Fast, continuous and coherent atom replacement in a neutral atom qubit array

Neutral atom quantum processors are a promising platform for scalable quantum computing. An obstacle to implementing deep quantum circuits is managing atom loss, which constitutes a significant fraction of all errors. Current approaches are either not capable of replacing lost atoms in the middle of a circuit -- and therefore restricted to fixed, short circuit depths -- or require more than an order of magnitude longer time than gate and measurement operations to do so. In this work, we demonstrate fast, continuous atom replacement leveraging the metastable ^171Yb qubit. A continuously loaded reservoir near the computation zone enables on-demand atom extraction with tweezers up to 500 times per second. New qubit arrays can be prepared 30 times per second when including single-atom preparation, non-destructive imaging and initialization. Importantly, existing qubits are completely undisturbed by the reloading process, owing to the extreme isolation of the metastable qubit from cooling and imaging light. This work establishes a complete foundation for implementing fast quantum circuits with unlimited depth, removing a final roadblock for fault-tolerant quantum computing with neutral atoms.