ConfigurableFakeBackend qubit_readout_error throws Invalid probabilities randomly

[dieser-max]

Environment

• Qiskit version: 'qiskit': '0.45.0', 'qiskit-aer': '0.12.0'
• Python version: 3.9.16
• Operating system: Windows-10

What is happening?

When choosing a low values for qubit_readout_error in a ConfigurableFakeBackend a Noise_Error will be thrown inconsistency. This does not happen with higher readout error values >10%.
This also seems not to be happening when the errors are defined with a list qubit_readout_error=[0.001, 0.002, 0.0001]

How can we reproduce the issue?

from qiskit import QuantumCircuit
from qiskit.providers.fake_provider import ConfigurableFakeBackend
from qiskit.providers.aer import AerSimulator
configurableFBE = ConfigurableFakeBackend('test', 3, qubit_readout_error=0.004)
qc = QuantumCircuit(3)
qc.measure_all()
backend = AerSimulator.from_backend(configurableFBE)
counts = backend.run(qc).result().get_counts()

Throws
NoiseError: 'Invalid probabilities: [1.0060265594601663, -0.006026559460166213] contains a negative probability.' ~70 % of the time

In qiskit-aer 0.13.0 additonally the description error from qiskit-aer/issues/1987 is thrown.

What should happen?

As long as a valid probability is provided, no negative probability should be returned.

Any suggestions?

No response

[rrodenbusch]

Regarding the NoiseError: In ConfigurableFakeBackend when qubit_readout_error is provided as a float then qubit_readout_error is used as the mean of a Guassian (normal) distribution to generate a list of nqubit readout error values. The default value of the standard deviation, std, for the Gaussian is std=0.01. So the probability of sampling a negative value from this distribution is significant which results in the NoiseError since the readout_error must be >= 0

Suggestion 1: Provide readout errors as a list

configurableFBE = ConfigurableFakeBackend('test', 3, qubit_readout_error=[0.004,0.004,0.004])

Suggestion 2: Provide a value for std so that the probability of a negative value from the Gaussian is minimal

configurableFBE = ConfigurableFakeBackend('test', 3, qubit_readout_error=0.004, std=0.001)