Ensure scalar parameter names are preserved during conversion

[natestemen]

Description: When converting Quil programs with single-value (scalar) parameters, ensure that the parameter name remains consistent by returning the symbolic name.

Motivation: When upgrading from pyquil v3 to v4, the output of a simple program changes subtly. If the starting pyquil program is

program = Program()
theta = program.declare("theta", memory_type="REAL")
program += RZ(theta, 0)

Then the diff between v3 program.out() and v4 program.out() is as follows.

-RZ(theta) 0
+RZ(theta[0]) 0

So far, this has nothing to do with cirq, but when calling cirq_rigetti.circuit_from_quil(program.out()) the instantiated cirq.Circuit now has a parameter name theta_0 which was never specified by the user.

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[natestemen]

I was able to reproduce this locally, but the CI failure made me look at it again. It seems that declared_size is not always set (i.e. it depends on how you instantiate the pyquil.Program: either via a quil string, or sequentially building it via pyquil operations). This makes it an unreliable signal for determining whether the variable is actually a scalar.

I'm a bit stumped as to what the best approach would be to resolve the underlying issue. Any advice would be appreciated.

[pavoljuhas]

@natestemen - it seems the CLA check is failing (the second comment here). Please fix, otherwise we are not able to consider the PR.

[natestemen]

Ah sorry about that! Didn't realize it was blocking a review. Resolved now.

[pavoljuhas]

@natestemen - after testing this a bit, I would argue the current cirq parameters naming is more close to the pyquil's program.out() behavior in your initial https://github.com/quantumlib/Cirq/pull/6755#issue-2574314732. The declaration of the theta scalar creates an array of size 1 so it's value is later referenced as theta[0].

Unless this is a bug in pyquil I feel it is better to leave the code as is. (cc @jselig-rigetti - can you please comment?)

BTW, the test quil code added in this PR renders the same for pyquil 3 and 4 with parameter expression theta[0]. Converting such program to a circuit with parameter theta_0 sounds sensible.

from pyquil import Program
print(Program("DECLARE theta REAL[1]\nRZ(theta) 0"))

output:

DECLARE theta REAL[1]
RZ(theta[0]) 0

[natestemen]

The main point of confusion is that parameter names are no longer consistent across SDKs. E.g. I define a a pyquil program with a parameter, I have to do some amount of logic to figure out what the parameter name is in the cirq circuit. That's a surprising behavior IMO.

That said, I understand where you're coming from, and the current implementation keeps things simple. I appreciate you taking a look!