  Transmon qubits experience open system effects that manifest as noise at a
broad range of frequencies. We present a model of these effects using the
Redfield master equation with a hybrid bath consisting of low and
high-frequency components. We use two-level fluctuators to simulate 1/f-like
noise behavior, which is a dominant source of decoherence for superconducting
qubits. By measuring quantum state fidelity under free evolution with and
without dynamical decoupling (DD), we can fit the low- and high-frequency noise
parameters in our model. We train and test our model using experiments on
quantum devices available through IBM quantum experience. Our model accurately
predicts the fidelity decay of random initial states, including the effect of
DD pulse sequences. We compare our model with two simpler models and confirm
the importance of including both high-frequency and 1/f noise in order to
accurately predict transmon behavior.
