How is quantum error applied to the qubits?

Question

I am trying to check the way qiskit has the noise implemented. I have read how is this theoretically done using quantum channels (see Nielsen and Chuang chapter 8) and I want to verify if qiskit follows the same procedure. I have started by the bit-flip error using the pauli_error function. Finally it creates a quantum error object which can be added to the NoiseModel object. However when adding this you have to use the add_all_qubit_quantum_error function and I do not really understand how this works. It is supposed to add the quantum error to all qubits when given a set of gates as an argument, as in the following example:

# Example error probabilities
p_reset = 0.03
p_meas = 0.1
p_gate1 = 0.05

# QuantumError objects
error_reset = pauli_error([('X', p_reset), ('I', 1 - p_reset)])
error_meas = pauli_error([('X',p_meas), ('I', 1 - p_meas)])
error_gate1 = pauli_error([('X',p_gate1), ('I', 1 - p_gate1)])
error_gate2 = error_gate1.tensor(error_gate1)

# Add errors to noise model
noise_bit_flip = NoiseModel()
noise_bit_flip.add_all_qubit_quantum_error(error_reset, "reset")
noise_bit_flip.add_all_qubit_quantum_error(error_meas, "measure")
noise_bit_flip.add_all_qubit_quantum_error(error_gate1, ["u1", "u2", "u3"])
noise_bit_flip.add_all_qubit_quantum_error(error_gate2, ["cx"])

print(noise_bit_flip)

However I do not know when the bit-flit is applied. Is it applied everytime one gate that can be decomposed in any of the ones of the list ["u1", "u2", "u3"]? What I need to know is how often it is applied and with which criterion.

Thank you for your help!

Answer 1

Qiskit simulator behaves like this for your code snippet: whenever it encounters one of u1, u2, or u3 gates (in the compiled circuit), it first applies the gate; then it performs an X gate according to the provided probability (0.05 in our case).