Towards multi-qubit readout simulations

Fault-tolerant quantum computing requires uniformly low error rates across all operations, including single- and two-qubit gates as well as qubit readout. Although recent works have demonstrated a readout fidelity of approximately 99.9 % in relatively small devices, achieving similar results in many-qubits devices remains challenging. This is especially true given recent advancements in understanding the limitations of fast and high-power readout. Indeed, placing many photons in a readout resonator can cause unexpected transitions to higher-energy states, a mechanism known as MIST or ionization. While this is well understood at the single- and two-qubit level, it is yet unclear how this problem manifests itself as system size increases. In this work, we develop a general framework to model measurement-induced ionization in many-qubit superconducting circuit devices to quantify how interactions with spectator modes influence this process. We use this approach to provide guidance for optimizing high-fidelity readout in scalable transmon architectures.