BQSKit is crashing when trying to compile a unitary using a gate set that has only VariableUnitaryGate

[alonkukl]

The following code makes BQSkit crash with pyo3_runtime.PanicException: not implemented. Probably QFactor is getting a circuit with gates it can't handle.

from bqskit import compile, MachineModel
from bqskit.compiler import Compiler
from bqskit.qis.unitary import UnitaryMatrix
from bqskit.ir.gates import VariableUnitaryGate


target = UnitaryMatrix.random(4)

model = MachineModel(4, gate_set={VariableUnitaryGate(3)})


comp = Compiler(num_workers=10)
synthesized_circuit = compile(target, max_synthesis_size=4, model=model, compiler=comp)

[alonkukl]

I think that the root cause of this crash is the way a layer generator is created from a gate set.

When a layer generator isn't given to QSearch (and to LEAP), then it asks the gate set object to generate a layer generator, using the function build_mq_layer_generator.

In our case, there is no single qubit gate, so the above function adds the U3 as a single qubit, which in turn QFactor can't handle.

BQSKit shouldn't have added a gate that wasn't part of the original gate set. This behaviour is documented The resulting layer generator will use arbitrary single-qudit gates. but it is wrong. It can produce a warning that a single qubit gate is missing, but it shouldn't add a gate to the gate set.

In my case, I can bypass this issue, by directly creating QSearch and providing it with the correct layer generator
WideLayerGenerator(VariableUnitaryGate(3), None)

[edyounis]

Most gate sets will not include a general single-qudit gate, so we introduce one during compilation stages that focus on multi-qudit gate optimization/transpilation. You are right; in this case and cases like this, we shouldn't be mocking with the gate set. We should check if the gate set contains multi-qudit general gates. If so, we don't need to use the layer generator with single-qudit gates added.

[alonkukl]

I have a different perspective.

If the user has explicitly provided a gate set, the tool shouldn't change it. The tool can give a warning or an error if the gate set isn't general enough or has other issues.

[edyounis]

In the case that I am referring to, the gate set is modified for specific compilation stages. The later stages reverse the modifications, so the user gate set is always honored.

[alonkukl]

OK, then your solution above looks good