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In order to build a custom noise model in Qiskit Aer, I requested the error parameters from different backends and got 3 (9) parameters for each single-qubit (two-qubit) gate as output. However, I could not find any documentation of what these numbers are actually specifying. Based on a previous stack-exchange question about Qiskit's basic_device_noise_model(), I would guess that the first parameter is giving the gate fidelity. Is this true? What are the other 2 (8) numbers specifying?

Here is an example for a single-qubit (id) and a two-qubit (cx) gate from the backend orense:

   {'type': 'qerror',
   'operations': ['id'],
   'instructions': [[{'name': 'id', 'qubits': [0]}],
    [{'name': 'z', 'qubits': [0]}],
    [{'name': 'reset', 'qubits': [0]}]],
   'probabilities': [0.9996936442814689,
    5.460519336879117e-05,
    0.0002517505251623531],
   'gate_qubits': [[0]]},

   {'type': 'qerror',
   'operations': ['cx'],
   'instructions': [[{'name': 'id', 'qubits': [1]}],
    [{'name': 'z', 'qubits': [0]}],
    [{'name': 'reset', 'qubits': [0]}],
    [{'name': 'z', 'qubits': [1]}],
    [{'name': 'z', 'qubits': [0]}, {'name': 'z', 'qubits': [1]}],
    [{'name': 'reset', 'qubits': [0]}, {'name': 'z', 'qubits': [1]}],
    [{'name': 'reset', 'qubits': [1]}],
    [{'name': 'z', 'qubits': [0]}, {'name': 'reset', 'qubits': [1]}],
    [{'name': 'reset', 'qubits': [0]}, {'name': 'reset', 'qubits': [1]}]],
   'probabilities': [0.9893181752693058,
    0.0004106923127677822,
    0.001895703775229592,
    0.002657456705309657,
    1.1031810267579292e-06,
    5.092144099539429e-06,
    0.005698491726413379,
    2.365595624250726e-06,
    1.0919290223224507e-05],
   'gate_qubits': [[0, 1]]},
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Each of those dicts are a QuantumError. You can learn more about the structure of a QuantumError here. That documentation is most likely what you were looking for.

Each instruction is an operation being applied on the qubit(s) in qubits. Each value in probabilities corresponds to the respective operation in instructions.

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