Leakage detection for a transmon-based surface code

Understand

Leakage outside of the qubit computational subspace, present in many leading experimental platforms, constitutes a threatening error for quantum error correction (QEC) for qubits.

• We develop a leakage-detection scheme via Hidden Markov models (HMMs) for transmon-based implementations of the surface code.
• By performing realistic density-matrix simulations of the distance-3 surface code (Surface-17), we observe that leakage is sharply projected and leads to an increase in the surface-code defect probability of neighboring stabilizers.
• Together with the analog readout of the ancilla qubits, this increase enables the accurate detection of the time and location of leakage.