.extensions.qiskit import IBMQEmulatorBackend

backend = IBMQEmulatorBackend('ibmq_belem') # Backend is supposed to only support 5 qubits

# Define a 7 qubit circuit that is already in the IBM gateset: {X, SX, CX, Rz} - no compilation needed
circ = Circuit(7).X(0).CX(0, 1).CX(0, 2).CX(0, 3).CX(0, 4).CX(0, 5).CX(0, 6).measure_all() 

result = backend.run_circuit(circ, n_shots=100)
print(result.get_counts) # get some length 7 bitstrings back from the simulator (shouldn't be possible)

However if I compile the circuit first with Backend.get_compiled_circuit an error message is thrown saying the circuit does not respect the MaxNQubitsPredicate. I'm not sure how this is enforced as it does not appear in Backend.required_predicates.

[CalMacCQ]

@cqc-alec @cqc-melf

Once #94 is merged can we consider this issue to be resolved? Or should we also be concerned that the only thing stopping a 7 qubit job running on a 5 qubit emulator is the MaxNQubitsPredicate on the pytket side?

[cqc-alec]

@cqc-alec @cqc-melf

Once #94 is merged can we consider this issue to be resolved? Or should we also be concerned that the only thing stopping a 7 qubit job running on a 5 qubit emulator is the MaxNQubitsPredicate on the pytket side?

I think we can close this issue, but I'd still like to understand why the qiskit runtime didn't balk at the circuit. So I think I will open another issue for that.

[cqc-alec]

On second thoughts -- let's leave this issue open to track that, since the description is still accurate. With no compilation, a 7=qubit circuit seems to run on a 5-qubit backend.