Folding functions should retain circuit properties (e.g., num shots) and device information (e.g., qubits, connectivity, etc.)

[rmlarose]

Our current folding techniques for Cirq build the folded circuit from an empty circuit. Circuits in Cirq can be tied to a device which tells information about qubits, connectivity, and gate operation times. As such, I think it's more appropriate to copy the input circuit and insert gates to do the folding. Thoughts?

[andreamari]

I think it's a good point. Moreover the same problem probably exists also for PyQuil circuits (e.g. num_shots can be a property of a circuit).

I agree with your solution, otherwise a simple alternative is also this one:
empty_circuit = input_circuit[0:0]
which creates an empty circuit with the same properties of the input and should work for both Cirq and PyQuil circuits.

[rmlarose]

Good point about pyQuil circuits and properties.

I tested

empty_circuit = input_circuit[0:0]

and it didn't copy over the shots.

I'll update the name of this issue to reflect the importance for pyQuil as well.

[andreamari]

It is true that empty_circuit = input_circuit[0:0] doesn't copy num_shots. But I also noticed that circuit_sum= input_circuit + input_circuit doesn't copy num_shots to circuit_sum !
So I wonder if this is a bug or a feature.

An empty circuit in PyQuil can be correctly created in this way
empty_circuit = input_circuit.copy_everything_except_instructions()

however, even in this case, num_shots would be lost as soon as we start making simple operations (e.g., additions).

[andreamari]

I will make a PR (#11 ) to improve the creation of empty circuits in PyQuil using copy_everything_except_instructions(). I am also adding some tests associated to this issue.

The problem with num_shots and similar properties remains. But I am quite sure that this is a bug of PyQuil. See, e.g., the __add__ and __getitem__ method of the Program class where it is evident that properties are not retained (https://github.com/rigetti/pyquil/blob/master/pyquil/quil.py).

[willzeng]

@andreamari does the issue appear specific to num_shots instructions? If so, perhaps dedicated code should exist to pop off this instruction?

Alternatively, we could code for with the assumption that the user does not choose numshots for things to be mitigated. When you run a given Program through mitigation it isn't clear what the numshots should do as the Mitigator will have discretion to choose optimal amounts of shots to take etc

[willzeng]

Of the two approaches I just suggested. I think I prefer stripping num_shots from an input program and emitting a warning to the user that it is being ignored.

[andreamari]

It is not just self.num_shots, there are also other properties like: self._defined_gates and self.native_quil_metadata, which are lost when using the + operator or slicing a circuit [ ].

I think I got what is the solution: one should only append instructions (or circuit slices) with the inst method or equivalently with +=. In this way all the properties of the input circuit are preserved. In other words: a = a + b is bad, while a += b should correctly preserve the properties of a.

[willzeng]

ah gotcha. This is an unexpected idiosyncrasy indeed. Does that mean that replacing all a=a+b with a+=b will fix the issue? If so, it is likely time for a pull request :)

[andreamari]

This issue is now solved by #11 .