Fast, low-photon-number readout using the arm qubit: Part I

Fast and high-fidelity qubit state measurement is essential for quantum information processing and error correction. In superconducting qubits, state-of-the-art readout typically relies on a dispersive interaction between the qubit and a readout resonator, but further speedups are constrained by limits on dispersive coupling strength and increased leakage at large photon numbers. We present a new readout approach based on the arm qubit architecture, which comprises a long-lived "data" mode coupled to a tunable "arm" mode [1]. The arm mode enables strong nonlinear coupling to a readout resonator, with numerical simulations predicting state-assignment fidelity of 99.99% in 27 ns with ~3.5 photons, assuming a quantum efficiency of 0.5. This design also eliminates the need for a Purcell filter and suppresses shot-noise dephasing during idle time. We report progress toward an experimental implementation of fast readout using the arm qubit.