Beyond two-level approximation in transmon arrays: Dark states and Leakage dynamics

Abstract:
Arrays of transmons have proven to be a viable medium for quantum information science and technology. Despite their success as qubit arrays, there remains yet untapped potential beyond the two-level approximation. Being experimentally controllable with good accuracy, the higher excited states provide an important resource for hardware-efficient many-body quantum simulations, quantum error correction, and quantum information protocols. The first part of the talk focuses on the dark and bright states of a transmon array in a waveguide. I show that how the bosonic many-body statistics enhances collective sub- and superradiance compared to that of qubit arrays and how to utilize side-port driving for local and non-local quantum gates for the dark states. The second part concentrates on the dynamics of the higher-excited states. An application is a passive leakage removal unit based on an array of coupled disordered transmons and last-site reset by feedback-measurement or dissipation. The unit effectively removes leakage with minimal effect to the coding qubit subspace by taking advantage of energy level disorder for qubit subspace and resonant leakage energy levels.