Change RB test parameters to make the test faster and more robust

[wshanks]

test_add_more_circuit_yields_lower_variance started failing with qiskit-aer 0.13.0. The test was only comparing the results of 3 RB sampels to 5 RB samples and looking for the case with more samples to have lower variance. These numbers of samples were likely not enough to ensure that the case with more samples always had lower variance across a wide range of random seeds. Here the high number of samples was increased to 30.

To make sure that the fit results were reasonable and that the test run time did not get too long, these additional changes were:

• Convert the test from two qubit RB to single qubit RB. This makes the simulation faster.
• Increase the single qubit gate error. This makes the qubit depolarize faster so that the RB curve can be traced out with less gates.

[wshanks]

Here is what the two cases look like now (starting to push the limit a bit of noisy data vs fast test time):

[wshanks]

And here the test code pulled out in case you want to play with the numbers in a notebook or script which is what I did:

from qiskit_aer import AerSimulator
from qiskit_aer.noise import NoiseModel, depolarizing_error
from qiskit_experiments.library import StandardRB


# depolarizing error
p1q = 0.15
p2q = 0.04
pvz = 0.0
pcz = 0.06

basis_gates = ["rz", "sx", "cx"]

# setup noise model
sx_error = depolarizing_error(p1q, 1)
rz_error = depolarizing_error(pvz, 1)

noise_model = NoiseModel()
noise_model.add_all_qubit_quantum_error(sx_error, "sx")
noise_model.add_all_qubit_quantum_error(rz_error, "rz")

noise_model = noise_model

# Need level1 for consecutive gate cancellation for reference EPC value calculation
transpiler_options = {
    "basis_gates": basis_gates,
}

backend = AerSimulator(noise_model=noise_model, seed_simulator=123)

exp1 = StandardRB(
    physical_qubits=(0,),
    lengths=list(range(1, 30, 3)),
    seed=444,
    backend=backend,
    num_samples=3,
)
exp1.analysis.set_options(gate_error_ratio=None)
exp1.set_transpile_options(**transpiler_options)
expdata1 = exp1.run().block_for_results()

exp2 = StandardRB(
    physical_qubits=(0,),
    lengths=list(range(1, 30, 3)),
    seed=123,
    backend=backend,
    num_samples=30,
)
exp2.analysis.set_options(gate_error_ratio=None)
exp2.set_transpile_options(**transpiler_options)
expdata2 = exp2.run().block_for_results()

[coruscating]

LGTM, thanks