Characterization of non-Markovian noise effects in superconducting qubits using pseudo-identity gates

In superconducting quantum computers available today, interactions between qubits and two-level system (TLS) defects in the device are known to be a significant source of noise. Qubit-defect interaction can manifest itself as non-Markovian noise in the dynamics of the qubit subsystem. Existing methods to identify such effects involve low-level noise spectroscopy experiments. We develop a method based on mirrored pseudo-identity gates to characterise qubit-TLS interactions, and include them in a noise model to describe the effect of the TLS defects on the quantum circuits. We run experiments on superconducting quantum computers and find that our method is well suited to characterize the interactions between TLS defects and a qubit, and that their presence is an important source of noise. We also use the method to characterize residual cross-talk interactions between neighbouring qubits. Including the non-Markovian components within our noise model allows us to significantly improve the accuracy of the predictions of the noise model when compared to experiment.