Leakage detection for a transmon-based surface code

Boris Mihailov Varbanov*, Francesco Battistel, Brian Michael Tarasinski, Viacheslav Petrovych Ostroukh, Thomas Eugene O’Brien, Leonardo DiCarlo, Barbara Maria Terhal

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

23 Citations (Scopus)
72 Downloads (Pure)


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. We restore the logical error rate below the memory break-even point by post-selecting out leakage, discarding less than half of the data for the given noise parameters. Leakage detection via HMMs opens the prospect for near-term QEC demonstrations, targeted leakage reduction and leakage-aware decoding and is applicable to other experimental platforms.

Original languageEnglish
Article number102
Number of pages13
JournalNPJ Quantum Information
Issue number1
Publication statusPublished - 2020


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