I wish to simulate the gate errors from IBM's various quantum computers. However I am not sure what the gate error rate physically means. An error rate of say 0.01 for a particular means one in every 100 trials the gate will not return the desired outcome. But if I wish to simulate this myself I need to know what outcome is produced. Is it simply a random outcome? For example if I am implementing a CNOT gate on the state |11>, what happens when the gate fails? What is the state after this?
Different quantum channels can cause the same gate error. One option is to assume that the gate error is caused by a depolarizing channel, followed by thermal relaxation. This is the assumption made in
NoiseModel.from_backend, whose documentation can help you: https://qiskit.org/documentation/stubs/qiskit.providers.aer.noise.NoiseModel.from_backend.html#qiskit.providers.aer.noise.NoiseModel.from_backend.