Modeling and suppression of noise in transmons: theory and experiment*

Currently available superconducting quantum processors with interconnected transmon qubits are noisy. The noise can be attributed to various sources such as open quantum system effects and spurious couplings (crosstalk). The ZZ-coupling between qubits in fixed frequency architectures is always present and contributes to both coherent and incoherent errors. We develop a procedure using dynamical decoupling to separate the errors resulting from crosstalk and demonstrate it through experiments performed on IBM quantum cloud processors. We then model the residual open quantum system effects numerically through circuit Hamiltonians. We use the Redfield master equation with a hybrid bath consisting of both high and low frequency components to fit the experimental free evolution decay for Haar-random initial states. We further reproduce the effects of actual time-dependent dynamical pulses on the IBM processors using the fitting parameters obtained for the free evolution case.