  Quantum Error Correction (QEC) is key to operating quantum processors
effectively at practical scales. QECs are designed for systems comprising
two-level systems, such as qubits, as their fundamental building block.
Unfortunately, qubits can leak to third and higher energy levels, making these
leaks challenging to detect and mitigate. If not addressed promptly, these
leakage errors can proliferate and undermine QEC, leading to significant
computational inaccuracies. Here, we present a high-fidelity three-level qubit
readout protocol that is simple to implement on dedicated hardware such as
FPGAs. Our design enables faster and higher-fidelity leakage detection over
approaches using conventional qubit-state discriminators.
