CU gate does not get its parameter assigned

[woodsp-ibm]

I ran this from an editable install of current main branch, under Python 3.9 on Linux.

A problem was raised around Qsikit Machine Learning and the EstimatorQNN here https://quantumcomputing.stackexchange.com/questions/32647/qiskit-machine-learning-qnnestimator-estimator-job-failed-when-using-cu-gates The original issue was using an Estimator with the EstimatorQNN but it seems the issue is more fundamental. I included a Sampler instead of the Estimator as the net outcome is the same.

In investigating it seems the cu parameter does not get assigned the value. u works as the above author noted.

from qiskit import QuantumCircuit
from qiskit.circuit import Parameter
from qiskit.primitives import Sampler

p_i = Parameter("input")
p_w = Parameter("weight")
qc1 = QuantumCircuit(2)
qc1.h(0)
qc1.ry(p_i ,0)
qc1.u(p_w, 0, 0, 0)
qc1.cu(p_w, 0, 0, 0, 0, 1)
qc1.rx(p_w, 0)
qc1.measure_all()

print(qc1)

qc2 = qc1.assign_parameters([0.7, 0.9])

print(qc2)
print(qc2.parameters)


sampler = Sampler()
job = sampler.run(qc1, [0.7, 0.9])
result = job.result()

This prints where the parameterized circuit is shown first and then that after the assign. As can be seen the weight param in the cu still appears to there though the circuit reports no parameters as seen by the empty view.

        ┌───┐┌───────────┐┌───────────────┐                   ┌────────────┐ ░ ┌─┐   
   q_0: ┤ H ├┤ Ry(input) ├┤ U(weight,0,0) ├─────────■─────────┤ Rx(weight) ├─░─┤M├───
        └───┘└───────────┘└───────────────┘┌────────┴────────┐└────────────┘ ░ └╥┘┌─┐
   q_1: ───────────────────────────────────┤ U(weight,0,0,0) ├───────────────░──╫─┤M├
                                           └─────────────────┘               ░  ║ └╥┘
meas: 2/════════════════════════════════════════════════════════════════════════╩══╩═

                                                                                0  1 
        ┌───┐┌─────────┐┌────────────┐                   ┌─────────┐ ░ ┌─┐   
   q_0: ┤ H ├┤ Ry(0.7) ├┤ U(0.9,0,0) ├─────────■─────────┤ Rx(0.9) ├─░─┤M├───
        └───┘└─────────┘└────────────┘┌────────┴────────┐└─────────┘ ░ └╥┘┌─┐
   q_1: ──────────────────────────────┤ U(weight,0,0,0) ├────────────░──╫─┤M├
                                      └─────────────────┘            ░  ║ └╥┘
meas: 2/════════════════════════════════════════════════════════════════╩══╩═
                                                                        0  1 
ParameterView([])

The sampler taking the original circuit and parameter values fails to run and raises an error

TypeError: ParameterExpression with unbound parameters ({Parameter(weight)}) cannot be cast to a float.

[wycp]

It seems that CUGate params getter retruns a temporary list
https://github.com/Qiskit/qiskit-terra/blob/bedecbdce563f4e83acc11c7ec5ca6878a36a4b7/qiskit/circuit/library/standard_gates/u.py#L286
once the list is indexed and assigned
https://github.com/Qiskit/qiskit-terra/blob/bedecbdce563f4e83acc11c7ec5ca6878a36a4b7/qiskit/circuit/quantumcircuit.py#L2842
temporary list is updated, but not CUGate fields.
Let's return some object with custom indexed asignmets implementation instead of temporary list...
I would be happy to continue fixing the issue as my first qiskit contribution.

[Cryoris]

I think this should get fixed by #9118, which fixes the parameter assignment issues with the CU gate 🙂

[wycp]

Indeed both issues have the same root cause, thank you for pointing it out.