DepolarizationChannel not completely depolarizing

[jin-sungk]

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Describe the bug

When I set the error probably to 1 using the cudaq.DepolarizationChannel to a given gate and measure the output, I expect the expectation value to be 0 in any basis. I observe a consistent of 0.338 after applying an x gate with an error rate of 1. Further testing with a Rx gate with varying angles that Z expectation correlates with the rotation angle, even with a depolarization rate of 1.

Steps to reproduce the bug

cudaq.set_target('density-matrix-cpu')

def rabi_noisy(time,shots):

    kernel=cudaq.make_kernel()

## set depolarization channel to 1 for rx gate
    noise_model=cudaq.NoiseModel()
    noise_model.add_channel("rx",[0],cudaq.DepolarizationChannel(1))

## vary the rx rotation angle
    qubits=kernel.qalloc(1)
    kernel.rx(time,qubits[0])
    kernel.mz(qubits[0])
    counts=cudaq.sample(kernel,shots_count=shots,noise_model=noise_model)
    exp_z=counts.expectation()
    return exp_z


angles = [x*0.1 for x in range(0,62)]
shots=10000

exp_z=[]

for angle in angles:
    exp_z.append(rabi_noisy(angle,shots))

Expected behavior

The expectation should be 0 for all Rx angles with DepolarizationChannel(1)

Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment

• CUDA Quantum version: 0.7.1
• Python version: 3.10.12
• C++ compiler:
• Operating system: macos Sonoma 14.5

Suggestions

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