Readout error mitigation for mid-circuit measurements and feedforward

Current-day quantum computing platforms are subject to readout errors, in which faulty measurement outcomes are reported by the device. These errors are especially detrimental on quantum circuits containing branch statements, where future quantum operations are determined on-the-fly by the outcomes of measurements made mid-circuit, as they cause incorrect branches of the circuit to be executed. Standard error mitigation methods for terminal measurements which act in post-processing are inapplicable in this context. Here we present a general method for readout error mitigation addressing these mid-circuit measurement errors. Our protocol works on arbitrary numbers of layers of mid-circuit measurements and feedforward, is unbiased, and does not increase the circuit depth nor two-qubit gate counts. Our scheme uses a form of gate twirling for symmetrization of the error channels and probabilistic bit-flips in feedforward data to average over an engineered ensemble of quantum trajectories. We experimentally demonstrate the effectiveness of our method, obtaining up to a ∼60% reduction in error on superconducting quantum processors for several types of feedforward circuits of practical interest, including dynamic qubit resets, shallow-depth GHZ state preparation, and multi-stage quantum state teleportation.

Based on preprint: arXiv:2406.07611