Learning Noise via Dynamical Decoupling of Entangled Qubits

Arrays of superconducting qubits with frequency tunable couplers are capable of producing interesting quantum behavior. Key to this is the ability to produce high-fidelity entangling interactions between qubits. The quality of these interactions is limited by noise caused by system-environment interaction. Characterizing noise in entangled systems is difficult not only because of the large numbers of degrees of freedom but also due to the limited and imperfect available control. This makes achieving good correspondence between theory and experiments difficult. By comparing the behavior of isolated and entangled qubits, we establish that the dominating source of noise during our resonant, excitation preserving two-qubit gates is frequency fluctuations of the coupler, not the qubits themselves. We drive pairs of entangled qubits through CPMG-inspired pulse sequences and observe steps in the decay curves that cannot be produced by Gaussian noise. Via detailed modeling, we establish that this noise is well described by a few stand-alone random telegraph fluctuators which interact with the coupler via its flux bias and have a correlation time on the order of 0.1ms. We also explore spatial inter-qubit correlations in this noise that could be detrimental to multi-qubit algorithms.