Hi @say4n! Thank you for your question!
The way you’re using it will have several problems because effectively MeasureThreshold doesn’t take wires as an argument, but also expval can only take an input that inherits from the PennyLane class Obervable, meaning you can’t use MeasureThreshold.
If you try this code you will see that MeasureThreshold is the wrong type.
import pennylane as qml
import strawberryfields as sf`
dev = qml.device("strawberryfields.fock", wires=2, cutoff_dim=10)
qml.Beamsplitter(0, 0, wires=[0, 1])
qml.Displacement(beta, 0, wires=0)
print("MT type: ",type(sf.ops.MeasureThreshold(select=)))
MeasureThreshold is very similar to MeasureFock so maybe this tutorial can help you.
Please let me know if this helps!